Hybrid Integrated Components for Optical Display Systems ... · Vladimir Kartashov, always have...

Hybrid Integrated Components

for Optical Display Systems --

Electroactive Polymer Based

Micro-Opto-Electro-Mechanical

Actuators

by

Guangmin Ouyang

Thesis submitted for the degree of Philosophiae Doctor

Department of Physics

University of Oslo

March 2012

iii

Abstract

The current worldwide market for TV and projectors is a multi-billion dollar business.

It is expected that laser based displays will gradually replace standard lamp projection

systems, attributed to their outstanding performance in: large colour gamut, high contrast

and long lifetime. One of the technical obstacles to overcome for wide acceptance of

lasers displays is the speckle noise, which is an image degrading phenomenon inherent in

the coherent laser light.

The demand of the market gives the motivation of the project BIA-286 (founded by

Research Council of Norway, Project number 182667): hybrid integrated components in

laser display systems for speckle reduction. As part of the project, the thesis focus on de-

speckle solutions based on electroactive polymer (EAP) technology and micro-electro-

mechanical systems (MEMS) technology. To be more specific, in the present work we are

looking for modulators, with working mechanism of polymer electro-mechanical

actuation, to apply in laser display systems for speckle reduction. The modulators are

expected to possess the merits such as compact, efficient, easy implantation and low cost.

Two working packages are included in the thesis: material investigation/optimization

and modulator design/fabrication. As one of most widely used EAPs, dielectric

elastomers (DEs) are investigated, since their physical properties can be tuned by

modification of their chemical structures. The target of DE material

investigation/optimization is obtaining a polydimethylsiloxane (PDMS) with fast

response time and requiring low driving voltage. In-depth understanding of PDMS, i.e.

the polymer micro-structure dependent electro-mechanical properties, is established

(Reported in Article I). The PDMS network structures are modified to achieve the desired

physical properties. PDMS with highly cross-linked network and minimum residual

groups show fast response time, but require high driving voltage.

We proceed to optimize PDMS electro-mechanical performance via network swelling,

as reported in Article II. Cross-linked PDMS is swollen by silicone oil. By introducing

short chain miscible solvent, polymer chains are surrounded by highly mobile solvents.

The ease of chain movement results in lower driving voltage requirement. Meanwhile, the

response time of PDMS increased, but the increase is rather moderate comparing with the

increase in Article I.

iv

Next, PDMS material optimization is carried out by filling PDMS with TiO2

nanoparticles (reported in Article III). The PDMS-TiO2 nanocomposite is prepared by in-

situ polymerization. The dielectric constant of PDMS-TiO2 composite increases with the

filler concentration. An optimization point is found at 5 wt% TiO2 particles. Comparing

with neat PDMS, the driving voltage is reduced (due to the increased dielectric constant),

and the response time is improved (due to the decreased chain mobility).

The second working package focuses on designing and fabricating of DE based

modulators, either as spatial light modulators (SLM) or de-speckle modulators. A SLM

with PDMS film sandwiched between two metal electrodes is reported in Article I. An air

gap structure is proposed and demonstrated in Article II. The reliability of the SLM is

improved by avoiding direct contact between the deformable PDMS surface and the rigid

electrodes. An alternate strategy for improving the modulator reliability is replacing the

top metal electrodes with transparent conductive polymer (PEDOT: PSS), as reported in

Article IV. This SLM has a sandwich structure PEDOT: PSS/PDMS/Gold electrode.

Reliability of the device is improved since PDMS is more compatible with PEDOT: PSS

than metals.

The DE material is applied in de-speckle modulators. A modulator by using dynamic

gratings for speckle reduction is proposed in Article V. To achieve a high de-speckle

efficiency, the DE material with response speed of 10 µs is employed. By dynamically

splitting the laser beam into different high orders and then passing through a diffuser,

varied speckle patterns are generated on the screen. The speckle noise is smoothed out by

adding these speckle patterns together in the integration time of human eye. A de-speckle

modulator with DE material as dynamic gratings is fabricated and demonstrated for 50 %

speckle reduction.

Besides the DE based modulator, we also look for other speckle reduction techniques.

Two piezoelectric benders have been employed for speckle reduction, as reported in

Article VI. Speckle noise is suppressed to 6 % and 9 % in free space geometry and

imaging geometry respectively, by dynamically changing the illumination angle of the

laser beam on a random phase diffuser.

v

Acronyms

CP Conductive Polymer PSS Poly(styrenesulfonate)

CPO Copper-Phthalocyanine Oligomer PU Polyurethane

CTE Coefficient of Thermal Expansion PVDF Polyvinylidene fluoride

DE Dielectric Elastomer P(VDF-TrFE)

Poly(vinylidenefluoride-co-trifluoroethylene)

DMD Digital Micromirror Device SC Speckle Contrast

DOE Diffractive Optical Element SEM Scanning Electron Microscopy

EAP Electroactive Polymer TIR Total Internal Reflection

ERFP Electrostrictive Relaxor Ferroelectric Polymers

FEM Finite Element Method

GLV Grating Light Valve

IG Ionic Gel

ITO Indium Tin Oxide

LED Light Emitting Diode

MEMS Micro-Electrical-Mechanical Systems

MOEMS Micro-Opto-Electro-Mechanical Systems

MW Molecular Weight

OLED Organic Light Emitting Diode

PANI Polyaniline

PDMS Polydimethylsiloxane

PEDOT Poly(3,4-ethylenedioxythiophene)

PZT Lead Zirconate Titanate

PMN-PT Lead Magnesium Niobate-Lead Titanate

vi

Preface

This thesis is submitted in fulfilment of the requirements for the degree of Philosophiae

Doctor at the University of Oslo, Norway.

The work presented in this thesis was performed at the Department of Micro and Nano

Systems Technology (IMST) at Vestfold University College, Horten. The project is

funded by the Research Council of Norway (NFR) and poLight AS under grant BIA-286

project number 182667, “Hybrid Integrated Components for Optical Display Systems”.

I would like to express my sincere gratitude to my supervisors Prof. Xuyuan Chen,

Prof. Kaiying Wang and Prof. Per Øhlckers. I am grateful for the inspiring guidance, the

fruitful discussions and support they offered. Many thanks to M. Nadeem Akram,

Vladimir Kartashov, always have good ideas for improving the research. Special thanks

to Prof. Einar Halvorsen and Zekija Ramic for their constant help in my study. Finally, I

would like to thank my parents and my fiancé for the support and understanding during

these years.

vii

List of articles

Journal publications

i. G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y. Chen, "A novel

tunable grating fabricated with viscoelastic polymer () and conductive polymer

(PEDOT)," Sensors and Actuators A: Physical, Vol. 158, Issue. 2, pp. 313-

319, 2010.

ii. G. Ouyang, Z. Tong, M. N. Akram, K. Wang, V. Kartashov, X. Yan, and X.

Chen, "Speckle reduction using a motionless diffractive optical element,"

Optics Letters, Vol. 35, Issue. 17, pp. 2852-2854, 2010.

iii. G. Ouyang, K. Wang, M. Akram, L. Henriksen, and X. Chen, "Optimization

of PDMS network for a fast response and sensitive actuation material applied

in a MEMS spatial light modulator," Applied Physics A: Materials Science &

Processing, Vol. 103, Issue. 2, pp. 381-388, 2011.

iv. G. Ouyang, K. Wang, L. Henriksen, and X. Chen, "Electroactive polymer-

based spatial light modulator with high reliability and fast response speed,"

Optical Engineering, Vol. 50, No. 124001, 2011.

v. G. Ouyang, K. Wang, and X. Y. Chen, "TiO2 nanoparticles modified

polydimethylsiloxane with fast response time and increased dielectric

constant," Submitted to Journal of Micromechanics and Microengineering,

2012.

vi. G. Ouyang, M. N. Akram, K. Wang, Z. Tong and X. Y. Chen, “Laser speckle

reduction based on angular diversity induced by piezoelectric benders,”

Submitted to IEEE photonics letters, 2012.

vii. M. N. Akram, Z. Tong, G. Ouyang, X. Chen, and V. Kartashov, "Laser

speckle reduction due to spatial and angular diversity introduced by fast

scanning micromirror," Applied Optics, Vol. 49, Issue. 17, pp. 3297-3304,

2010.

viii

Conference proceedings:

i. G. Ouyang, K. Wang, M. N. Akram and X. Y. Chen, "Fabrication and

characterization of polymer based spatial light modulators, " Proc. SPIE 7426,

742604-8, 2009.

ii. G. Ouyang, K. Wang, L. Heriksen, M. N. Akram, and X. Y. Chen,

"PEDOT:PSS-- Material property characterization and spatial light modulator

fabrication, " Proc. IMAPS Nordic, pp. 112-114, 2009.

iii. G. Ouyang, Z. Tong, W. Gao, K. Wang, M. N. Akram, V. Kartashov and X.

Y. Chen, "Polymer Based Multiple Diffraction modulator for Speckle

Reduction, " Proc. SPIE 73871, 73871F, 2010.

iv. G. Ouyang, K. Wang, and X.Y. Chen, "Enhanced electro-mechanical

performance of TiO2 nanoparticle modified polydimethylsiloxane (PDMS) as

electroactive polymers," Solid-State Sensors, Actuators and Microsystems

Conference (TRANSDUCERS), pp. 614-617, 2011.

v. G. Ouyang, Z. M. Tong, M. N. Akram, V. Kartashov, K. Y. Wang, and X. Y.

Chen, "Speckle reduction in laser displays using a MEMS tandem modulator,

" Solid-State Sensors, Actuators and Microsystems Conference

(TRANSDUCERS), pp. 538-541, 2011.

vi. Guangmin Ouyang, Kaiying Wang, Xuyuan. Chen, "Enhanced electro-

mechanical performance of TiO2 nanoparticle modified polydimethylsiloxane

(PDMS) as electroactive polymers, " EuroEAP 2011, Pisa, 2011.

vii. Guangmin Ouyang, Kaiying Wang, Lars Henriksen, and Xuyuan Chen, "High

sensitivity and fast response electroactive polymer via network swelling,"

Micromechanics and Micro systems Europe Workshop 2011, Tønsberg, 2011.

viii. Guangmin Ouyang, Kaiying Wang, and Xuyuan Chen, "Enhanced electro-

mechanical performance of TiO2/PDMS nanocomposite and its application in

MEMS tuneable gratings," Micromechanics and Micro systems Europe

Workshop 2011, Tønsberg, 2011.

ix. Kaiying Wang, Guangmin Ouyang, Muhammad N. Akram, Lars Henriksen

and Xuyuan Chen, "Surface and electromechanical properties of

polydimethylsiloxane films prepared by a modified molding technique," Proc.

SPIE 7381, 73810C, 2009.

ix

x. Kaiying Wang, Guangmin Ouyang, Xuyuan Chen, "Surface modification and

wettability of silicone PDMS film," Electronic System-Integration Technology

Conference (ESTC), Berlin, 2010.

xi. Akram M. N., Kartashov V., Wang K., Ouyang G.., Chen X., Laser speckle

reduction using dynamic polymer-based diffraction grating spatial phase

modulator, International symposium on photoelectronic detection and

imaging, Proc. SPIE 7382, 73822H (2009).

xii. M. Nadeem Akram, Guangmin Ouyang, Wenhong Gao, Zhaomin Tong and

Xuyuan Y. Chen, "Speckle reduction in line-scan laser projectors using binary

phase codes: theory and experiments, " Proc. SPIE 7387, 73871P, 2010.

xiii. Z. M. Tong, G. M. Ouyang, W. H. Gao, M. N. Akram, V. Kartashov, K. Y.

Wang, and X. Y. Chen, "Simulation of laser speckle reduction by using an

array of diffraction gratings, " Proc. SPIE 7387, 73871S, 2010.

xiv. Zhaomin Tong, Muhammad Nadeem Akram, Guangmin Ouyang, and Xuyuan

Chen, Vladimir Kartashov, "Laser Speckle Reduction Using MEMS Scanning

Mirror, " SID Symposium Digest of Technical Papers, Volume 41, Issue 1, pp.

1909-1912, 2010.

xv. Wenhong Gao, Zhaomin Tong, Muhammad Nadeem Akram, Guangmin

Ouyang, Xuyuan Chen, "Construction of Electrical Actuated two Dimensional

Phase Mask for Speckle Contrast Reduction in a Full Frame Laser Projector

System, " Speckle 2010, Brazil, 2010.

xvi. Wenhong Gao, Vladimir Kartashov, Guangmin Ouyang, Zhaomin Tong,

Muhammad Nadeem Akram and Xuyuan Chen, "The matrices used to reduce

speckle in laser projector system, " Proc. SPIE 7387, 73871R, 2010.

xvii. Wenhong Gao, Muhammad Nadeem Akram, Guangmin Ouyang, Zhaomin

Tong and Xuyuan Chen, "Speckle reduction in line-scan laser projectors using

binary phase codes: theory and experiments," Proc. IDW10, Vol.1, pp.1569-

1572, 2010.

xviii. Xu Dang, Guangmin Ouyang, Zhaomin Tong, Nan Feng, Dong Yang, Akram

Muhammad Nadeem, Kaiying Wang and Xuyuan Chen, "Dynamic periods

tandem grating for speckle reduction, " Micromechanics and Micro systems

Europe Workshop 2011, Tønsberg, 2010.

xi

Contents

Abstract ..................................................................................................................... iii

Acronyms .................................................................................................................... v

Preface ....................................................................................................................... vi

List of articles ........................................................................................................... vii

1 Introduction ....................................................................................................... 1

1.1 Overview of the electroactive polymer technology ................................... 1

1.2 Dielectric elastomer: actuation mechanism, configuration and challenges 3

1.3 Applications of dielectric elastomers ......................................................... 5

1.3.1 Dielectric elastomer for MEMS spatial light modulator ........................ 6

1.3.2 Dielectric elastomer for MEMS de-speckle modulator ......................... 8

1.4 Aims and tasks of the study ...................................................................... 14

1.5 Outline of the thesis .................................................................................. 15

2 Summary of the articles .................................................................................. 16

2.1 Article I..................................................................................................... 16

2.2 Article II ................................................................................................... 21

2.3 Article III .................................................................................................. 24

2.4 Article IV .................................................................................................. 29

2.5 Article V ................................................................................................... 32

2.6 Article VI .................................................................................................. 36

3 Conclusions ...................................................................................................... 41

References ................................................................................................................. 43

Article I ..................................................................................................................... 51

Article II ................................................................................................................... 52

Article III .................................................................................................................. 53

Article IV .................................................................................................................. 54

Article V .................................................................................................................... 55

Article VI .................................................................................................................. 56

Hybrid Integrated Components for Optical Display Systems -- Electroactive Polymer Based Micro-Opto-Electro-Mechanical Actuators

1

1 Introduction

1.1 Overview of the electroactive polymer technology

Electroactive polymers (EAPs) have attracted considerable interests in applications of

electro-mechanical actuators. Traditional actuators include electro-magnetic actuators,

motors, hydraulic cylinders and pneumatic actuators. The relatively big size, heavy

weight, complex transmission and high cost of production limit their application area and

motivate the research on alternative technologies [1]. EAPs are polymers that exhibit a

change in size or shape when stimulated by an electric field and are emerging as a new

class of actuation material. The advantage of the EAP based modulators involve

lightweight, low cost, durability, easy fabrication, simple actuation configuration, etc [2].

EAPs can be broadly categorized into two major classes: electronic EAPs and ionic

EAPs. Further subdivision based on their actuation mechanism and type of material

involved is also possible. Dielectric elastomers (such as silicones [3-6], polyurethanes [3,

7], acrylics [4, 8, 9]), electrostrictive relaxor ferroelectric polymers (such as

polyvinylidene fluoride [3] and poly(vinylidenefluoride-co-trifluoroethylene)) [10-12]

and liquid crystal elastomers [13-15] can be put under electronic classification. The

actuation of electronic EAPs is generally from Maxwell stress or electrostriction. Ionic

gels (such as polyacrylic acid gels [16, 17] ), ionic polymer-metal composites [18-20] and

conductive polymers (such as polypyrroles [21, 22] and polyanilines [23, 24] ) fall under

the ionic classification. The actuation mechanism for ionic EAPs involves the diffusion or

mobility of ions. A comparison of EAP material properties is given in Table. 1.

The actuation mechanisms and configurations vary with the type of EAPs. Taking

electrostrictive relaxor ferroelectric polymers as an example, when the voltage is applied,

the polar groups start to align with regard to the electric field and result in crystal

elongation. This deformation is reversible when electric field in opposite direction is

applied. Poly(vinylidenefluoride-co-trifluoroethylene) has been investigated by Zhang’s

group [25]. A microfluidic pump is realized using this material, as shown in Figure 1. In

this microfluidic pump, the polymer diaphragm bends up and down under with applied

electric field, pumping the fluid through the channel along the preferred flow direction.


2

Tabl

e 1.

Cla

ssifi

catio

n of

EA

P m

ater

ials

and

thei

r phy

sica

l pro

perti

es c

ompa

rison

.

Ref

eren

ces

[3]

[3]

[3, 7

]

[4, 8

]

[3]

[12]

[16,

17]

[23]

Res

pons

e sp

eed

Fast

Fast

Fast

Med

ium

Fast

Fast

Slow

Slow

Die

lect

ric

cons

tant

2.8

2.8

6 4.8

8.4

50

--

35

Ela

stic

m

odul

us

(MPa

)

0.7

0.5

0.5

3 --

380

--

900

Spec

ific

dens

ity

1 1.5

1.1

1 1.8

1.8

≈1

≈1

Max

imum

pr

essu

re

(MPa

)

1.36

0.39

1.6

7.2

4.8

43

0.3

450

Max

imum

st

rain

(%)

32

38

22

380

0.1

4.3

>40

10

EA

P m

ater

ials

DE-

- Sili

cone

DE-

- Flu

oros

ilico

ne

DE-

-Pol

yure

than

e

DE-

-Acr

ylic

ERFP

--PV

DF

ERFP

—P(

VD

F-

TrEF

)

IG--

Pol

yele

ctro

lyte

CP-

- PA

NI

Ele

ctro

nic

EA

PS

Ioni

c

EA

PS


3

Figure 1. Schematic drawing of the microfluidic pump using P(VDF-TrFE) as the active polymer

material (see Ref. [25] ).

Detailed explanation of the actuation mechanisms and configurations for other EAPs

refers to [1, 3, 26]. The present research focuses on dielectric elastomer (DE), which is

going to be introduced in the following section.

1.2 Dielectric elastomer: actuation mechanism and configurations

Dielectric elastomers have been reported to have large achievable strain/stress, fast

response speed, long life time, high reliability and durability [27]. Some types of DE,

such as polydimethylsiloxane (PDMS), is chemical and environmental inert. It has

resistance to high temperature, UV radiation and chemical attack. Moreover, the physical

properties of DEs can be tuned based on application requirements, by modifying its

micro-structure. Attributing to these merits, DE materials are are progressively emerging

as one of the best-performing materials and they are investigated in these work.

The actuation mechanism of DE is electro-mechanical, that a mechanical movement of

DE is achieved by applying electrical energy. Under electric field, DE deforms either by

Maxwell stress or electrostriction, or both. The Maxwell stress effect arises from

Coulomb electrostatic interactions among free charges between two electrodes. The

change of electric field distribution inside the DE is manifested as strain [28]. For DE

with elastic modulus Y and relative dielectric constant εr, subjected to an electric field E,

the Maxwell stress induced strain SMaxwell, which is in the direction perpendicular to the

electrodes, is calculated by:

SMaxwell = - ε0εrE2/Y (1)


4

where ε0=8.85 pF/m is the dielectric constant of vacuum, the minus sign indicates that it is

always a compressive strain. The field induced deformation in silicones and acrylics

elastomers is primarily due to the Maxwell stress effects [9, 27].

Electrostriction arises due to the change of material dielectric properties under electric

field. The electrostriction induced strain Selectrostriction is calculated by:

Selectrostriction= - Q ε02(εr-1)2E2 (2)

where Q is the electrostrictive coefficient. From the equation, there is a direct coupling

between electric polarization and mechanical strain response. Electrostriction contributes

largely to the electromechanical response of polyurethanes.

In this work, one of the silicones, PDMS, are focused. Therefore it is the Maxwell

stress induced strain which is mostly concerned.

The basic element of DE based actuator is shown in Figure 2, where a DE layer is

sandwiched between two electrodes. At least one of the electrodes is compliant to couple

to the DE deformation. For Maxwell stress induced strain, when the electric field is

applied via electrodes, the electrostatic attraction between opposite charges and the

repulsion of like charges on the electrodes generate stresses on DE. The DE contract in

thickness and expand in area [26].

Figure 2. Typical configuration of DE based actuators. The DE actuated by means of electrostatic

forces applied via electrodes on the sides of DE material.

The basic element as shown in Figure 2 can be incorporated into a wide variety of

configurations. Several representative dielectric elastomer actuator configurations are

given in Figure 3. The DE films can be stretched over a frame, rolled into a scroll, formed

into a tubular shape, or laminated on a flexible substrate to form unimorphs and

bimorphs. The most suitable configuration depends on the application requirements and

the properties of DE films.


5

Figure 3. Representative dielectric elastomer actuator configurations (see Ref. [17] ).

Unimorph and bimorph actuators are benders. In unimorph benders, the DE films are

sandwiched between two electrodes. As one end of the DE film is fixed, when voltage is

applied, the free end of the DE film bends according to the polarity of the applied field. A

bimorph is stacking of two unimorph benders. Unimorph and bimorph benders have

application potential in low force robotic grippers. Diaphragm actuators can exploit both

directions of planar expansion of the DE films. This kind of actuators is promising in

applications such as pumps and loudspeakers. Spider and bowtie actuators are designed to

couple both planar directions into a single linear direction. They have been used in robotic

leg applications such as the self-contained hexapod robot [29]. Spring rolls are interesting

from the perspective that they are capable of providing a combination of large linear

strains with relatively large output forces [30, 31]. It capable of both bending and

elongation [32].

1.3 Applications of dielectric elastomers

Many applications have involved the DE actuation technology. The best example is

given by Swiss Federal Laboratories in 2005 EAP arm wrestling competition [33]. The

present robot, as shown in Figure 4, is driven by an array of rolled DE actuators. 250


6

rolled DE actuators with small diameters are arranged in two groups according to the

human muscle configuration in order to achieve an arm-like bidirectional rotation

movement. The robot is weight 15-20 kg and powered by a computer-controlled high

voltage amplifier. Though the robot lost the arm wrestling competition against human

component, it demonstrated the very bright feature for DE as artificial muscles.

Figure 4. (a) The spring roll DE actuator in robot. (b) Demonstration of arm wrestling robot. (see

Ref. [33] ).

Attributing to its light weight, simple actuation mechanism, easy processing, reliable

and bio-compatible, DE material is an excellent candidate in compact MEMS devices. In

the present work, the DE technology has been employed in MEMS modulator for optics

applications, i.e. for light modulation and speckle reduction. Details of these two

applications are introduced in this section.

1.3.1 Dielectric elastomer for MEMS spatial light modulator

Traditional SLMs are primarily based on silicon technology. Grating light valve (GLV)

[34], which is originally developed at Stanford University, is made by an 1D array of tiny

movable silicon nitride ribbons mounted on a silicon base. Digital micromirror device

(DMD) [35], which is invented by Texas Instruments, uses a 2D array of tilting silicon

micromirrors. Spatial optical modulator (SOM) [36], invented by Samsung, uses an array

of movable ribbons made of polarized lead zirconate titanate (PLZT). Commercialized

products based on these technologies are existing in the market, but the complex

processing steps result in high cost of the device.

DE materials have been considered as promising candidates in MEMS spatial light

modulators. The configuration and operation principle of a typical DE based SLM is

illustrated in Figure 5. A thin DE film is sandwiched between top reflective electrode and

rigid substrate with bottom electrodes. If no voltage is applied, the polymer has a plane


7

surface. The top electrode works as a flat mirror, reflecting the incoming light. If voltage

is applied through the top and bottom electrodes, the polymer surface deforms into a

sinusoidal shape according to the electric field distribution. From the reflective top

electrodes, higher diffraction orders of the light are propagated with different angles. By

selecting/blocking different diffraction orders, the light intensity of the incoming lights

can be modified.

Figure 5. Operation principle of the EAP based spatial light modulator. (a) no voltage is applied;

(b) voltage is applied.

DE based SLMs have many advantages comparing with traditional silicon based

SLMs. Firstly, the fabrication process is simple, therefore the yield is increased and the

cost is reduced. Secondly, a continuously reflective membrane is used on the top of DE,

giving 100% optical fill factor. Thirdly, since the number of pixels is determined by the

bottom electrodes, the resolution of the SLM is improved by enlarging the number of the

bottom electrodes.

Many DE based SLMs have been investigated [37-44]. One technical obstacle in

fabrication is the surface quality of metal films on DE. Buckles in metal film was reported

in [44], as shown in Figure 6. The cause is explained as the big thermal expansion

coefficient (CTE) difference between metal films and PDMS. During metal evaporation,

the metal has a high temperature and causes the heating of PDMS. After cooling to room

temperature, PDMS shrinks, resulting in the formation of buckles. An optimized

fabrication procedure is proposed by Sakarya’s group in [37, 38]. Two silicon chips are

bonded together with an intermediate 5 µm DE layer in between. The top silicon is

deposited with 50 nm silicon nitride layer as etch stop and 80 nm aluminium layer for

reflectivity and conductivity. A reflective surface with high optical quality is obtained


8

when the bulk silicon of the top chip is etched away. A fabricated SLM chip together with

the far-field diffraction patterns are shown in Figure 7. The SLM works at 10 Hz. A 2D

deformable micromirror device are reported by Kuck’s group [45]. Their technology is

compatible with CMOS technology for bottom electrodes matrix addressing. A 512 x 464

pixel SLM has been successfully fabricated. Silicones are applied as the electroactive

polymers to form gratings. The driving voltage is 250 V DC for bias voltages and 15 V

AC for signal voltage. Working frequency at 1 kHz is reported for this device.

Figure 6. Buckles in thin metal film on PDMS (See Ref. [44] ).

Figure 7. (a) Photography of a packaged SLM. (b) Far-field diffraction pattern of a semi-actuated

(up) and a full-actuated SLM (down).

1.3.2 Dielectric elastomer for MEMS de-speckle modulator

DE based modulator is suitable for optical modifications, such as speckle reduction. A

short introduction about speckle phenomenon and state of the art de-speckle technologies

are given in this section. Advantages of DE based de-speckle modulator are discussed.

In comparison of conventional lamps, the use of lasers in projection display provide

many advantages such as wide colour gamut, long life time, highly directional light, high

light intensity, etc. The imaging quality produced by laser projector is unbeatable even by


9

the latest LED, OLED technology. It has been considered as the light source for next

generation. However, one major technique obstacle, which prevents applications of laser

displays, is the speckle phenomenon. Speckle is induced by the light scattering and

interference of coherent radiation from a screen with roughness in the scale of optical

wavelength. A typical speckle pattern produced by green laser is illustrated in Figure 8.

The constructive and destructive interference pattern observed on the screen masks the

image information and therefore its reduction is highly desirable in projection displays.

The fundamental theory of speckle formation, its statistical properties and de-speckle

methods are described thoroughly by Goodman in [46]. Speckle phenomenon is evaluated

by the speckle contrast SC. The definition, given by Goodman, is:

푆퐶 = 휎/퐼 ̅ (3)

where σ is the standard deviation and 퐼 ̅ is the mean value of light intensity in the speckle

pattern. A speckle contrast lower than 4% is desired to avoid disturbance a human

observer [47]. This is challenging since a fully coherent and polarized laser produces

speckle contrast of SC=1.

Figure 8. A typical speckle pattern generated by green laser on a de-polarized screen.

Considerable efforts have been made to minimize the speckle noise [47-61]. An

efficient strategy is creating a number of speckle patterns in a short time. The averaging

function of human eye can smooth the speckle noise by adding all these speckle patterns

together in the integration time of human eye. Theoretically if N number of independent

speckle patterns are generated within 30 ms, a typical value of human eye integration

time, the speckle contrast is suppressed to SC=1 / N0.5 [46]. Dynamic speckle patterns can

be generated by inserting moving or rotating diffusers into the beam or by moving the

projection screen itself. Several configurations based on the moving diffusers and screens


10

method have been reported, including moving/rotating random phase diffusers [47, 54],

moving screen and rotating diffuser [55], two spatially separated screens that are in

relative motion to each other [56]. Alternative strategy for speckle reduction includes:

using a static phase plate based on binary phase code [57] or on Barker code [58, 59],

using ultrasonic wave induced diffraction grating [60] and using several partially coherent

laser beams [61], etc.

Trisnadi [53] has reported a method to reduce speckle using moving diffusers with

optimized binary phase patterns. The predetermined phase patterns are obtained by

mapping the rows or columns of a Hadamard matrix [62] to the diffuser, as shown in

Figure 9. In this example, 16 phase patterns are produced based on 16×16 Hadamard

matrix.

In order to apply this technology into laser projection display with GLV as display

chip, a 64×64 Hadamard matrix is applied for producing diffuser with 64 phase patterns.

The diffuser is made by etching of the phase patterns in a fused-silica wafer. It is placed

at a plane conjugate to the GLV and is set in transverse oscillatory motion by a voice coil.

Within the integration time of the detector, laser beam goes through 64 phase patterns.

The measured speckle image, as shown in Figure 10(b), is an average of 64 images. By

using the diffuser, the speckle contrast reduced from 0.70 to 0.09, which is close to the

theoretical value 0.07/640.5.

Figure 9. Mapping from the 16×16 Hadamard matrix to sixteen phase patterns. (see Ref. [62] )


11

Figure 10. Images and sampling of intensity fluctuation of (a) the original speckle and (b)

the reduced speckle. (see Ref. [62])

Speckle reduction is achieved by applying the listed de-speckle techniques. However

for those methods which need mechanical vibration or rotation of the diffuser or screen, a

motor should be involved in the system. This increases the noise level and decrease the

system reliability. It is also difficult for practical implementation. The methods based on

binary phase code or Barker code are only applicable in line-scan projectors. Trisnadi‘s

method requires a huge diffuser with pre-determined phase patterns, since each pixel in

the screen should be corresponded to a diffuser. When a laser array is used, the

illumination optics is complicated and the cost increases.

In this work, DE based MEMS modulators are employed for speckle reduction. The

modulator has similar structure as spatial light modulators. It utilizes polymer dynamic

gratings for speckle reduction. When light illuminated at the polymer grating, a

diffraction pattern is generated. The diffraction patterns passes through a random phase

plate, projects to the screen and creates a speckle pattern. The speckle pattern is

determined by the shape of the grating. By dynamically change the shape of the grating

(i.e. changing the spatial frequency, the orientation), the speckle pattern is changing with

time, accordingly. Adding these dynamic speckle patterns together in the integrating time

of human eye, the speckle noise is reduced.

Attributing to the merits of DE material, the DE based de-speckle modulator have

several advantages. Firstly, the DE is electro-active and no motor or mechanical

vibration/rotation is needed. Therefore, the reliability is improved and the noise is

minimized. Secondly, the de-speckle modulator is compact and lightweight because it is

fabricated by MEMS micro-machining technologies. The system implantation is more


12

flexible. Thirdly, the fabrication process is simple and straight forward. The production

cost is reduced. Last but not least, the modulators are made of chemical and

environmental inert material. According to experiments in [63] , it can stand 80 °C, 80%

humidity, and long-time laser exposure without any degradation, providing the capability

to work in harsh environments.

1.4 Challenges of DE and DE based modulators

Though promising, there are still several technical obstacles for DEs and DE based

modulators to overcome for their wide application. The first challenge is the high electric

field requirement. Typical operating voltages for DE films (10-100 µm in thickness)

range from 500 V to 10 kV [26]. Pelrine’s group [5] suggested that compact voltage

amplifiers could be designed to solve this problem. However, the high driving voltages

inevitably brings safety issues, especially in biomedical applications. Many efforts have

been asserted aiming at low driving voltage of DEs. A comprehensive study was carried

by Zhang’s group [64]. Different hardeners were added into silicone to form DE with

varied physical properties. The study concluded that the silicone which has the lowest

elastic modulus and highest dielectric constant exhibits the highest strain response. In

order to reduce the driving voltage while maintaining the same strain, the elastic modulus

of DE should be decreased and the dielectric constant should be enhanced. Fillers with

high dielectric constant are widely used to mix with DE materials to enhance the

dielectric constant. Zhang’s group reported in [64] that silicone with 20 wt% copper-

phthalocyanine oligomer (CPO) fillers shows almost 80% increase in dielectric constant

(from 3.27 to 5.85). The reduction of driving voltage is 28%. Szabo’s group increased the

dielectric constant of silicone from 3 to 8 by mixing 70 wt% PMN-PT particles inside

silicone matrix [65]. Enhancement of silicone dielectric constant from 6 to 8 was obtained

by Carpi’s group, by mixing 30 wt% of TiO2 particles into silicone [2].

The slow response speed of DEs is another challenge in developing DE actuators.

Though DEs have been considered having the fastest response speed among EAP

materials, the response speed is still relatively slow comparing with silicon based

modulators and actuators. The response speed of DEs, which is typically in the range of

milliseconds to seconds [37, 64], is originated from its intrinsic viscoelastic properties.

The response speed in this range maybe good enough for some DE applications such as in

energy harvesters, robots, pumps, and loudspeakers. However, a faster response speed is

necessary for applications like spatial light modulators (SLM), tuneable gratings, optical


13

switches and variable optical attenuator [66]. For example, in DE based spatial light

modulators, a response speed in microsecond is necessary to eliminate the motion-blur in

HD line-scan display systems. Techniques for improving the response speed involve

reducing the viscosity of DEs. For cross-linked polymers, this is achieved by increasing

the cross-linking density. Higher cross-linked elastomer exhibits less viscosity and reacts

faster to the stimulation. However, the elastic modulus increases with the cross-linking

density, meaning a higher driving voltage is needed to activate the DEs. This is a trade-off

situation between response speed and driving voltage. Optimisation of the polymer

network structure is desired to achieve both the response speed and low driving voltage.

Other challenges in developing DE actuators include selecting of suitable compliant

electrodes. In DE actuator configurations, the electrodes contact with DE and couple to

the deformation. It is important to find compliant thin electrodes to deform with the DE

and without generating an opposing stress or losing conductivity during DE actuation. In

early stage of DE actuator development, thin metal films were deposited on the surface of

DE material as electrodes. This has been approved impractical. Cracking on metal films

during DE deformation was observed because of the stiff metal films and the big CTE

difference between metal and polymer. Kofod’s group investigated a number of

compliant electrodes for DE actuators, including carbon black, rubber electrodes, glue

electrodes, grease electrodes and dust electrodes [67]. It turns out that grease electrodes

shows best performance in DE actuators. Nevertheless the messy nature of grease

electrodes and their uniformity problems have driven researchers to investigate more

sophisticated alternatives. The most recent development of compliant electrodes is single

wall carbon nanotubes, which has been presented by Yuan’s group in [68].

The choice of DEs in different applications is also important. To date, the development

of DEs focuses on two types: acrylics and silicones. These two types of DEs show great

electro-mechanical performance under applied electric field. Generally speaking, actuator

designed using acrylic elastomer is impractical since its property is largely dependent on

temperature. Silicone, on the other hand, shows resistance to temperature variation and

environmental degradation. Its physical properties remain constant over a wide

temperature range. High actuation efficiency is a further advantage of silicone over

acrylic attributed to its lower viscosity. At last, silicones are more biocompatible than

most carbon-based polymers [69], and therefore are more suitable for biomedical

applications.


14

1.5 Aims and tasks of the study

The main objective of the project is to design and fabricate optical components for

display systems. Specifically, we are looking for modulators, which are compact, efficient

and low cost, to apply in laser display systems for speckle reduction. The solution is a

micro-opto-electro-mechanical systems (MOEMS) modulator based on micromaching

and dielectric elastomer technology. As part of the project, the present research focuses

on two fields: DE material investigation and modulator design/fabrication. The aim of the material investigation, is finding a DE material, which exhibits high

electro-mechanical performance and suitable as actuation material for MEMS modulators.

The tasks include:

Literature survey and theoretical analyse of DE material properties, actuation

mechanisms, configurations, challenges and solutions.

DE electro-mechanical properties characterization: Measurement system design,

construction and calibration; material properties testing.

DE electro-mechanical properties optimization: PDMS micro-structure tuning via

cross-linking density controlling, swelling, and dielectric nanoparticles mixing.

The DE materials with optimized properties are applied in MEMS modulators. The

target is to design and fabricate a DE based MEMS modulator, which can be implemented

into laser display systems for speckle reduction. The MEMS spatial light modulators

based on DE technology is another outcome. The tasks in this part include:

Study and literature survey of the speckle fundamentals, lasers, state of the art of

de-speckle methods.

Finite element method (FEM) simulations of modulators performance: structure

dependence, material properties dependence, driving condition dependence, etc.

Component design and fabrication: designing of the modulator structure based on

FEM simulations; planning the fabrication process and performing the device

fabrication in clean-room using micromachining technology.

Speckle reduction characterization: using the fabricated de-speckle modulator in

constructed optical systems for speckle reduction; comparing the experiments with

the theoretic value; optimization in system level.

Spatial light modulator characterization.


15

1.6 Outline of the thesis

The thesis is organized based on the published and submitted journal articles. The first

chapter “Introduction” gives an overview about state of the art technologies, the

motivation and the scope of the research. In Chapter 2 “Summary of the articles”, the

published and submitted journal articles are collected. All articles are briefly described

and discussed. Each article represents a stage of the work. These articles are organized in

a symmetrical way to show the progress of the research step by step. The full-length

articles are enclosed in the end of the thesis. In Chapter 3 “Conclusions”, the

investigations are summarized and the contribution of this work to science is concluded.


16

2 Summary of the articles

In this section, four published and two submitted journal articles are collected and

summarized to present the research content during Ph.D studies. The research consists of

two packages: the investigation of DE material is focused in Article I, Article II and

Article III; the DE applications in SLM and in de-speckle modulator are descripted in

Article IV and Article V. Another technique to reduce speckle noise by using

piezoelectric material is reported in Article VI. Collected articles are briefly descripted

based on the research motivation, experiments, results and discussion. The collected

articles are organized in terms of the research stage, rather than the publication date. The

full-length articles are enclosed in the end of the thesis.

2.1 Article I

G. Ouyang, K. Wang, M. N. Akram, L. Henriksen and X. Y. Chen, “Optimization of

PDMS network for a fast response and sensitive actuation material applied in a

MEMS spatial light modulator,” Applied Physics A, Vol.103, Issue.2, pp.381-388,

2011.

Motivation:

The motivation of this work is study of DE materials and improves their

electromechanical actuation performance. Because the DE is going to be applied in

MEMS modulators for light modulation, a fast response time of DE is required to ensure

the modulation efficiency. Moreover, a low driving voltage is always desired from the

safety and system simplicity points of view. These two challenges, which are not just to

DEs but to all the EAPs, are the primary motivations in this work.

Another motivation is establishing the material characterization systems. In order to

perform materials characterization, reliable and precise measurement systems is

necessary. Elastic modulus and response time are the most interested properties in this

work and corresponding measurements system is important.


17

Experiments:

Among all DEs, PDMS is chosen as the actuation material, attributed to its fast

response, softness, and chemical/environmental resistance. Most importantly, its physical

properties can be tuned by modifying its network structure. The target is obtaining a

PDMS with fast response time and low elastic modulus (therefore, need low voltage to

achieve the same strain, as indicated by Equation.1). The solution is PDMS micro-

structure modification. PDMS elastomers have three-dimensional networks, which are

formed by hydrosilylation process [70]. In this work, the 3D network of PDMS is formed

by vinyl-terminated linear PDMS cross-linked with trimethylsiloxy-terminated

methyhydrosiloxane-dimethylsiloxane copolymer in the presence of platinum complex

catalyst. The electro-mechanical properties of PDMS depend on both the cross-linked

network and the residual chemical groups. The PDMS cross-link density and the amount

of chain entanglements in PDMS network is modified by changing the cross-linker

concentration, chain length of linear PDMS, functionality of cross-linker.

The measurement systems for response time and elastic modulus characterization are

constructed. The response time is measured by an optical-mechanical system, as shown in

Figure 11. The testing polymer is deposited on a prism coated with an indium tin oxide

(ITO) layer (works as one electrode). A metal needle (works as another electrode) is

suspended close to the polymer film. With the voltage applied, a small spot on the

polymer (the area under the needle) experience Maxwell stress and deform. The

deformation results in light diffraction and scattering, which is detected by the photo-

detector. The response time is obtained by comparing the detected signal to applied

voltage in an oscilloscope.

Figure 11. Diagram of the response time measurements. (a) Measurement setup; (b) Applied AC

voltage and detected light intensity in oscilloscope.


18

The elastic modulus of the polymer is obtained by Hertz equation [71] using a

mechanical measurement system, as illustrated in Figure 12. A metal probe with sphere

indenter is fixed with a load cell. The probe is controlled to move perpendicularly to the

polymer surface by a control stage. When the indenter touches the polymer surface, the

contacting force is detected by the load cell. According to Hertz equation, there is a linear

relationship between indentation depth of power 3/2 and the force on indenter. The elastic

modulus is a function of the gradient and it is acquired by fitting indentation depth with

regard to detected force.

Figure 12. Diagram of the elastic modulus measurements. (a) Measurement setup; (b) The

calculated elastic modulus based on the relationship between h3/2 and contacting force.

Results and discussion:

The influence of the cross-linker concentration, linear PDMS chain length and

functionality of cross-linkers on response time and elastic modulus are presented in Table

1, Table 2 and Table 3, respectively. From Table 1, a fastest PDMS elastomer (9 μs) is

obtained at 10% cross-linker concentration, indicating that an ideal network is formed

with minimum residual groups. According to the experiments, this network is not formed

when the function groups of cross-linker (Si-H) are equal to that of the linear PDMS

(C=C), but is achieved in the presence of extra Si-H groups. This is attributed to

oxidation/hydrolysis side reactions of Si-H groups, via reaction with other compounds

such as oxygen and moisture in air [72].


19

Table 1. Dependences of response time and elastic modulus on cross-linker concentration.

Samples Cross-linker

Concentration

C=C:Si-H Response

time (μs)

Elastic

modulus (kPa)

Curing

time (min)

A1 3% 1:0.4 * * *

A2 4% 1:0.5 11200 8 84

A3 5% 1:0.6 980 32 63

A4 8% 1:1.0 65 278 41

A5 10% 1:1.2 9 601 22

A6 20% 1:2.4 100 481 6

A7 40% 1:4.8 700 122 2

A8 45% 1:5.3 800 79 2

A9 60% 1:7.1 x 3 1

A10 65% 1:7.7 x 1 1

A11 67% 1:8.0 * * *

* A gel-like elastomer cannot be formed.

x The response time is impossible to measure since the polymer is extremely sticky and

soft.

From Table 2, the response time increases with molecular weight of linear PDMS. It is

due to the increased chain lengths between the cross-linking sites. In other words, the

cross-link density is lower with increased chain length of linear PDMS, and polymer with

lower cross-link density is slower. When long chain linear PDMS mix with cross-linker,

the long-range chain-chain interactions, called “chain entanglements”, dominated the

polymer network. The consequence of the entanglements domination (B4, B5) is the

significantly increase response time [73]. Therefore a short linear PDMS is the choice to

form a cross-linking dominated PDMS network, if fast response speed is desired.

Table 2. Dependences of response time and elastic modulus on linear PDMS chain length.

Samples Molecular

weight of linear

PDMS (g/mol)

Response time

(μs)

Elastic modulus

(kPa)

Curing time

(min)

B1 800 * * *

B2 6000 5 712 28

B3(A5) 9400 9 601 22

B4 28000 190 380 7

B5 62700 500 175 1


20


According to Table 3, with the decrease of cross-linker functionality (C1C4), the

cross-link density is smaller; therefore a slower and softer PDMS is obtained. The

relationship between response time and elastic modulus show agreement with the

“Voiget-Kelvin model” [74], in which the response time is inversely proportional to the

elastic modulus. Table 3. Dependences of response time and elastic modulus on the functionality of cross-linker.

Samples Molecular weight

of cross-linker

(g/mol)

Functionalit

y of cross-

linker

Response

time (μs)

Elastic

modulus

(kPa)

Curing

time

(min)

C1 1050 9.2 8 781 2

C2 1950 8.9 8 697 2

C3(A5,B3) 1950 4.8 9 601 22

C4 1950 2.2 200 79 48

C5 1950 1.1 * * *


Based on the experiments, PDMS elastomer formed by short chain vinyl-terminated

PDMS cross-linking with high functionality cross-linker has the fastest response speed.

Such polymer has densely cross-linked networks with minimum residual groups. They are

good candidates for MEMS modulators. The PDMS elastomer, which has response time

of 9 µs and elastic modulus of 601 kPa is prepared and applied as the actuation material

in a MEMS spatial light modulator.

A SLM, as shown in Figure 13(a), is fabricated using micro-machining technology. In

this SLM, the actuation material PDMS is sandwiched between rigid glass substrate and

thin gold film. Under the applied voltage (250V DC and 100 V AC), the surface of

elastomer deforms, forming a sinusoidal grating. The functionality of this PDMS based

SLM is demonstrated by illuminating it with a laser beam. As shown in Figure 13(b), the

polymer grating successfully diffracted light into higher orders.


21

Figure 13. Photographs of (a) Fabricated SLM chips using PDMS as actuation material; (b)

Diffraction patterns from the SLM.

2.2 Article II

G. Ouyang, K. Wang, L. Henriksen, and X.Y. Chen, “High sensitivity and fast

response Polydimethylsiloxane via network swelling and its application in MEMS

spatial light modulator,” Optical engineering, Vol. 50, No. 124001, 2011.

Motivation:

Based on Article I [75], the response time and elastic modulus of PDMS can be tuned

by modifying its network structure. A PDMS with fast response time is achieved, but with

the cost of increased elastic modulus. Typically PDMS with fast response speed has

highly cross-linked network and maintain high elastic modulus. Another strategy, aiming

to obtain both fast response speed and low elastic modulus, or to obtain fast response

speed with slightly increased elastic modulus, should be explored for improving the

performance of PDMS in MEMS modulator.

The structure of SLM proposed in Article I has a metal film directed deposited on

PDMS deformable surface. This is not a good choice since the metal film may bring

reliability problems due to fatigue and creep, not to mention that the film itself

mechanically constrain the deformation of PDMS. Improvement of the SLM structure is

desired in the device reliability point of view.


22

Experiments:

The solution for material optimization is PDMS micro-structure modification via

network swelling. Cross-linked PDMS is swollen by silicone oil with molecular weight

around 700. Samples with solvent concentration varied from 0% to 90% are prepared and

characterized.

The SLM is designed to avoid the direct contact between PDMS deformable surface

and rigid electrodes. An air-gap structure is proposed. The air gap between PDMS and

signal electrodes allows free deformation of PDMS and avoids fatigue and creep of the

electrodes. The SLM is fabricated using micro-machining technology. The fabrication

process is descripted in Figure 14.

Figure 14. SLM fabrication process flow. (a) Au and TiW interdigital electrodes patterning. (b)

Electrodes wire-bonding and wire protection. (c) Glass beads and conductive adhesives

deposition. (d) PDMS deposition. (e) PDMS squeezing. Fibers are used to define the PDMS

thickness. (f) PDMS curing and peeling. Because of the hydrophilic surface of the glass plate,

PDMS elastomer is only attached to the prism. (g) Chip mounting. The ITO electrodes on the

prism are connected to the chip by conductive adhesive.


The dependence of response time and elastic modulus on solvent concentration is

plotted in Figure 15. By introducing short chain miscible solvent into PDMS, the polymer

network is swollen. Because the polymer chains are surrounded by highly mobile


23

solvents, the chain movement is promoted. The ease of chain movement results in the

decrease of elastic modulus. Meanwhile, the response time of PDMS increased, but this

effect is rather moderate. To show the advantage of this optimization method, the

properties of PDMS via network swelling are compared with the results from Article I

[75]. As shown in Figure 16, in order to obtain a PDMS with similar response time, the

PDMS tuned by network swelling has much lower elastic modulus.

Figure 15. Response time and elastic modulus of swollen PDMS with regard to solvent

concentration.

Figure 16. Comparison of response time and elastic modulus of PDMS between this work and

Article I.

The PDMS, which has elastic modulus 200 kPa and response speed 34 µs, is applied in

a SLM. The schematic of the device is shown in Figure 17. It is driven by 200 V DC

voltage and 60 V AC voltages to the ITO and interdigital electrodes, respectively. The

response time of the modulator is measured as 35 µs. The SLM shows no degradation or


24

breakdown after millions of actuation cycles. It functioned well after storage in room

temperature for a month, indicating good reliability of the device.

Figure 17. Schematic of the air-gap SLM structure: (a) Top view; (b) Cross-section view.

2.3 Article III

G Ouyang, K Wang, and X Y Chen, “TiO2 nanoparticles modified polydimethylsiloxane

with fast response time and increased dielectric constant,” Submitted to Journal of

Micromechanics and Microengineering, 2012.

Motivation:

From Article I [75] and Article II [76], we see that the fast response time obtain with

the cost of increase of elastic modulus. The aim for DE material properties optimization is

to achieve fast response and low driving voltage. From Equation. (1), the low driving

voltage is accomplished either by decreasing the elastic modulus, or increasing the

dielectric constant of DE actuation material. Mixing of polymers with dielectric or

piezoelectric fillers to form polymer composites has been recognized as an efficient way

to increase the dielectric constant of polymers. According to [77], when fillers with

dielectric constant εfiller mixed with polymer with dielectric constant εpolymer, the dielectric

constant of the composite εcomposite is in the range of: εpolymer < εcomposite < εfiller. The work


25

present in this article is to prepare fillers modified PDMS, which has enhanced dielectric

constant, to reduce the driving voltage requirement.

Experiments:

As the host polymer, PDMS with three dimensional networks is formed by crosslinking

linear vinyl-terminated PDMS with trimethylsiloxy-terminated methyhydrosiloxane-

dimethylsiloxane copolymer. TiO2 nanoparticles are selected as fillers due to its high

dielectric constant, chemical inertness, non-toxicity and easy dispersion in silicones [2].

The PDMS-TiO2 nanocomposite is prepared by in-situ polymerization. Firstly, TiO2

particles are mixed with linear PDMS in the presence of polyether-modified silicone,

which works as dispersant. These components are mixed by high energy ball-milling for

24 hours. Secondly, the cross-linker and catalyst are added into the solution sequentially,

followed by 2 minutes manual stirring after each step. Finally, samples are cured at room

temperature for 30 minutes.


Inhomogeneous dispersion due to pronounced agglomeration tendency of nanoparticles

is a common phenomenon for synthesizing polymer nanocomposites [27]. Two

techniques have been applied to suppress the agglomeration. High-energy ball-milling is

employed during mixing, providing high impact and shear energy to break agglomerates.

Moreover, polyether-modified silicones, which is a good dispersant for dispersing TiO2

into silicones, is used for homogenous dispersion and avoiding particles re-

agglomeration.

The size distribution of the dispersed particles in PDMS solution is characterized by

particle size analyser and SEM. The results are given in Figure 18 and Figure 19

respectively. Both experiments indicate that the particles in PDMS solutions have size

around 500 nm.


26

Figure 18. Particle size distribution of nanoparticles in PDMS solution with and without the

dispersant, measured by particle size analyser.

Figure 19. SEM image of TiO2 nanoparticles.

Light transmittance spectra for the nanocomposites films (after curing) with different

particle concentrations are plotted in Figure 20. The transmittance decreases with particle

concentration. At the UV range (300 nm – 400 nm), the light loss is dramatic for all films,

due to the absorption of TiO2 fillers at this wavelength range [16]. When it comes to the

visible wavelength (400nm – 800 nm) the light loss is mostly due to the Mie scattering.


27

Figure 20. The transmittance spectra of 10 µm nanocomposite films with varied TiO2

concentrations.

The dependence of nanocomposite elastic modulus on TiO2 particle concentration is

plotted in Figure 21. The influence of particles to the elastic modulus is attributed to two

factors: softening effects due to the interfering of cross-linking (major effect for TiO2

concentration lower than 5 wt%) and hardening effects due to the particle-polymer

interaction (major effect for TiO2 concentration higher than 5 wt%). At 5 wt% particle

concentration, the two effects are balanced with each other, and the elastic modulus is

more or less the same as neat PDMS elastomer.

Figure 21. The elastic modulus of nanocomposites with varied TiO2 concentrations.

The effect of particle concentration on the response time is given in Figure 22. With

increase of the particle concentration, the PDMS response time increase and then


28

decrease. An optimization point is found at 5 wt% particle concentration. Comparing with

neat PDMS, the response time of PDMS with 5 wt% fillers improves from 9 µs to 6 µs,

while the elastic modulus is more or less the same.

Figure 22. The response time of nanocomposites with varied TiO2 concentrations.

The dependence of the dielectric constant on particle concentration is shown in Figure

23. A sharp increase of the dielectric constant from 2.97 to 3.85 is observed when the

concentration of TiO2 particles increases from 0 wt% to 5 wt%. Thereafter, the dielectric

constant gradually increased to 4.4 with particles concentration reaching to 30 wt%. The

increasing of dielectric constant is attributed to the high dielectric constant of TiO2.

The measured dielectric constant of nanocomposite is compared with theoretical values

calculated from several mixing rules: Maxwell-Wagner equation [78, 79], Sillars equation

[80], Looyenga equation [81], and Lichtenecker equation [82]. As seen from Figure 23,

none of the theoretical model predicts the dielectric constant change of nanocomposite

well. The reason could be due to: these mixing rules hold for the dielectric constant at

high frequency range, while in this work the dielectric constant has been measured at 100

Hz. At low frequency, the Maxwell-Wagner relaxation process [83] arises and influences

the dielectric constant.


29

Figure 23. Experimental and theoretical dielectric constant of nanocomposites with varied TiO2

concentrations.

2.4 Article IV

G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y. Chen, “A novel tunable

grating fabricated with viscoelastic polymer (PDMS) and conductive polymer

(PEDOT),” Sensors and actuators A: Physical, Vol.158, Issue.2, pp.313-319, 2010.

Motivation:

This work is performed after the study in Article I. Motivation of the work is to design

and fabricate a novel SLM with improved reliability. Beside the air-gap structure as

reported in Article II, a SLM, in which transparent a conductive polymer is utilized as

electrode, is proposed. Attributing to the stability and compatibility with other polymer

materials, conductive polymer PEDOT: PSS is employed as the top electrode. Moreover,

PEDOT: PSS is a cheap material and gives opportunity for easy deposition by screen

printing [84] , spin coating [85], roll-to-roll coating [86].

Another motivation is to investigate the conductive polymer. Optical and electrical

properties of the PEDOT: PSS as well as its processing technique is studied. The

knowledge of conductive polymer is not only useful for SLMs but for all MEMS

modulators, especially in some biological applications where an all-organic modulator is

desired.


30

Experiments:

In the SLM, PDMS, working as actuation material, is sandwiched between rigid glass

substrate and conductive polymer PEDOT: PSS. The transmission and sheet resistance of

the conductive polymer are measured for different thickness of PEDOT: PSS films.

The device is fabricated using micro-machining technology. The fabrication process is

simple and straight forward. PDMS is an originally hydrophobic material exhibiting a

static contact angel of 110o [87]. A surface treatment of PDMS to make it hydrophilic is

necessary for the subsequent PEDOT: PSS deposition. Plasma treatment is commonly

applied to change the wettability of polymers [88-90]. SF6, Ar and O2 plasmas are tried

one after another for PDMS surface modification. The wettability is evaluated by

measuring the contact angle between water drops and the modified surface. The surface

morphology of PDMS after the plasma treatment is also characterized.


Unlike the SLM reported in Article I where the illumination light is reflected at the top

metal surface, the device proposed in this work has transparent top electrodes. If no

voltage is applied, the polymer surface is flat. Since both PEDOT: PSS and PDMS are

transparent, incoming light goes through these layers with the same optical path and

reflects at the underlying gold electrode. The reflected light is mainly in the 0th diffraction

order. If the driving voltage is applied, the polymer surface deforms into a sinusoidal

shape. When incoming light penetrates the transparent layers, which has varied thickness

at different location, a spatially varying optical path of the light beam is obtained. The

polymer layers act like a phase grating and diffract light into higher diffraction orders.

A prototype of the SLM has been fabricated as shown in Figure 24. PEDOT: PSS films

with thickness of 100 nm are deposited on PDMS surface. As a preliminary test, the

modulator has interdigital bottom electrodes and it works for one pixel. Hence there are

only two signal voltages which are applied through the left and right bottom electrodes,

respectively. The grating profile on PEDOT: PSS surface under 250 V driving voltage is

measured by interferometer and shown in Figure 25(a). The value of the peak-to-peak

deformation is approximately 200 nm.


31

Figure 24. A prototype of the SLM. The centre of the chip is covered by PDMS and PEDOT:

PSS. The underlying interdigital electrode is zoomed in and shown in the bottom-right corner.

Figure 25. (a) Grating profile on PEDOT: PSS surface measured by an interferometer. The relief

depth is exaggerated for observation; (b) Simulation results of surface grating from COMSOL.

The colour grade represents the magnitude of the electric potential.

A 3D model simulation is performed by coupling electrical domain and mechanical

domain in COMSOL Multiphysics [91]. The simulation results are obtained by solving

the Maxwell’s equations and the Hook’s law in electrical domain and mechanical domain,

respectively. The simulated PDMS grating under applied voltage is shown in Figure

25(b). The simulation results are compared with experimental results in terms of PDMS

surface relief depth. As shown in Figure 26, the experiments agree with the simulations.


32

Figure 26. Experimental results (▲) and simulation results (♦) of PDMS relief depth with regard

to the driving voltage.

The response time of the device is measured as rise time 240 μs and fall time 250 μs.

The response speed is determined both by the time needed for polymer deformation, and

by the time constant of the RC circuit. In this work, the PDMS is synthesized to have

response time of as 10 μs, while the retardation time of the RC circuit, which is calculated

by τ =RC, is 230 μs. The slow RC charging process is originated from the high resistance

of the thin conductive polymer film. If the PEDOT: PSS is tuned to have high

conductivity, as reported in [86, 92, 93], the RC charging time should be in the range of

nanoseconds.

The stability of the device is tested by continuously driving the device by 200 V and 1

kHz rectangular wave for eight hours. The test shows that the modulator works well after

millions times of actuation. No breakdown or reduction in the relief depth is observed.

Eight hours continuously laser exposure does not bring any degradation of the modulator.

The modulator works well after two months stored at room temperature.

2.5 Article V

G. Ouyang, Zhaomin Tong, M. Nadeem Akram, Kaiying Wang, Vladimir Kartashov,

and Xuyuan Chen, “Speckle reduction using a motionless diffractive optical element,”

Optics letters," Vol. 35, Issue.17, pp. 2852-2854, 2010.

Motivation:

As described in the introduction part, laser display has many advantages (e.g. colour

performance, brightness, contrast and lifetime) comparing with conventional lamp


33

displays. However, the speckle phenomenon significantly degrades the imaging quality

and need to be suppressed. Considerable efforts have been made to solve the problem.

However, the proposed solutions are either expensive or not reliable.

A motionless diffractive optical element (DOE) is proposed in this work for speckle

reduction. The key component in the device is dynamic polymer gratings, which is

formed by DE material actuated under AC voltage. To achieve a satisfied de-speckle

efficiency, the DE material with response speed in micro-seconds is necessary. The

PDMS with optimized electro-mechanical properties using the reported techniques in

Article I to III is a perfect candidate. Attributing to the merits of DE material, the de-

speckle modulator have several advantages. The modulator is lightweight, compact, and

therefore easy for implementation. The fabrication is simple, hence the cost is low. The

reliability is improved since no mechanical vibration/rotation is used in the system.

Experiments:

The schematic of the modulator is shown in Figure 27(a). The de-speckle modulator

has similar structure as the air-gap SLM, except in this case there are three separated

polymer gratings. The polymer gratings have different orientations and spatial

frequencies, which are determined by the shape of the underlying interdigital electrodes.

Using micro-machining technology, a MOEMS de-speckle modulator based on DE

material is fabricated and shown in Figure 27(b), where the modulator is assembled in a

fixture for characterization.

Figure 27. (a) Schematic of the dynamic DOE. Picture in the bottom shows the different

electrodes in each grating. The dimensions are not to scale. (b) A fabricated DOE assembled in a

fixture with electrical connection.


34

The working principle of the de-speckle modulator for speckle reduction is creating N

number of speckle patterns and reduce the speckle contrast to SC=1 / N0.5 in the

integration time of human eye. In the modulator, each grating can be switched “on” and

“off” independently. In the “off” state, the PDMS surface is flat and the incident light is

reflected at the angle of the total internal reflection (TIR); while in the “on” state, a phase

grating is generated on the PDMS surface, splitting light into several diffraction orders.

When one or several gratings are actuated, the incoming light is reflected sequentially and

is split into a multitude of diffraction orders. There are totally N=2m diffraction patterns

for the modulator with m gratings. The diffraction patterns are scattered by a diffuser,

homogenized by a light pipe and projected to the screen as speckle patterns. By actuating

PDMS in high frequency, numbers of speckle patterns are generated fast and averaged out

in human eye. As seen in this application, if N=1000 number of speckle patterns need to

be created within the integration time of human eye (around 30 ms), the response speed of

PDMS should be at least 30 µs. It is important to optimize PDMS to achieve the response

speed requirement. The PDMS used in this work has response time 10 μs and elastic

modules 550 kPa. It is driving under 200 V DC bias voltage and 50 V AC signal voltage.


A DOE with two gratings is fabricated, and characterized for speckle reduction in the

measurement setup as shown in Figure 28. A laser beam illuminates the modulator with

an angle slightly larger than TIR-angles for the polymer/air and the glass/air interfaces.

When voltage is applied, four diffraction patterns appeared sequentially on the first

diffuser, corresponding to M=22 states. Photographs of these diffraction patterns are

shown in Figure 29. Simulation of the modulator function to generate diffraction patterns

is done in ZEMAX. As shown in Figure 29, the experimental results agree with

simulations. Output diffraction spots of the modulator are scattered by the first diffuser,

homogenised by the light pipe and illuminated at the second diffuser. The speckle field on

the second diffuser is imaged by the CCD camera.


35

Figure 28. Experiment setup for speckle contrast characterization.

Figure 29. Comparison of multiple diffraction patterns from experiments and simulations.

The speckle field registered by the CCD camera when all gratings are switched off is

shown in Figure 30(a), and the speckle contrast is 0.73; while the speckle filed when four

different diffraction patterns appear during the exposure time (1/32 s) is shown in Figure

30(b) with C = 0.37. Therefore, the speckle contrast reduction is (0.37/0.73) x 100% ≈

51%. The original speckle contrast 0.73 is close to the expected value 1/21/2 as it should

be when the narrowband laser is scattered by a depolarizing screen. By tuning on the de-

speckle modulator, N=22 number of speckle patterns are generated and detected by the

CCD. According to simulations of the designed DOE, there is no overlapping between

any of the four diffraction patterns, while the experiments demonstrate too that the

overlapping is negligible. Thus the theoretical value of the reduced speckle contrast is

0.73*1/40.5 = 0.365, which shows fair agreement with the experiments.


36

Figure 30. Images and sampling of intensity fluctuation of the original speckle (a) and the

reduced speckle (b). The CCD level 256 corresponds to the brightest spot in the speckle pattern,

while 0 corresponds to the darkest spot in the speckle pattern.

2.6 Article VI

G. Ouyang, M. N. Akram, K. Wang, Z. Tong, X. Y. Chen, “Laser speckle reduction

based on angular diversity induced by piezoelectric benders,” Submitted to IEEE

photonics letters, ,2012.

Motivation:

Besides the DE based modulator, we also look for other speckle reduction techniques.

Piezoelectric materials have been considered as good candidates, because it has stable

performance, cheap and resistance to high temperature. Two piezoelectric benders have

been employed in this work for speckle reduction. By dynamically changing the

illumination angle of the laser beam on diffuser, different speckle patterns are generated

sequentially and smoothed out in the integration time of human eye.

The measured speckle contrast depends on measurement conditions. Many parameters

affect the measured speckle contrast, such as distance between camera and screen, F

number of the lens, settings of the camera, etc. System calibration is necessary to fairly

evaluate the speckle reduction efficiency. Examples of how to determine the

measurement parameters are given both in free space geometry and imaging geometry.


37

Another motivation of this work is to investigate the “compound speckle” phenomenon

for better understanding of de-speckle method in system level.

Experiments:

The experimental setup to implement speckle reduction in the free space propagation is

shown in Figure 31. He-Ne laser (633 nm) with beam spot size 0.5 mm is employed as

light source. The piezoelectric benders are deposited with thin gold film, which works as

reflective mirrors. Two piezoelectric benders are placed perpendicularly to each other and

driven by ±100 V AC voltage. Laser beam is reflected at two benders, and it is steered in

both X and Y directions when the benders are actuated. The moving laser beam is

collected by a condenser lens, and falls on a transmitting diffuser with different angles.

After passing through the diffuser, laser beam propagates in free space and falls on a

CCD sensor, which has pixel size of 5.6 μm × 5.6 μm.

Figure 31. Speckle contrast measurement setup of free space geometry.

Imaging geometry is constructed as shown in Figure 32. Unlike free space geometry,

the scattered light after diffuser 1 is collected and homogenized by a light pipe. Diffuser 2

is placed at the output of the light pipe, to further homogenize the beam. The object,

which is a transparent plastic sheet printed with a letter “S”, is placed right after diffuser

2. The CCD camera is equipped with an imaging lens.


38

Figure 32. Speckle contrast measurement setup of imaging geometry.


When the piezoelectric benders are actuated, the moving laser beam is focused at a spot

on diffuser via the condenser lens. The spatial position of the laser spots on diffuser

remains the same while the illumination angle is changed. The laser with different

illumination angle generates different speckle patterns, which are averaged out by the

CCD camera during its integrating time. In order to find the suitable measurement

parameters, the system calibration is done based on three criterions: 1). The speckle size

detected in CCD sensor should be at least ten times bigger than with the CCD pixel size,

to avoid the spatial averaging due to CCD pixel. 2). The CCD camera should be operated

in its linear regime. 3). In free space geometry, ideally the original SC is equal to 1

without any de-speckle method applied. In practice, however, the SC should be 1/20.5

(0.71) even without the motion of piezoelectric benders, due to the narrowband laser

scattered at a depolarizing screen [46].

In free space geometry, the one dimensional “width of speckle” is given by λz/D [94],

where λ is the laser wavelength, z is the propagation distance between diffuser and CCD

sensor, D is the diameter of the scattering spot. An approximation is for speckle size is Ac

= (λz/D)2 [46]. The speckle size is calculated as 63.3 × 63.3 μm, which is big enough to

avoid the CCD spatial averaging. The camera settings and light intensity are adjusted to

make the camera operate in it linear regime and the obtained SC is close to 0.71. The

captured speckle image with and without the actuation of the piezoelectric benders are

shown in Figure 33(a) and Figure 33(b), respectively. The SC is reduced from 0.71 to

0.06.


39

For imaging geometry, the first two criterions as in free space geometry are employed.

The third criterion is not applicable due to “compound speckle” phenomenon [46]. When

light passes through diffuser 1, and falls on a finite-sized diffuser 2, there is a

compounding of speckle statics. The SC of compound speckle can be bigger than 1. The

speckle generated from diffuser 1 has bigger size, or “coarse speckle”; while the speckle

generated from diffuser 2 has smaller size, or “fine speckle”. The fine speckle size is

calculated by [(1+M)×λ×F#]2 [94], where M = f1/f2 is the optical magnification. To

obtain sufficiently large fine speckle, F# and M are set as the maximum value 16 and 2,

respectively. The speckle size is 30.4 μm × 30.4 μm, which is big enough to avoid the

CCD spatial averaging. The captured speckle image with and without the actuation of the

piezoelectric benders are shown in Figure 33(c) and Figure 33(d). The SC is reduced from

0.87 to 0.16. The original SC is 0.87 rather than 0.71, which is due to the compound

speckle phenomenon. In this case, both the fine and coarse speckle is detected by the

CCD sensor. However, the coarse speckle is too big to distinguish. The size of coarse

speckle is calculated size by [(λz/D)× M]2, where z is the propagation distance within the

light pipe. We assume that z is three times longer than the length of light pipe. To observe

the coarse speckle clearly, the optical configuration is adjusted to: M = 6/25. The coarse

speckle size is calculated as 159.5 μm × 159.5 μm. It is clear that the coarse speckle, as

shown in Figure 33(e), is much bigger than the fine speckle, since the letter “S” is the

same as in Figure 33(c). The coarse speckle is reduced from 0.72 to 0.09 by piezoelectric

benders in imaging geometry.


40

Figure 33. Speckle reduction in free space geometry: (a) original speckle SC=0.71; (b) reduced

speckle SC=0.06. Speckle reduction in imaging geometry with both fine and coarse speckle: (c)

original speckle SC=0.87; (d) reduced speckle SC=0.16. Speckle reduction in imaging geometry

with only coarse speckle: (e) original speckle SC=0.71; reduced speckle SC=0.09.


41

3 Conclusions

Two properties, which are the fast response time and the low driving voltages, have

been focused in PDMS material investigation. By modifying the chain length of linear

PDMS, the cross-linker density and the functionality of cross-linkers, it is found out that

PDMS with highly cross-linked network and minimum residual groups shows fast

response time, but requires high voltage to actuate (Article I). The synthesized PDMS has

response time 9 µs and elastic modulus 601 kPa. The SLM, which is based on this type of

PDMS, show response time of 9 µs and it is driven by 250V DC and 100 V AC. In order

to reduce the driving voltage requirement of PDMS without substantial increase of the

response time, PDMS network is modified via network swelling (Article II). The ease of

chain movement results in reduced driving voltage requirement. PDMS with 50% solvent

have response time 34 µs and elastic modulus 200 kPa. The SLM based on this PDMS

has response time of 35 µs and driven by 200 V DC and 60 V AC. In the next step, the

optimization of PDMS is carried out by filling PDMS with TiO2 nanoparticles (Article

III). The dielectric constant of PDMS-TiO2 composite increases from 3.0 (neat PDMS) to

4.4 (PDMS with 30 wt% TiO2). PDMS modified by adding 5 wt% TiO2 particles has

response time 6 µs, elastic modulus 567 kPa and dielectric constant 3.8. The fabricated

SLM shows response time 6 µs and driven by 200 V DC voltage.

A SLM with sandwich structure, that PDMS film is between two metal electrodes, is

proposed (Article I). This SLM shows advantages in light weight and easy fabrication, but

may have reliability issue. An improved SLM with air-gap structure is designed and

demonstrated (Article II). It has better reliability attributed to the absence of the contact

between deformable PDMS and metal top electrodes. An alternate strategy for improving

the reliability of SLM is replacing the metal top electrodes with conductive polymer

(Article IV). The device reliability is achieved due to the improved compatibility between

PDMS and electrodes.

PDMS based de-speckle modulator consists of many dynamic polymer gratings

(Article V). An air-gap structure, similar as the SLM reported in Article II, is applied and

ensures reliability of the modulator. Speckle noise is successfully suppressed to 50%

when two dynamic gratings are used. Another de-speckle device investigated in this work

is piezoelectric benders (Article VI). By varying the incident angle of the illumination

beam with two piezoelectric benders, the speckle noise is reduced to 6%.


42

The investigation and exploring of this research work is valuable from both the DE

material and MEMS modulator points of view. Firstly, measurement setups for polymer

response time and elastic modulus characterization have been constructed. The setups are

possible to re-construct in other labs as substitutes for sophisticated/expensive equipment.

Secondly, the understanding of PDMS electro-mechanical properties with regard to its

micro-structure has been obtained. This is very useful for tuning PDMS to obtain desired

physical properties via chemical methods. Thirdly, the techniques for synthesizing

polymer-particle composite and suppressing particle agglomeration have been described.

This is a guide for further PDMS properties tuning by mixing with other particles.

Fourthly, three SLM with different structures and working principles have been reported.

It gives ideas about how to design a polymer based SLM and how to solve the consequent

issues. Last but not the least, a de-speckle modulator by dynamic polymer gratings have

been demonstrated. It shows advantages in compactness, reliability, lightweight and easy

processing, comparing with existing de-speckle technologies. This point out a new way to

design and manufacture de-speckle modulators by DE technology and MEMS

technology.


43

References

[1] A. O'Halloran, F. O'Malley, and P. McHugh, "A review on dielectric elastomer

actuators, technology, applications, and challenges," Journal of Applied Physics, vol. 104,

pp. 071101-10, 2008.

[2] F. Carpi and D. D. Rossi, "Improvement of electromechanical actuating

performances of a silicone dielectric elastomer by dispersion of titanium dioxide powder,"

Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 12, pp. 835-843, 2005.

[3] R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, and S. Chiba, "High-field

deformation of elastomeric dielectrics for actuators," Materials Science and Engineering:

C, vol. 11, pp. 89-100, 2000.

[4] R. D. Kornbluh, R. Pelrine, Q. Pei, S. Oh, and J. Joseph, "Ultrahigh strain response

of field-actuated elastomeric polymers," Newport Beach, CA, USA, 2000, pp. 51-64.

[5] R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, "High-Speed Electrically Actuated

Elastomers with Strain Greater Than 100%," Science, vol. 287, pp. 836-839, February 4,

2000 2000.

[6] C. Löwe, X. Zhang, and G. Kovacs, "Dielectric Elastomers in Actuator Technology,"

Advanced Engineering Materials, vol. 7, pp. 361-367, 2005.

[7] L. Petit, B. Guiffard, L. Seveyrat, and D. Guyomar, "Actuating abilities of

electroactive carbon nanopowder/polyurethane composite films," Sensors and Actuators

A: Physical, vol. 148, pp. 105-110, 2008.

[8] P. G. A. Madden, J. D. W. Madden, P. A. Anquetil, N. A. Vandesteeg, and I. W.

Hunter, "The relation of conducting polymer actuator material properties to performance,"

Oceanic Engineering, IEEE Journal of, vol. 29, pp. 696-705, 2004.

[9] W. Ma and L. E. Cross, "An experimental investigation of electromechanical

response in a dielectric acrylic elastomer," Applied Physics A: Materials Science &

Processing, vol. 78, pp. 1201-1204, 2004.

[10] C. Huang, R. Klein, F. Xia, H. Li, Q. M. Zhang, F. Bauer, and Z. Y. Cheng,

"Poly(vinylidene fluoride-trifluoroethylene) based high performance electroactive

polymers," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 11, pp. 299-

311, 2004.


44

[11] S. Guo, X.-Z. Zhao, Q. Zhou, H. L. W. Chan, and C. L. Choy, "High electrostriction

and relaxor ferroelectric behavior in proton-irradiated poly(vinylidene fluoride-

trifluoroethylene) copolymer," Applied Physics Letters, vol. 84, pp. 3349-3351, 2004.

[12] Q. M. Zhang, V. Bharti, and X. Zhao, "Giant electrostriction and relaxor ferroelectric

behavior in electron- irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer,"

Science, vol. 280, pp. 2101-2104, 1998.

[13] D. L. Thomsen, P. Keller, J. Naciri, R. Pink, H. Jeon, D. Shenoy, and B. R. Ratna,

"Liquid Crystal Elastomers with Mechanical Properties of a Muscle," Macromolecules,

vol. 34, pp. 5868-5875, 2001/08/01 2001.

[14] C. M. Spillmann, B. R. Ratna, and J. Naciri, "Anisotropic actuation in electroclinic

liquid crystal elastomers," Applied Physics Letters, vol. 90, pp. 021911-3, 2007.

[15] W. Lehmann, H. Skupin, C. Tolksdorf, E. Gebhard, R. Zentel, P. Kruger, M. Losche,

and F. Kremer, "Giant lateral electrostriction in ferroelectric liquid-crystalline

elastomers," Nature, vol. 410, pp. 447-450, 2001.

[16] I. W. Hunter and S. Lafontaine, "A comparison of muscle with artificial actuators,"

in Solid-State Sensor and Actuator Workshop, 1992. 5th Technical Digest., IEEE, 1992,

pp. 178-185.

[17] R. D. Kornbluh, R. Pelrine, Q. Pei, R. Heydt, S. Stanford, S. Oh, and J. Eckerle,

"Electroelastomers: applications of dielectric elastomer transducers for actuation,

generation, and smart structures," San Diego, CA, USA, 2002, pp. 254-270.

[18] K. Asaka, K. Oguro, Y. Nishimura, M. Mizuhata, and H. Takenaka, "Bending of

Polyelectrolyte Membrane–Platinum Composites by Electric Stimuli I. Response

Characteristics to Various Waveforms," Polymer Journal, vol. 27, pp. 436-440, 1995.

[19] S. Nemat-Nasser and Y. Wu, "Comparative experimental study of ionic polymer--

metal composites with different backbone ionomers and in various cation forms," Journal

of Applied Physics, vol. 93, pp. 5255-5267, 2003.

[20] B. J. Akle, M. D. Bennett, and D. J. Leo, "High-strain ionomeric–ionic liquid

electroactive actuators," Sensors and Actuators A: Physical, vol. 126, pp. 173-181, 2006.

[21] S. Hara, T. Zama, W. Takashima, and K. Kaneto,

"Tris(trifluoromethylsulfonyl)methide-doped polypyrrole as a conducting polymer

actuator with large electrochemical strain," Synthetic Metals, vol. 156, pp. 351-355, 2006.

[22] B. R.H, "Conducting polymer artificial muscles," Synthetic Metals, vol. 78, pp. 339-

353, 1996.


45

[23] R. Baughman, L. Shacklette, R. Elsenbaumer, E. Pichta, and C. Becht, Conjugated

Polymeric Materials: Opportunities in Electronics, Optoelectronics and Molecular

Electronics. Netherlands: Kluwer Academic Publishers, 1990.

[24] K. Kaneto, M. Kaneko, Y. Min, and A. G. MacDiarmid, "“Artificial muscle”:

Electromechanical actuators using polyaniline films," Synthetic Metals, vol. 71, pp. 2211-

2212, 1995.

[25] F. Xia, S. Tadigadapa, and Q. M. Zhang, "Electroactive polymer based microfluidic

pump," Sensors and Actuators A: Physical, vol. 125, pp. 346-352, 2006.

[26] P. Brochu and Q. Pei, "Advances in Dielectric Elastomers for Actuators and

Artificial Muscles," Macromolecular Rapid Communications, vol. 31, pp. 10-36, 2010.

[27] F. Carpi, G. Gallone, F. Galantini, and D. De Rossi, "Silicone–Poly(hexylthiophene)

Blends as Elastomers with Enhanced Electromechanical Transduction Properties,"

Advanced Functional Materials, vol. 18, pp. 235-241, 2008.

[28] I. Krakovsky, T. Romijn, and A. P. de Boer, "A few remarks on the electrostriction

of elastomers," Journal of Applied Physics, vol. 85, pp. 628-629, 1999.

[29] Y. Bar-Cohen, Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality,

Potential, and Challenges, 2nd ed.: SPIE Publications, 2004.

[30] Q. Pei, R. Pelrine, M. A. Rosenthal, S. Stanford, H. Prahlad, and R. D. Kornbluh,

"Recent progress on electroelastomer artificial muscles and their application for

biomimetic robots," San Diego, CA, USA, 2004, pp. 41-50.

[31] Q. Pei, R. Pelrine, S. Stanford, R. Kornbluh, and M. Rosenthal, "Electroelastomer

rolls and their application for biomimetic walking robots," Synthetic Metals, vol. 135-136,

pp. 129-131, 2003.

[32] P. Qibing, R. Marcus, S. Scott, P. Harsha, and P. Ron, "Multiple-degrees-of-freedom

electroelastomer roll actuators," Smart Materials and Structures, vol. 13, p. N86, 2004.

[33] K. Gabor, L. Patrick, and W. Michael, "An arm wrestling robot driven by dielectric

elastomer actuators," Smart Materials and Structures, vol. 16, p. S306, 2007.

[34] D. M. Bloom, "Grating Light Valve: revolutionizing display technology," 1997, pp.

165-171.

[35] L. J. Hornbeck, "Deformable-mirror spatial light modulator," in Proceedings of

SPIE, 1989, pp. 86-102.

[36] S. Yun, J. Song, I. Yeo, Y. Choi, V. Yurlov, S. An, H. Park, H. Yang, Y. Lee, K.

Han, I. Shyshkin, A. Lapchuk, K. Oh, S. Ryu, J. Jang, C. Park, C. Kim, S. Kim, E. Kim,

K. Woo, J. Yang, E. Kim, J. Kim, S. Byun, S. Lee, O. Lim, J. Cheong, Y. Hwang, G.


46

Byun, J. Kyoung, S. Yoon, J. Lee, T. Lee, S. Hong, Y. Hong, D. Park, J. Kang, W. Shin,

S. Lee, S. Oh, B. Song, H. Kim, C. Koh, Y. Ryu, H. Lee, and Y. Baek, "Spatial optical

modulator (SOM): high-density diffractive laser projection display," San Jose, CA, USA,

2007, pp. 648710-7.

[37] S. Sakarya, G. Vdovin, and P. M. Sarro, "Spatial light modulators based on

micromachined reflective membranes on viscoelastic layers," Sensors and Actuators A:

Physical, vol. 108, pp. 271-275, 2003.

[38] S. Sakarya, G. Vdovin, and P. M. Sarro, "Technology of reflective membranes for

spatial light modulators," Sensors and Actuators, A: Physical, vol. 97-98, pp. 468-472,

2002.

[39] R. Gerhard-Multhaupt, W. Brinker, H. J. Ehrke, W.-D. Molzow, H. Roeder, T. H.

Rosin, and R. Tepe, "Viscoelastic spatial light modulators and schlieren-optical systems

for HDTV projection displays," Santa Clara, CA, USA, 1990, pp. 69-78.

[40] R. Gerhard-Multhaupt, "Thin polymeric layers for spatial light modulators polymeric

Layers." vol. 85, P. Wünsche, Ed., ed: Springer Berlin / Heidelberg, 1991, pp. 133-142.

[41] W. Brinker, W. Wirges, G. Przyrembel, R. Gerhard-Multhaupt, J. Klemberg-Sapieha,

L. Martinu, D. Poitras, and M. R. Wertheimer, "Metallized viscoelastic control layers for

light-valve projection displays," Displays, vol. 16, pp. 13-20, 1995.

[42] J.-S. Wang, I. W. Jung, and O. Solgaard, "Fabrication method for elastomer spatial

light modulators for short wavelength maskless lithography," Sensors and Actuators A:

Physical, vol. 114, pp. 528-535, 2004.

[43] R. Tepe, "Theoretical analysis of an electrically addressed viscoelastic spatial light

modulator," Journal of the Optical Society of America A, vol. 4, pp. 1273-1282, 1987.

[44] D. Cristea, P. Obreja, M. Kusko, E. Manea, and R. Rebigan, "Polymer

micromachining for micro- and nanophotonics," Materials Science and Engineering C,

vol. 26, pp. 1049-1055, 2006.

[45] H. Kück, W. Doleschal, A. Gehner, W. Grundke, R. Melcher, J. Paufler, R.

Seltmann, and G. Zimmer, "Deformable micromirror devices as phase-modulating high-

resolution light valves," Sensors and Actuators, A: Physical, vol. 54, pp. 536-541, 1996.

[46] J. W. Goodman, Speckle Phenomena in Optics Roberts and Company Publishers,

2006.

[47] L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, "Speckle Reduction in

Laser Projection Systems by Diffractive Optical Elements," Applied Optics, vol. 37, pp.

1770-1775, 1998.


47

[48] A. Ozcan, A. Bilenca, A. E. Desjardins, B. E. Bouma, and G. J. Tearney, "Speckle

reduction in optical coherence tomography images using digital filtering," J. Opt. Soc.

Am. A, vol. 24, pp. 1901-1910, 2007.

[49] F. Riechert, G. Bastian, and U. Lemmer, "Laser speckle reduction via colloidal-

dispersion-filled projection screens," Appl. Opt., vol. 48, pp. 3742-3749, 2009.

[50] A. A. Andreev, T. B. Andreeva, I. N. Kompanets, M. V. Minchenko, and E. P.

Pozhidaev, "Speckle-noise suppression due to a single ferroelectric liquid-crystal cell,"

Journal of the Society for Information Display, vol. 17, pp. 801-807, 2009.

[51] W. Ha, S. Lee, Y. Jung, J. Kim, and K. Oh, "Speckle reduction in multimode fiber

with a piezoelectric transducer in radial vibration for fiber laser marking and display

applications," San Jose, CA, USA, 2008, pp. 68731V-8.

[52] W.-S. Ha, S.-J. Lee, K.-H. Oh, Y.-M. Jung, and J.-K. Kim, "Speckle Reduction in

Near-field Image of Multimode Fiber with a Piezoelectric Transducer," J. Opt. Soc.

Korea, vol. 12, pp. 126-130, 2008.

[53] J. I. Trisnadi, "Hadamard speckle contrast reduction," Optics Letters, vol. 29, pp. 11-

13, 2004.

[54] S. Lowenthal and D. Joyeux, "Speckle Removal by a Slowly Moving Diffuser

Associated with a Motionless Diffuser," Journal of the Optical Society of America A, vol.

61, pp. 847-851, 1971.

[55] S. C. Shin, S. S. Yoo, S. Y. Lee, C.-Y. Park, S.-Y. Park, J. W. Kwon, and S.-G. Lee,

"Removal of hot spot speckle on laser projection screen using both the running screen and

the rotating diffuser," Displays, vol. 27, pp. 91-96, 2006.

[56] E. G. Rawson, A. B. Nafarrate, R. E. Norton, and J. W. Goodman, "Speckle-free

rear-projection screen using two close screens in slow relative motion," Journal of the

Optical Society of America A, vol. 66, pp. 1290-1294, 1976.

[57] M. N. Akram, V. Kartashov, and Z. Tong, "Speckle reduction in line-scan laser

projectors using binary phase codes," Optics Letters, vol. 35, pp. 444-446, 2010.

[58] V. Yurlov, A. Lapchuk, S. Yun, J. Song, and H. Yang, "Speckle suppression in

scanning laser display," Applied Optics, vol. 47, pp. 179-187, 2008.

[59] V. Yurlov, A. Lapchuk, S. Yun, J. Song, I. Yeo, H. Yang, and S. An, "Speckle

suppression in scanning laser displays: aberration and defocusing of the projection

system," Applied Optics, vol. 48, pp. 80-90, 2009.


48

[60] L. Wang, T. Tschudi, M. Boeddinghaus, A. Elbert, T. Halldorsson, and P. Petursson,

"Speckle reduction in laser projections with ultrasonic waves," Optical Engineering, vol.

39, pp. 1659-1664, 2000.

[61] S. An, A. Lapchuk, V. Yurlov, J. Song, H. Park, J. Jang, W. Shin, S. Karpoltsev, and

S. K. Yun, "Speckle suppression in laser display using several partially coherent beams,"

Optics Express, vol. 17, pp. 92-103, 2009.

[62] J. Hadamard, "Résolution d'une question relative aux déterminants," Bulletin des

Sciences Mathématiques vol. 17, pp. 240-246, 1893.

[63] V. Kartashov, L. Henriksen, J. H. Ulvensoen, B. Svardal, T. Svortdal, R. Berglind,

and G. Hedin, "Image improvement in a laser projection display with a spatial light

modulator with a deformable polymer," Journal of the Society for Information Display,

vol. 17, pp. 581-587, 2009.

[64] X. Zhang, M. Wissler, B. Jaehne, R. Breonnimann, and G. Kovacs, "Effects of

crosslinking, prestrain, and dielectric filler on the electromechanical response of a new

silicone and comparison with acrylic elastomer," San Diego, CA, USA, 2004, pp. 78-86.

[65] J. P. Szabo, J. A. Hiltz, C. G. Cameron, R. S. Underhill, J. Massey, B. White, and J.

Leidner, "Elastomeric composites with high dielectric constant for use in Maxwell stress

actuators," San Diego, CA, USA, 2003, pp. 180-190.

[66] A. Malthe-sørenssen, E. Zimmer, T. Natestad, and B. Jacobson, "Method and device

for variable optical attenuator," US Patent, 2005.

[67] G. Kofod, "Dielectric elastomer actuators," Ph.D Thesis, The Technical University of

Denmark, 2001.

[68] W. Yuan, T. Lam, J. Biggs, L. Hu, Z. Yu, S. Ha, D. Xi, M. K. Senesky, G. Gruner,

and Q. Pei, "New electrode materials for dielectric elastomer actuators," San Diego,

California, USA, 2007, pp. 65240N-12.

[69] K. Doris and H. Hartwig, "Polymers for biomedical applications: improvement of the

interface compatibility " Advances in Polymer Science vol. 149, pp. 1-57, 2000.

[70] B. Marciniec, Comprehensive Handbook on Hydrosilylation Oxford: Pergamon,

1992.

[71] C. U. Ansel and K. F. Saul, Advanced Strength and Applied Elasticity, 4th ed.:

Prentice Hall, 2003.

[72] A. C. C. Esteves, J. Brokken-Zijp, J. Laven, H. P. Huinink, N. J. W. Reuvers, M. P.

Van, and G. de With, "Influence of cross-linker concentration on the cross-linking of

PDMS and the network structures formed," Polymer, vol. 50, pp. 3955-3966, 2009.


49

[73] M. Doi and S. F. Edwards, The Theory of Polymer Dynamics USA Oxford

University Press, 1988.

[74] J. M. G. Cowie and V. Arrighi, Polymers: Chemistry and Physics of Modern

Materials, 3rd ed.: CRC Press, 2007.

[75] G. Ouyang, K. Wang, M. Akram, L. Henriksen, and X. Chen, "Optimization of

PDMS network for a fast response and sensitive actuation material applied in a MEMS

spatial light modulator," Applied Physics A: Materials Science & Processing, vol. 103,

pp. 381-388, 2011.

[76] G. Ouyang, K. Wang, L. Henriksen, and X. Chen, "Electroactive polymer-based

spatial light modulator with high reliability and fast response speed," Optical

Engineering, vol. 50, pp. 124001-4, 2011.

[77] F. Carpi, G. Gallone, F. Galantini, and D. D. Rossi, "Enhancing the dielectric

permittivity of elastomers," in Dielectric Elastomers as Electromechanical Transducers,

ed Amsterdam: Elsevier, 2008.

[78] J. C. Maxwell, in Electricity and Magnetism. vol. 1, ed Oxford: Clarendon Press,

1892, p. 452.

[79] K. W. Wagner, Die Isolierstoffe der Elektrotechnik. Berlin: Springer, 1924.

[80] R. Sillars, Journal Institute of Electrical Engineers, vol. 80, p. 378, 1937.

[81] H. Looyenga, "Dielectric Constants of Heterogeneous Mixtures," Physica, vol. 31

pp. 401-406, 1965.

[82] K. Lichtenecker and K. Rother, "Die Herleitung des Logarithmischen

Mischungsgesetzes aus Allgemeinen Prinzipien der Stationaren Stromung," Physics

Zeitschrift, vol. 32, pp. 255-260, 1931.

[83] L. K. H. v. Beek, "Dielectric behaviour of heterogenous systems," in Progress in

Dielectrics, J. B. Birks, Ed., ed London: Heywood Books, 1967, p. 69.

[84] K. Ren, S. Liu, M. Lin, Y. Wang, and Q. M. Zhang, "A compact electroactive

polymer actuator suitable for refreshable Braille display," Sensors and Actuators A:

Physical, vol. 143, pp. 335-342, 2008.

[85] Y.-M. Chang, L. Wang, and W.-F. Su, "Polymer solar cells with poly(3,4-

ethylenedioxythiophene) as transparent anode," Organic Electronics, vol. 9, pp. 968-973,

2008.

[86] A. Denneulin, A. Blayo, J. Bras, and C. Neuman, "PEDOT:PSS coating on specialty

papers: Process optimization and effects of surface properties on electrical performances,"

Progress in Organic Coatings, vol. 63, pp. 87-91, 2008.


50

[87] M. E. Vlachopoulou, P. S. Petrou, S. E. Kakabakos, A. Tserepi, and E. Gogolides,

"High-aspect-ratio plasma-induced nanotextured poly(dimethylsiloxane) surfaces with

enhanced protein adsorption capacity," Journal of Vacuum Science and Technology B:

Microelectronics and Nanometer Structures, vol. 26, pp. 2543-2548, 2008.

[88] M. T. Khorasani, H. Mirzadeh, and S. Irani, "Plasma surface modification of poly (l-

lactic acid) and poly (lactic-co-glycolic acid) films for improvement of nerve cells

adhesion," Radiation Physics and Chemistry, vol. 77, pp. 280-287, 2008.

[89] S. K. Øiseth, A. Krozer, B. Kasemo, and J. Lausmaa, "Surface modification of spin-

coated high-density polyethylene films by argon and oxygen glow discharge plasma

treatments," Applied Surface Science, vol. 202, pp. 92-103, 2002.

[90] M. E. Vlachopoulou, P. S. Petrou, S. E. Kakabakos, A. Tserepi, K. Beltsios, and E.

Gogolides, "Effect of surface nanostructuring of PDMS on wetting properties,

hydrophobic recovery and protein adsorption," Microelectronic Engineering, vol. 86, pp.

1321-1324, 2009.

[91] C. AB, "COMSOL Multiphysics Model Library: 3D Cantilever Beam,," ed, 2007.

[92] S. L. Lai, M. Y. Chan, M. K. Fung, C. S. Lee, and S. T. Lee, "Concentration effect of

glycerol on the conductivity of PEDOT film and the device performance," Materials

Science and Engineering B: Solid-State Materials for Advanced Technology, vol. 104, pp.

26-30, 2003.

[93] Y. H. Ha, N. Nikolov, S. K. Pollack, J. Mastrangelo, B. D. Martin, and R. Shashidhar, "Towards a Transparent, Highly Conductive Poly(3,4-ethylenedioxythiophene)," Advanced Functional Materials, vol. 14, pp. 615-622, 2004. [94] G. Cloud, "Optical Methods in Experimental Mechanics," Experimental Techniques, vol. 34, pp. 15-19, 2010.


51

Article I

G. Ouyang, K. Wang, M. Akram, L. Henriksen, and X. Chen, "Optimization of

PDMS network for a fast response and sensitive actuation material applied in a

MEMS spatial light modulator," Applied Physics A: Materials Science & Processing,

Vol. 103, Issue. 2, pp. 381-388, 2011.

Appl Phys ADOI 10.1007/s00339-011-6348-6

Optimization of PDMS network for a fast response and sensitiveactuation material applied in a MEMS spatial light modulator

G. Ouyang · K. Wang · M.N. Akram · L. Henriksen ·X.Y. Chen

Received: 11 October 2010 / Accepted: 10 February 2011© Springer-Verlag 2011

Abstract The cross-linked networks of linear vinyl-termi-nated poly(dimethylsiloxane) (PDMS) with trimethylsiloxy-terminated methyhydrosiloxane–dimethylsiloxane copoly-mer (cross-linker) were investigated by tuning the cross-linker concentration, the chain length of linear PDMS andthe functionality of the cross-linker. Response time andelastic modulus of the cross-linked PDMS elastomer werecharacterized by home-made measurement systems and an-alyzed based on their network topologies. An optimizedPDMS elastomer, which has response time of 9 µs and elas-tic modulus of 601 kPa, was utilized as the actuation mater-ial in a MEMS spatial light modulator (SLM). The fabrica-tion processes were described. This polymer based SLM hasshown fast response time (9 µs) and successfully diffractedlight into higher orders with 84% first-order diffraction effi-ciency.

G. Ouyang · K. Wang · M.N. Akram · X.Y. Chen (�)Vestfold University College, RI, Raveien 197, Borre 3184,Norwaye-mail: [email protected]: +47-33031100

G. Ouyange-mail: [email protected]

K. Wange-mail: [email protected]

M.N. Akrame-mail: [email protected]

L. HenriksenpoLight AS, Horten 3179, Norwaye-mail: [email protected]

1 Introduction

Polymer has promising features in applications such asMEMS actuators and modulators, attributed to its light-weight, easy fabrication and low production cost. In theseapplications, the most important performance of the poly-mer is the mechanical deformation under electric force,which is characterized by the response time and the sensi-tivity. When the polymer is applied in a spatial light mod-ulator (SLM) as actuation material, fast response time andhigh sensitivity are always preferred to achieve high mod-ulation efficiency of light. Several polymers have been in-vestigated as candidate materials in the SLMs, for exam-ple, polymethylmethacrylate (PMMA) [1], SU-8 [2] andP(VDF-TrFE) based terpolymer [3]. However, the slow re-sponse is the biggest technical obstacle which limits thepractical application of polymer based modulators.

Poly(dimethylsiloxane) (PDMS) is one of the most im-portant polymers with unique properties, such as elastic-ity, durability, and resistance to high temperature, UV ra-diation and chemical attack [4]. These properties make thecross-linked PDMS suitable for a wide range of applica-tions, such as in mechanic devices [5], in electrical/opticaldevices [6–8] and in biomedical devices [9–11]. More im-portantly, the response time and sensitivity of the PDMSelastomer (cross-linked PDMS) can be tuned by modify-ing its network topology. Therefore it is a promising can-didate for SLMs. The theory of PDMS elasticity has beenwell established [12–14]. Normally, the PDMS elastomerhas three-dimensional networks, which are formed by hy-drosilylation reaction. The physical-mechanical propertiesof the elastomer depend on the cross-linked network struc-ture and the residual chemical groups. Therefore, it is ofgreat interest to understand and control polymer networktopology for PDMS material engineering.

mailto:[email protected]





G. Ouyang et al.

Scheme 1 Hydrosilylationreaction

Fig. 1 Diagram of the responsetime measurements.(a) Measurement setup;(b) applied AC voltage anddetected light intensity. Theresponse time is defined as thetime required for the lightintensity to rise (fall) from 10%(90%) to 90% (10%) of its peakvalue

In this work, vinyl-terminated linear PDMS were cross-linked through hydrosilylation reaction by multifunctionalcross-linkers in the presence of platinum complex catalyst.The aim is to synthesis PDMS material which has fast re-sponse time and high sensitivity. The sensitivity was indi-cated by the elastic modulus for this application. PDMS withdifferent network topologies were prepared, by changing theconcentration of the cross-linker, the chain length of the lin-ear PDMS, and the cross-linker functionality, one at a time,with other parameters fixed. A PDMS elastomer with opti-mized network structure was successfully synthesized andfurther fabricated as the actuation material in a SLM. Com-paring with other polymers [1–3], this modulator had simi-lar sensitivity under applied electric force while the responsetime was significantly improved.

2 Experimental

2.1 Materials

Linear vinyl-terminated poly(dimethylsiloxane) with mole-cular weight from 800 g/mol to 62,700 g/mol and cross-linkers, which are trimethylsiloxy-terminated methy-hydrosiloxane–dimethylsiloxane copolymer, were purchasedfrom Gelest, Inc. The cross-linking process was catalyzedby platinum complex previously dissolved in xylene, whichwere from UCT Specialties, LLC, at room temperature.

Hydrosilylation reaction has been commonly used to pre-pare PDMS elastomer [15], in which two vinyl groups ofthe linear PDMS react with silane groups of the cross-linkerforming a three-dimensional cross-linked network. This re-action is expressed in Scheme 1.

The rate of the reaction and properties of the formedPDMS elastomer depend on several parameters, e.g. the

functionality and concentration of the cross-linker, the mole-cular weight of the linear PDMS, the curing tempera-ture and the catalyst. In this work, methyhydrosiloxane-dimethylsiloxane copolymer was added to vinyl-terminatedlinear PDMS. After stirring for approximately 2 min,0.1 wt% platinum complex catalyst was added. Then themixture was intensively stirred for 30 s and cured in air atroom temperature.

2.2 Measurement setups

Two properties of the PDMS elastomer, which are theresponse time and the elastic modulus, were focused inthis work and characterized by home-made measuring sys-tems.

The response time is in a range of micro-seconds, whichwas measured by a system equipped with a laser and aphoto-detector [16], as shown in Fig. 1. In this measurementsetup, the testing PDMS (20 µm thin film) was depositedon a prism coated with a transparent indium tin oxide (ITO)layer. A metal needle was suspended 5 µm above the PDMSfilm and its position was controlled precisely by a controlstage. During measurements, a laser illuminated the PDMSon the prism. If no voltage was applied, the PDMS had aflat surface and the incident laser was totally reflected fromPDMS/air interface, which was further blocked by a stopper.Once the voltage was applied between the ITO layer and theneedle, a small spot on the PDMS (the area under the nee-dle) experienced an electric force and started to deform. Thedeformation led to light diffraction and scattering, whichwas detected by the photo-detector. Using this method, thedynamic deformation of the PDMS was represented as thechanging light intensity detected by the photo-detector. Theplots in Fig. 1 show how the detected signal changed withregard to the applied 300 V plus voltage with 1 ms width.

Optimization of PDMS network for a fast response and sensitive actuation material applied in a MEMS

Fig. 2 Diagram of the elasticmodulus measurements.(a) Measurement setup; (b) thecalculated elastic modulus basedon the relationship between h3/2

and F

When the pulse front came, the PDMS had a rapid defor-mation and then moved slowly to the maximum value. Thesimilar behavior was observed when the pulse was released.The response time can be then extracted from the profile ofdetected light intensity. This measurement setup has beenproved to be very effective and accurate in the evaluation ofthe response time of polymers [16]. It provides rich infor-mation on the response behavior of polymer material overa wide range of frequencies (1 Hz–1 GHz). However, itshould be kept in mind that several factors may influence themeasurement accuracy, e.g. capacitance between the needleand PDMS surface, the resistance of the ITO layer and therise/fall time of the function generator. The time lag intro-duced by these factors was calculated in a range of nano-seconds in this work, which is therefore neglected.

An accurate and convenient measurement system wasconstructed to measure the elastic modulus of the synthe-sized PDMS, as shown in Fig. 2. In this setup, a metal probewith sphere indenter was fixed with a high precision loadcell. The probe was controlled to move perpendicular to thePDMS surface by a control stage. The elastic modulus wasobtained based on Hertz equations [17]:

1 − ν2p

Ep

+ 1 − ν2p

Ei

= 4h32

3F

(1

Rp

− 1

Ri

)− 12

(1)

where Ep , Ei , νp , νi , Rp , Ri are the elastic modulus, Pois-son’s ratio, radius of the contact surface of the testing poly-mer and the indenter, respectively; F is the contact forcefrom the indenter; h is the indentation depth. In this case,a sphere indenter, which has Ri equal to 1 mm, is pressedonto a PDMS plane, which has Rp as infinity. The Pois-son ratio νp of PDMS is 0.5. The indenter is made of ruby,which has an elastic modulus Ei around 300 GPa. Since Ei

is much bigger than Ep , it is considered as infinity. Equation(1) is then rewritten as

1

Ep

= 16

9FR

12i h

32 (2)

In (2), there was a linear relationship between h3/2 and F .By moving the probe and recording the contact force, the

relationship between the indentation depth and the force canbe found, as plotted in Fig. 2. The gradient K , which is ob-tained by linear fitting the plots using least square method,was equal to 9/(16R

1/2i Ep), from which the Ep can be cal-

culated.

3 Results and discussion

3.1 Influence of the cross-linker concentration

The influence of the cross-linker concentration on PDMSresponse time and elastic modulus was investigated. Lin-ear PDMS with molecular weight equal to 9400 g/mol, wasmixed with the cross-linker, which has functionality 4.8,molecular weight 1950 g/mol. The response time and elas-tic modulus of synthesized PDMS elastomer with differentcross-linker concentrations are shown in Table 1.

It is observed that a gel-like elastomer can be formedwithin 4% and 65% weight percent of cross-linker, whichare defined as the upper and lower critical gel point, re-spectively. With increase of the cross-linker concentra-tion, the response time of the PDMS elastomer decreases(A2→A4), reaches the minimum point (A5), and thenincreases (A6→A8). The change of the response timeis attributed to the change of network topology. Whilecross-linker concentration was 10%, a fastest PDMS elas-tomer (9 µs) was obtained, indicating that an ideal three-dimensional cross-linked network was formed, with min-imum dangling or pendant chains. This network can re-spond to the applied force rapidly. Besides this point, lessor more cross-linker lead to defectiveness of the network,therefore slow down the response. According to the exper-iments, a perfect network was not formed when the num-ber of function groups of cross-linker (Si–H) were equal tothat of the linear PDMS (C=C), but was achieved in thepresence of extra Si–H groups. This is attributed to oxida-tion/hydrolysis side reactions of Si–H groups (the reactionformula is expressed as (3) and (4)), via reaction with other

G. Ouyang et al.

Table 1 Dependences ofresponse time and elasticmodulus on cross-linkerconcentration

∗ A gel-like elastomer cannot beformed; × The response timewas impossible to measure sincethe polymer was extremelysticky and soft

Samples Cross-linkerconcentration

C=C:Si–H Response time(µs)

Elastic modulus(kPa)

Curing time(min)

A1 3% 1:0.4 ∗ ∗ ∗A2 4% 1:0.5 11,200 8 84

A3 5% 1:0.6 980 32 63

A4 8% 1:1.0 65 278 41

A5 10% 1:1.2 9 601 22

A6 20% 1:2.4 100 481 6

A7 40% 1:4.8 700 122 2

A8 45% 1:5.3 800 79 2

A9 60% 1:7.1 × 3 1

A10 65% 1:7.7 × 1 1

A11 67% 1:8.0 ∗ ∗ ∗

compounds such as oxygen and moisture [18].

2R–Si–H + O2Catalyst−→ 2R–Si–O–H (3)

R–Si–H + H2OCatalyst−→ R–Si–O–H + H2 (4)

From Table 1, the response time increases significantlywith the presence of extra linear PDMS (A2, A3), while theincrease rate of the response time with extra cross-linker(A6, A7, A8) is slower as compare to the former. This isbecause the chain length of the linear PDMS (molecularweight = 9400 g/mol) is much longer than the cross-linker(molecular weight = 1950 g/mol). The dangling or pendantlong chains respond slower to the applied force than thatof shot chains, and thus significantly increase the responsetime.

The elastic modulus of the cross-linked polymer is alsochanged with the concentration of the cross-linker, due tothe changed network topology. A hardest elastomer is ob-tained when a perfect three-dimensional network is formed,while any defectiveness of the network decreases the elas-tic modulus. From Table 1, the elastic modulus is inverselyproportional to the response time. The results agree with the“Voigt–Kelvin model” [19].

The curing time of the cross-linked PDMS decreases withthe increase of the cross-linker concentration, indicating thatno matter the perfectness of the formed network, additionalcross-linker always increase the curing speed.

3.2 Influence of the linear PDMS chain length

The influence of the linear PDMS chain length to thephysical-mechanical properties of the PDMS elastomerwas studied. Linear PDMS with molecular weight from800 g/mol (short chain) to 62,700 g/mol (long chain) weremixed by cross-linker with molecular weight 1950 g/moland functionality 4.8. The molar ratio of function groups

between linear PDMS (C=C) and cross-linker (Si–H) wasfixed at 1:1.2 to ensure an ideal network. The response timeand elastic modulus of the formed PDMS elastomers areshown in Table 2.

It can be observed in Table 2 that the response time in-creases from 5 to 500 µs with increasing molecular weight.It is attributed to the increased chain lengths between thecross-linking sites. In another words, the cross-link den-sity is lower with increased chain length of linear PDMS(the cross-link density is defined as the averaged number ofcross-links per unit volume), and polymer with lower cross-link density is slower.

It is worth to mention that when long chain linear PDMSmixed with cross-linker, the long-range chain-chain inter-actions, called “chain entanglements”, dominated the poly-mer network. The consequence of the entanglements domi-nation (B4, B5) is the significantly increase response time. Itis explained thus: the entanglements forms a “tube” that re-stricts the lateral movement of chains and allows one dimen-sional diffusion in the “tube” as a mechanism for relaxation.The relaxation of these trapped entanglements is a relativeslow process via a sliding motion of the chains along eachother [20]. In applications, where a fast response time is de-sired, the linear PDMS with short chains is a better choiceto form a cross-linking dominated PDMS network.

3.3 Influence of the cross-linker functionality

Trimethylsiloxy-terminated methyhydrosiloxane–dimethyl-siloxane copolymer with different number of Si–H groupsper chain was mixed with vinyl-terminated linear PDMSwith molecular weight 9400 g/mol. The molar ratio of C=Cand Si–H was fixed at 1:1.2 to form an ideal network. Thecorresponding response time and elastic modulus of thecross-linked PDMS are listed in Table 3.

With the decrease of cross-linker functionality (C1→C4),the cross-link density is smaller, therefore a slower and


Table 2 Dependences ofresponse time and elasticmodulus on linear PDMS chainlength

∗ A gel-like elastomer cannot beformed

Samples Molecular weightof linear PDMS(g/mol)

Response time(µs)


Curing time(min)

B1 800 ∗ ∗ ∗B2 6000 5 712 28

B3(A5) 9400 9 601 22

B4 28,000 190 380 7

B5 62,700 500 175 1

Table 3 Dependences ofresponse time and elasticmodulus on the functionality ofcross-linker

∗ A gel-like elastomer cannot beformed

Samples Molecular weightof cross-linker(g/mol)

Functionalityof cross-linker

Response time(µs)


Curing time(min)

C1 1050 9.2 8 781 2

C2 1950 8.9 8 697 2

C3 (A5, B3) 1950 4.8 9 601 22

C4 1950 2.2 200 79 48

C5 1950 1.1 ∗ ∗ ∗

Table 4 Contributions ofchemical cross-links andentanglements to the elasticmodulus

Samples Elastic modulus(kPa)

Contributionfrom chemicalcross-links(kPa)

Contributionfromentanglements(kPa)

Percentageof entangle-mentscontribution

B2 712 689 23 3%

B3(C3) 601 440 161 27%

B4 380 148 232 61%

B5 175 66 109 62%

C1 781 592 189 24%

C2 697 584 113 16%

C4 79 69 10 12%

softer PDMS was obtained. Moreover, the relationship be-tween response time and elastic modulus show agreementwith the “Voigt–Kelvin model”. The curing time of thecross-linked PDMS increases with the decreased function-ality of the cross-linker, as expected. For sample C5, thecross-linker is nearly monofunctional. In this case, a three-dimensional network cannot be formed.

3.4 Influence of entanglements

Many efforts have been made to investigate the influence ofthe entanglements on physical-mechanical properties of thecross-linked PDMS [20–22]. In the case of small polymerdeformation, a phenomenological equation, which was de-veloped by Langley [21], Dossin and Graessley [22], can be

used to calculate the contribution of entanglements to theelastic modulus [14].

G =(

1 − 2

f

)ρ

MRT + G0

NTE (5)

where G is the shear modulus equal to E/3 in the case of theincompressible polymer, f is the functionality of the cross-linker, ρ is the density, M is the molecular weight, R is thegas constant, T is the temperature, G0

N is the plateau mod-ulus of the melt of uncross-linked polymer and TE is thetrapping factor. The first term in (5) is the modulus con-tributed from chemical cross-links, while the second term isfrom entanglements. Based on this model, the contributionof cross-links to the elastic modulus was calculated. Usingthe measured elastic modulus, the contribution of entangle-ments was obtained from (5), as shown in Table 4.

The contribution of entanglement to elastic modulus be-come obvious when long chain linear PDMS were cross-

G. Ouyang et al.

Fig. 3 A PDMS based SLM.(a) Schematic drawing of theSLM structure;(b) manufactured SLM chip;(c) cross-section of the SLMand electrical connection

linked. For sample B2, B3(C3), C1, C2 and C4 the elas-tic modulus is mostly determined by chemical cross-links,while for sample B4 and B5, entanglements start to dom-inate the polymer network. Among all entanglements inpolymer network, only the fraction of entanglements be-tween elastically active chains, called “the trapped entan-glements”, contributes to the elastic modulus [20]. The in-creased percentage of the entanglements contribution indi-cates an increased fraction of the trapped entanglements.These trapped entanglements slow down the polymer re-sponse to the applied force, as discussed previously. There-fore, a trapped entanglements dominated polymer is muchslower than a cross-link dominated PDMS. These resultsagreed with the discussion in Sect. 3.2: an abrupt responsetime increase was observed when the molecular weight oflinear PDMS reached 28,000 g/mol.

To conclude, PDMS elastomer formed by short chainvinyl-terminated PDMS cross-linking with high function-ality cross-linker has fastest response under electric force.Hence such polymer is a good candidate for polymer basedmodulators. In the next section, the PDMS elastomer wasapplied as the actuation material in a SLM, by cross-linkingthe vinyl-terminated PDMS, which has molecular weight9400 g/mol, with the cross-linker functionality 4.8.

4 Application of the optimized PDMS in spatial lightmodulator

Spatial light modulator is a widely used device for modify-ing the optical wavefront properties in projection displays,printing and optical communications. In this work, a fast andsensitive PDMS, which has intrinsic response time 9 µs andelastic modulus 601 kPa, was applied as the actuation mate-rial in a SLM.

The SLM was successfully fabricated using microma-chining technology. First, a 250 nm thick titanium–tungsten

adhesion layer was sputtered on a glass substrate. After-wards, a 150 nm thick gold layer was sputtered on thetitanium–tungsten layer. These two layers were then pat-terned as interdigital bottom electrodes by reactive ion etch-ing and wet etching, respectively. Afterwards, the synthe-sized PDMS was deposited by spin coating to form a filmwith thickness of 4 µm. Finally, a 50 nm thick gold film wasdeposited on the PDMS by sputtering, which worked bothas an electrode and reflective mirror. The schematic draw-ing as well as the fabricated SLM are shown in Figs. 3a, 3b.The top gold film had a continuous reflective surface. Thismirror-like surface is necessary to ensure good light modu-lation efficiency.

The working principle of this SLM have been describedin previous publication [23]. In brief, grating can be gen-erated on polymer surface by applying voltage through thetop gold film and bottom interdigital electrodes, as shownin Fig. 3c. Incident light illuminated at the grating and dif-fracted into higher orders. By selectively picking/blockingthese orders, the intensity of the light can be modulated.Performances of the SLM depend on both PDMS propertiesand structure dimensions. The following design parametersof the SLM were obtained from finite element method sim-ulations in COMSOL targeting a relatively big relief depthof PDMS: PDMS thickness 4 µm; period of gold electrodesin a range of 50 µm to 100 µm, size of the chip 20 × 12 mm.

When no voltage is applied, the polymer surface is flat,reflecting incoming light as a mirror; while the driving volt-age is applied, a sinusoidal grating is formed on PDMS/Ausurface, diffracting incoming light into many higher orders.Surface topographies of the SLM was measured and recon-structed by an interferometer in these two cases, as shownin Fig. 4. In order to achieve a large relief depth, 250 VDC voltage and ±100 V AC voltage was applied to the topand bottom electrodes, respectively. The peak-to-peak defor-mation was 170 nm, giving 84% first-order diffraction effi-ciency at 635 nm wavelength. Higher diffraction efficiencycan be achieved by apply higher voltage or choose a softer


Fig. 4 Surface topography ofthe top gold. (a) A flat surfacewith roughness 1.2 nm wasobserved when no drivingvoltage was applied;(b) a sinusoidal grating wasformed when driving voltagewas applied

Fig. 5 Output light of the SLM. (a) Reflected light; (b) diffracted light

PDMS elastomer. By utilize the grating, incident light wasdiffracted into higher orders, as observed in Fig. 5. The re-sponse time was 9 µs, which was exactly the same as the in-trinsic response time of the PDMS, indicating that the timelag in the circuit is negligible. Comparing with other poly-mer based SLMs, which have response time 4 ms [2], 2 ms[24], 1 ms [23], our SLM is much faster thanks to the opti-mized PDMS network. The faster response of the SLM givesmany advantages in its practical applications. For example,the efficiency can be improved in date storage applications;the motion-blur of fast moving objects in action scenes canbe eliminated in display applications.

5 Conclusions

A fast response and sensitive PDMS elastomer was syn-thesized by cross-linking vinyl-terminated linear PDMSwith methyhydrosiloxane-dimethylsiloxane copolymer atthe function group ratio C=C:Si–H=1:1.2. The presenceof extra Si–H function group was necessary due to in-evitable oxidation/hydrolysis side reactions. Linear PDMSwith shorter chain length or cross-linker with more func-tion groups per volume created a denser polymer networkwhich showed faster response time and higher elastic mod-ulus. The relationship between response time and elasticmodulus can be roughly fitted to Voigt–Kelvin model. Theinfluence of the entanglements became obvious in the poly-mer network when the molecular weight of linear PDMSreached 28,000 g/mol.

The optimized PDMS elastomer with intrinsic responsetime 9 µs and elastic modulus 601 kPa was utilized as the ac-tuation material in a SLM. Under the applied electric force,the surface of elastomer was deformed, forming a sinusoidalgrating. The grating successfully diffracted light into higherorders with 84% first-order diffraction efficiency. Compar-ing with other technologies, the SLM fabricated using ourPDMS elastomer has shown significant improvement in re-sponse time.

Acknowledgements The authors are grateful to Dr. V. Kartashovfor fruitful discussions. This research work is funded by the ResearchCouncil of Norway under grant BIA-286 project number 182667.

References

1. H.P. Pham, V.D. Dao, S. Amaya, R. Kitada, Y. Li, S. Sugiyama,IEEJ Trans. Sens. Micromach. 126(7), 306–311 (2006)

2. T. Aida, Y. Habu, T. Kato, Proc. SPIE 6993, 699301–699308(2008)

3. D.Y. Jeong, E.K. Hong, J. Korean Phys. Soc. 48(2), 229–232(2006)

G. Ouyang et al.

4. S.J. Clarson, J.A. Semlyen, Siloxane Polymers (Englewood Cliffs,Prentice Hall, 1993)

5. D.-W. Lee, Y.-S. Choi, Microelectron. Eng. 85(5–6), 1054–1058(2008)

6. Y.J. Chuang, T.H. Liao, P.R. Chen, K.Y. Hung, J. Micromech. Mi-croeng. 20(8), 129901 (2010)

7. G. Ouyang, K. Wang, L. Henriksen, M.N. Akram, X.Y. Chen,Sens. Actuators A, Phys. 158(2), 313–319 (2010)

8. G. Ouyang, Z. Tong, M.N. Akram, K. Wang, V. Kartashov, X. Yan,X. Chen, Opt. Lett. 35(17), 2852–2854 (2010)

9. S.M. Azmayesh-Fard, E. Flaim, J.N. McMullin, J. Micromech.Microeng. 20(8) (2010)

10. T. Fujii, Microelectron. Eng. 61–62, 907–914 (2002)11. K.S. Phillips, K.M. Kang, L. Licata, N.L. Allbritton, Lab. Chip.

10(7), 864–870 (2010)12. M.A. Llorente, J.E. Mark, Macromolecules 13(3), 681–685

(1980)13. J.E. Mark, J. Am. Chem. Soc. 92(25), 7252–7257 (1970)14. S.K. Patel, S. Malone, C. Cohen, J.R. Gillmor, R.H. Colby, Macro-

molecules 25(20), 5241–5251 (1992)15. B. Marciniec, Comprehensive Handbook on Hydrosilylation

(Pergamon, Oxford, 1992)

16. L.Z. Rogovina, V.G. Vasil’ev, G.L. Slonimskii, M.V. Sobolevskii,P.V. Ivanov, Mech. Compos. Mater. 29(3), 307–313 (1993)

17. A.C. Ugural, S.K. Fenster, Advanced Strength and Applied Elas-ticity, 4th edn. (Prentice Hall, Upper Saddle River, 2003)

18. A.C.C. Esteves, J. Brokken-Zijp, J. Laven, H.P. Huinink,N.J.W. Reuvers, M.P. Van, G. de With, Polymer 50(16), 3955–3966 (2009)

19. J.M.G. Cowie, V. Arrighi, Polymers: Chemistry and Physics ofModern Materials, 3rd edn. (CRC Press, Boca Raton, 2007)

20. M. Doi, S.F. Edwards, The Theory of Polymer Dynamics (Claren-don, Oxford, 1999)

21. N.R. Langley, Macromolecules 1(4), 348–352 (1968)22. L.M. Dossin, W.W. Graessley, Macromolecules 12(1), 123–130

(1979)23. G. Ouyang, K. Wang, M.N. Akram, X.Y. Chen, Proc. SPIE 7426,

742601–742604 (2009)24. R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q.B. Pei, S. Chiba,

Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 11(2), 89–100(2000)


52

Article II

G. Ouyang, K. Wang, L. Henriksen, and X. Chen, "Electroactive polymer-based

spatial light modulator with high reliability and fast response speed," Optical

Engineering, Vol. 50, No. 124001, 2011.

Optical Engineering 50(12), 124001 (December 2011)

Electroactive polymer-based spatial light modulatorwith high reliability and fast response speed

Guangmin OuyangKaiying WangVestfold University CollegeRaveien 197Horten, 3179, NorwayE-mail: [email protected]

Lars HenriksenPoLight ASHorten, 3179, Norway

Xuyuan ChenVestfold University CollegeRaveien 197Horten, 3179, Norway

Abstract. An electroactive polymer (EAP)-based MOEMS spatial lightmodulator (SLM), which shows high reliability and fast response speed,is reported in this work. The reliability is achieved by designing the SLMwithout direct contact between electrodes and deformable EAP surface;while the fast response speed is obtained by optimizing the microstructureof the EAP material. The concept of SLM, material optimization approach,fabrication processes, as well as characterization methods, are described.The SLM is driven by relatively low voltage, which is 200 V dc and 60 Vac and the response time is 35 μs. The manufactured SLM shows nodegradation or breakdown after millions of actuation cycles, indicatinga good reliability of the device. C©2011 Society of Photo-Optical InstrumentationEngineers (SPIE). [DOI: 10.1117/1.3659216]

Subject terms: spatial light modulators; polymers; micro-optics; diffractive opticalelements.

Paper 110937RR received Aug. 5, 2011; revised manuscript received Oct. 11,2011; accepted for publication Oct. 19, 2011; published online Dec. 5, 2011.

1 IntroductionA spatial light modulator (SLM) is a device that imposesspatially-varying modulation on a beam of light. Many dif-ferent of SLMs have been commercialized, such as gratinglight valve,1 digital micromirror device,2 and spatial opticalmodulator.3 These devices are all based on silicon technol-ogy. Organic materials, for example electroactive polymers(EAPs), are promising alternates for SLMs. EAPs have manydesirable features, e.g., lightweight, durability, easy fabrica-tion, and low production cost.4 It has been utilized in manyoptical devices such as tunable gratings,5 micro-optical zoomlens,6 optical attenuators,7 and spatial light modulators.8 InEAP-based SLMs, gratings, which are formed by EAP ma-terial under electric field, are used to modulate the light. Theelectric field induced strain is either generated from Maxwellstress, arising from Coulomb electrostatic interactions amongfree charges on electrodes, or from the electrostrictive stress,arising from strain-induced permittivity change of the mate-rial, or both. Currently, several technical obstacles limit thepractical applications of EAP-based SLMs. First, to drive theEAP under an electric field, a sandwich structure, in whichthe EAP is between two electrodes, is commonly employed.At least one of the electrodes has to comply with the defor-mation of the EAP. Such electrodes suffer from reliabilityproblems due to fatigue and creep, not to mention that theelectrodes themselves mechanically constrain the deforma-tion of the EAP. Second, due to the viscoelastic propertyof polymers, the EAP-based devices normally have slowresponse speed (in the range of milliseconds to seconds).Finally, a high driving voltage, typically on the order of kilo-volts, is required to drive the devices.9

In order to improve the performance of an EAP-basedspatial light modulator, the SLM structures were designedto avoid direct contact between the deformable EAP surfaceand electrodes. Polydimethylsiloxane (PDMS) was used asthe EAP material and it was optimized to have a fast response

0091-3286/2011/$25.00 C© 2011 SPIE

speed and high sensitivity by swelling of its network. Thefast response speed of the PDMS resulted in higher SLMmodulation efficiency, while the high sensitivity of PDMSlowered the requirement of driving voltages.

2 PDMS Processing and CharacterizationThe property of the PDMS is determined by its microstruc-ture. PDMS elastomer has a three-dimensional cross-linkednetwork, formed by linear vinyl-terminated PDMS reactingwith multifunctional cross-linker. To obtain both the responsespeed and sensitivity, the PDMS networks were swollen bysolvents. First, the linear PDMS and the cross-linker weremixed by manual stirring for 3 min. The molar ratio betweenfunction groups of linear PDMS and cross-linkers were setas 1:1.2 to achieve a PDMS network with minimum dan-gling or pendant chains.10 Second, silicon oil (solvent) withmolecular weight around 700 was added into the PDMS so-lution to swell the PDMS network. Samples with solventconcentration varying from 0% to 90% were prepared andcharacterized. Third, a catalyst was added into the mixture,followed by stirring thoroughly for 5 min. Finally, the sam-ples were de-bubbled in room temperature and cured in anoven at 60◦C.

The dependence of response time and elastic modulus ofswollen PDMS on solvent concentration are plotted in Fig. 1.The unswollen (0% solvent concentration) PDMS had a re-sponse time of 9 μs and elastic modulus 601 kPa. This highelastic modulus and fast response time indicated a rigid struc-ture which was formed by a tightly cross-linked network. Dueto the high elastic modulus, the sensitivity of the PDMS wasrelatively low, i.e., high voltage was required to drive the EAP.By introducing a short chain miscible solvent into PDMS, thepolymer network was swollen. Because the polymer chainswere surrounded by highly mobile solvents, the chain move-ment was promoted. The ease of chain movement resulted inthe decrease of elastic modulus, meaning a promoted sensi-tivity. Meanwhile, the response time of PDMS increased, butthis effect was rather moderate. To show the advantage of this

Optical Engineering December 2011/Vol. 50(12)124001-1


Ouyang et al.: Electroactive polymer-based spatial light modulator. . .

Fig. 1 Response time and elastic modulus of swollen PDMS with regard to solvent concentration.

optimization method, the properties of PDMS via networkswelling were compared with Ref. 10, in which the PDMSproperties were tuned by modifying the cross-link density.As shown in Fig. 2, in order to obtain a PDMS with similarresponse time, the PDMS tuned by network swelling had amuch higher sensitivity. The improvement was attributed toless trapped entanglements between polymer networks.10

3 Electroactive Polymer-Based SpatialLight Modulator

The optimized PDMS, which had an intrinsic response timeof 34μs and elastic modulus 200 kPa, was applied to fabricatea SLM. The structure of the SLM as well as the electrical con-nections is shown in Fig. 3. The PDMS film is deposited onthe transparent indium tin oxide (ITO) top electrode, whichis coated on the prism. There is an air gap between the inter-

Fig. 2 Comparison of response time and elastic modulus of PDMSbetween this work (PDMS tuned by network swelling) and Ref. 10(PDMS tuned by cross-link density modifying).

digital bottom electrodes and the PDMS deformable surface.The air gap allows free deformation of the PDMS and avoidsthe fatigue and creep of the electrodes. If no voltage is ap-plied, the PDMS surface is flat and incident light reflectson polymer/air interface at the angle of the total internal re-flection. When voltage is applied through the ITO layer andinterdigital electrodes, a phase grating in the form of period-ical relief is generated on the polymer surface. Incident lightis diffracted into many higher orders from the gratings. Per-formances of the SLM depend on both polymer properties

Fig. 3 Schematic of (a) top and (b) cross-section view of the SLM.



Fig. 4 SLM fabrication process flow. (a) Au and TiW interdigital elec-trodes patterning. (b) Electrodes wire-bonding and wire protection.(c) Glass beads and conductive adhesives deposition. (d) PDMS de-position. (e) PDMS squeezing. Fibers are used to define the PDMSthickness. (f) PDMS curing and peeling. Because of the hydrophilicsurface of the glass plate, the PDMS elastomer was only attached tothe prism. (g) Chip mounting. The ITO electrodes on the prism wereconnected to the chip by conductive adhesive.

and structure dimensions. The following design parametersof the SLM are obtained from finite element simulationsin COMSOL,11 targeting a relatively large relief depth ofPDMS: PDMS thickness 25 μm, air gap 5 μm, period ofinterdigital electrodes in a range of 60 to 80 μm, size of thechip 11 mm × 18 mm.

The modulator has been fabricated with micromachiningtechnology, as described in Fig. 4. Typically, a driven electricfield up to 150 V/μm is required to actuate EAP films.9 TheSLM reported in this work is driven by 200 V dc voltageand 60 V ac voltages to the ITO and interdigital electrodes,respectively. The low driving voltage requirement (260 V on25 μm thick PDMS film) is attributed to the high sensitivityof PDMS. When the SLM was not actuated, the incident lightwas not diffracted. i.e., only zeroth order of the light came outof the SLM, as shown in Fig. 5(a). The measured power of theoutput beam was 1.84 mW. When the SLM was actuated, theincident light was decomposed into several output beams, asshown in Fig. 5(b). The power distribution of the decomposedbeams is shown in Table 1, and the sum of all orders was

Fig. 5 Measured far-field diffraction patterns from (a) a nonactuatedSLM, and (b) an actuated SLM. Simulation of far-field diffraction pat-terns from (c) a nonactuated SLM, and (d) an actuated SLM.

Table 1 Experimental and simulation results of optical power distri-bution when the SLM is actuated.

Diffraction orders 0 + 1 − 1 + 2 − 2 + 3 − 3 + 4 − 4

Power (μW)experiments

110 538 532 221 227 54 58 19 20

Power (μW) theory 106 596 596 212 212 27 27 2 2

1.78 mW. The small deviation from a nondiffracted beam(1.84 mW) could be due to scattering and higher orderswhich were not taken into account. Simulations in ZEMAXwere done to obtain far-field diffraction patterns using scalardiffraction theory.10 The results were compared with exper-iments, as shown in Figs. 5(c) and 5(d) and Table 1. Thedeviation between experiments and theory, especially forhigh orders, is due to the shape of the gratings, which isnot a perfect sinusoidal. The response time of the modulatorwas measured as 35 μs, which was close to the intrinsic re-sponse time of the PDMS. The divergence can be generatedfrom the timing delay in the circuit. The fast response speedgives many advantages in practical applications of SLMs.For example, the motion-blur of fast moving objects in ac-tion scenes can be eliminated in display applications. TheSLM shows no degradation or breakdown after millions ofactuation cycles. It functioned well after storage in roomtemperature for a month, indicating good reliability of thedevice.

4 ConclusionA MEMS spatial light modulator was fabricated using micro-machining technology with swollen PDMS as the actuationmaterial. The SLM is driven by 200 V dc voltage and 60 Vac voltage and the response time is 35 μs. Compared withother polymer-based SLMs,5,11–14 the advantages of the pro-posed SLM are: fast response speed, reliability, low drivingvoltage, and low light loss. The improved reliability of theSLM is achieved by designing the SLM without direct con-tact between electrodes and deformable EAP surface, whilethe fast response speed is attributed to the optimized PDMSnetwork structure.

AcknowledgmentsThe authors are grateful to Dr. M. N. Akram and Dr. V.Kartashov for fruitful discussions. This work is funded by theResearch Council of Norway under Grant BIA-286 projectnumber 182667.

References1. D. M. Bloon, “The grating light valve: revolutionizing display technol-

ogy,” Proc. SPIE 3013, 165–171 (1997).2. L. J. Hornbeck, “Digital light processing update: status and future ap-

plications,” Proc. SPIE 3634, 158–170 (1999).3. S. K. Yun, J. H. Song, and I. J. Yao, “Spatial optical modulator (SOM):

high-density diffractive laser projection display,” Proc. SPIE 6487,648710 (2007).

4. F. Carpi and D. D. Rossi, “Improvement of electromechanical actuatingperformances of a silicone dielectric elastomer by dispersion of tita-nium dioxide powder,” IEEE Trans. Dielectr. Electr. Insul. 12, 835–843(2005).

5. G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y. Chen,“A novel tunable grating fabricated with viscoelastic polymer (PDMS)and conductive polymer (PEDOT),” Sensor Actuators A 158, 313–319(2010).


http://dx.doi.org/10.1117/12.273868

http://dx.doi.org/10.1117/12.349351

http://dx.doi.org/10.1117/12.714699

http://dx.doi.org/10.1109/TDEI.2005.1511110

http://dx.doi.org/10.1016/j.sna.2010.02.006


6. H. Kim, J. Park, N. H. Chuc, H. R. Choi, J. D. Nam, Y. Lee, H. S. Jung,and J. C. Koo, “Development of dielectric elastomer driven micro-optical zoom lens system,” Proc. SPIE 6524, 65241V (2007).

7. A. Malthe-Sørrensen, E. Zimmer, T. Naterstad, and B. Jacobsen, U.S.Patent 6,897,995 (2005).

8. V. Kartashov, L. Henriksen, J. H. Ulvensøen, B. Svardal, T. Svortdal,R. Berglind, and G. Hedin, “Image improvement in a laser projectiondisplay with a spatial light modulator with a deformable polymer,” J.Soc. Inf. Display 17(7), 581–587 (2009).

9. A. O. Halloran, F. O. Malley, and P. McHugh, “A review on dielectricelastomer actuators, technology, applications and challenges,” J. Appl.Phys. 104, 071101 (2008).

10. J. W. Goodman, Introduction to Fourier Optics, 3rd Ed, Roberts &Company, Englewood (2005).

11. G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y. Chen,“Optimization of PDMS network for a fast response and sensitive actu-

ation material applied in a MEMS spatial light modulator,” Appl. Phys.A 103, 381–388 (2011).

12. G. Ouyang, Z. Tong, W. Gao, K. Wang, M. N. Akram, V. Kartashov, andX. Y. Chen, “Polymer-based multiple diffraction modulator for specklereduction,” Proc. SPIE 7387, 73871F (2010).

13. S. Sakarya, G. Vdovin, and P. M. Sarro, “Spatial light modulators onmicromachined reflective membranes on viscoelastic layers,” SensorActuat. 108, 271–275 (2003).

14. R. Gerhard-Multhaupt, W. Brinker, H. J. Ehrke, W. D. Molzow, H.Roeder, T. H. Rosin, and R. Tepe, “Viscoelastic spatial light modulatorsand Schlieren-optical systems for HDTV projection displays,” Proc.SPIE 1255, 69–78 (1990).

Biographies and photographs of the authors are not available.


http://dx.doi.org/10.1117/12.716872

http://dx.doi.org/10.1889/JSID17.7.581

http://dx.doi.org/10.1889/JSID17.7.581

http://dx.doi.org/10.1063/1.2981642

http://dx.doi.org/10.1063/1.2981642

http://dx.doi.org/10.1007/s00339-011-6348-6

http://dx.doi.org/10.1007/s00339-011-6348-6

http://dx.doi.org/10.1117/12.869886



http://dx.doi.org/10.1117/12.19865

http://dx.doi.org/10.1117/12.19865


53

Article III

G. Ouyang, K. Wang, and X. Y. Chen, "TiO2 nanoparticles modified

polydimethylsiloxane with fast response time and increased dielectric constant,"

Submitted to Journal of Micromechanics and Microengineering, 2012.

TiO2 nanoparticles modified polydimethylsiloxane with fast response time and increased dielectric constant

G Ouyang, K Wang, and X Y Chen

Faculty of Science and Engineering, Vestfold University College, N-3103 Borre, Norway

Abstract: Polymer nanocomposite has shown great potential and impact on broad range of techniques since its properties can be tailored by nano-fillers. This paper reports a polymer nanocomposite with enhanced electro-mechanical performance by mixing TiO2 nanoparticles into polydimethylsiloxane. The nanocomposites with particle concentration up to 30 wt% were prepared. High energy ball milling and polyether-modified silicone dispersant was used to suppress the agglomeration and ensure a stable dispersion. Properties of the nanocomposites, i.e. elastic modulus, response time, dielectric constant and optical transmittance, were tuned by modifying the particles concentration. Optimized electro-mechanical properties were observed at 5 wt% TiO2 particles. This nanocomposite was applied to a MEMS spatial light modulator, which was fabricated using micro-machining technology. The device was driven by 200 V DC voltage and had response time of 6 µs.

Keywords: Nanocomposite; Electroactive polymers; MEMS; Spatial light modulator.

1. Introduction

Electroactive polymers (EAPs) have recently attracted considerable interests in applications of electro-mechanical modulators and actuators, attribute to their lightweight, reliability, durability and easy processing [1]. They have been applied in various devices, such as eyeball actuators [2], tunable gratings [3], lighter-than-air vehicles [4], inchworm robots [5], tactile display [6], etc. Despite the listed advantages of EAPs, several technical obstacles need to be overcome for their wide applications. A high driving electric field up to 150 V/μm, is required to drive the EAPs [7]. This brings safety issues associated with high voltages. The response speed of EAPs is slow, typically in the range of millisecond. A number of approaches have been explored aiming at improving electro-mechanical performance of EAPs, by increasing their dielectric constant and/or decreasing their elastic modulus [8-12]. Under the applied electric field, EAPs deform either by Maxwell stress, arising from Coulomb electrostatic interactions among free charges on electrodes, or electrostriction, arising from strain-induced permittivity change of the material, or both. The Maxwell stress induced strain SMaxwell, and the electrostriction induced strain Selectrostriction is calculated by Equation. (1) and Equation. (2), respectively [13].

SMaxwell = - ε0εrE2/Y (1) Selectrostriction= - Qε0

2(εr-1)2E2 (2) where the ε0=8.85 pF/m is the dielectric constant of the vacuum, εr is the relatively permittivity, E is the electrical field and equal to the applied voltage divided by the EAP films thickness, Y is the elastic modulus and Q is the electrostrictive coefficient. From the equations, in order to achieve a same strain, the material with lower elastic modulus or/and higher dielectric constant requires lower

driving voltage. Therefore a reduced driving voltage requirement for EAPs is accomplished by increasing their dielectric constant and/or decreasing their elastic modulus. Reducing the elastic modulus of EAPs involves polymer network modification [8]. Increasing of the dielectric constant can be achieved by mixing fine particles into polymer host [9-12].

In this work, nano-sized TiO2 particles were applied to modify the polymer microstructure. Polydimethylsiloxane (PDMS) was chosen as the host polymer because of its remarkable durability and resistance to temperature, UV radiation and weathering [21]. Titanium dioxide particles were used as fillers owing to its relatively high permittivity, chemical inertness, non-toxicity and easy dispersion in silicone [1]. Previous applications of TiO2-polymer nanocomposite were mainly in photocatalytic and UV absorption. The material investigations focused on the mechanical properties [14, 15], thermal properties [14, 16], optical properties [16-19], and dielectric properties [1, 15, 20]. Investigations regarding the response time, when TiO2-polymer nanocomposite is actuated under electric field, are limited. The presented work aims to broaden the application area of the material and to use it as electroactive polymers in compact and light weight MEMS devices, such as spatial light modulators (SLM), variable optical attenuators and optical switches. Fine TiO2 particles were dispersed in PDMS with the help of polyether-modified silicone. The nanocomposites were characterized for their response time, elastic modulus, dielectric constant and optical transmittance. Comparing with pure PDMS, the nanocomposites with optimized TiO2 concentration have faster response speed and increased dielectric constant. The nanocomposite was applied to a MEMS spatial light

modulator. The SLM was fabricated using micro-machining techniques. The device was driven by relatively low voltage and shown fast response time.

2. Experimental

As the host polymer, PDMS with three dimensional networks was formed by hydrosilylation reaction. Linear vinyl-terminated PDMS (molecular weight 9400 g/mol, from Gelest Inc.) reacted with cross-linkers, which were trimethylsiloxy-terminated methyhydrosiloxane- dimethylsiloxane copolymer (molecular weight 1950 g/mol, functionality 4.8, from Gelest Inc.). The ratio of function groups C=C:Si–H was 1:1.2. The presence of extra Si–H function groups is necessary because of the inevitable oxidation/hydrolysis side reactions [22]. The cross-linking process was catalyzed by platinum complex previously dissolved in xylene (from UCT Specialties, LLC). The reaction formula is given in figure 1, where the C=C groups in linear PDMS react with the Si–H groups in cross-linker. The fillers, which were titanium dioxide nanoparticles, have primary size of 25 nm (anatase, from Sigma-Aldrich Co.,).

There are many ways to disperse inorganic nanoparticles into organic polymers matrix. One can firstly modify the particles to change their surface properties and then disperse them into polymer solutions, as reported in [23, 24]. Or certain dispersant can be used to help the particle dispersion when mixing the particles with polymer solution, such as in [18, 19]. Surface modification of the particles is rather complicated and time consuming. After modification, a process to remove residual chemicals is necessary. This method is normally applied for high volume particle processing. Directly mixing the particles into solution with the help of dispersant, on the contrary, is simple and requires single processing step. If the correct dispersant is chosen, particles agglomeration is suppressed and a stable and evenly dispersed particles in solution can be obtained. In this work, fine TiO2 particles were dispersed in PDMS with the help of polyether-modified silicone. Firstly, TiO2 particles (weight percentage from 1% to 30%) were mixed with linear PDMS in the presence of polyether-modified silicone (KF-6015 from ShinEtsu). The ratio between polyether-modified silicones and nanoparticles was 1:5 by weight. These components were mixed by high energy ball-milling for 24 hours. Secondly, the cross-linker and catalyst were added into the solution sequentially, followed by 2 minutes manual stirring after each step. The ratio between linear PDMS and cross-linker was 9:1 by weight and the catalyst was 100 ppm. Finally, samples were cured at room temperature for 30 minutes. The size distribution of dispersed particles in PDMS solution was characterized by particle size analyzer Malvern ZetaSizer Nano S. The solution was diluted to 0.1 wt% to meet the requirement of the device. The particle distribution was also characterized by scanning electron microscope (SEM, Philips XL30). The nanocomposite solution, which was well mixed but without adding catalyst for cross-

linking, was deposited on silicon wafers. The samples were placed in oven at 40oC overnight. The wafer with TiO2 nanoparticles on its surface was used for SEM measurements.

Figure 1. Chemical reaction between linear vinyl-terminated PDMS and trimethylsiloxy- terminated methyhydrosiloxane-dimethylsiloxane copolymer, in presence of Pt catalyst. The elastic modulus of the nanocomposite was obtained by Hertz equation using a mechanical measurement system, as shown in figure 2a. In this setup, a metal probe with sphere indenter is fixed with a load cell. The probe is controlled to move perpendicularly to the nanocomposite surface by a control stage. When the indenter touches the nanocomposite surface, the contacting force is detected by the load cell. According to Hertz equation, there is a linear relationship between indentation depth of power 3/2 and the force on indenter [8]. The elastic modulus is a function of the gradient and it is acquired by fitting indentation depth with regard to detected force, as plotted in figure 2b.

Figure 2. Diagram of the elastic modulus measurements. (a) Measurement setup; (b) The calculated elastic modulus based on the relationship between indentation depth of power 3/2 (h3/2) and detected force on indenter (F).

The response time of the nanocomposite was measured

using an optical measurement system, as shown in figure 3a. In this setup, the testing nanocomposite is deposited on a prism coated with an indium tin oxide layer (works as one electrode). A metal needle (works as another electrode) is suspended close to the nanocomposite film. When the voltage is applied to the electrodes, a small spot on the

nanocomposite (the area under the needle) deforms. The deformation results in light diffraction and scattering, which is detected by the photo-detector. The response time is obtained by comparing the detected signal to applied voltage in an oscilloscope, as plotted in figure 3b. More details of the elastic modulus and response time measurement setups are referred to [8].

Figure 3. Diagram of the response time measurements. (a) Measurement setup; (b) Applied AC voltage and detected light intensity in oscilooscope. The response time is defined as the time required for the light intensity to rise (fall) from 10%(90%) to 90%(10%) of its peak value.

The dielectric constant was characterized by measuring the capacitance of nanocomposites using TTi LCR400 precision LCR meter. The testing samples were molded into cylinders with thickness of 5.7 mm and diameter of 26 mm. Both sides of the testing samples were sputtered with 50 nm gold film, to ensure a good electrical contact to the LCR meter. Light transmittance was measured for 10 μm thick nanocomposite films which were deposited on glass plates. Hitachi U-1100 spectrophotometer was employed for the measurements. The characterized wavelength range was between 300 nm and 800 nm. A nanocomposite with 5 wt% TiO2 was applied in a MEMS spatial light modulator. The SLM was fabricated using micro-machining technologies. The fabrication process is shown in figure 4. A 250 nm thick titanium–tungsten adhesion layer was sputtered on a glass substrate. Afterwards, a 150 nm thick gold layer was sputtered on the TiW layer. Both TiW and Au layer were deposited by MRC 603 DC Sputter. These two layers were patterned as interdigital bottom electrodes by wet etching sequentially. The Au layer was etched by KI+I2+H2O at room temperature for 20 sec and the TiW layer was etched by H2O2 at 50oC for 60 sec. A photoresist layer (Microposit S1813, 2.2 μm thick) was patterned to protect the bonding pads. Afterwards, the synthesized nanocomposite was spin coated on the chip surface with spin speed of 1200 rpm. The thickness of nanocomposite layer was 4 μm, measured by Veeco Dektak Profilometer. A 50 nm thick gold film was sputtered on the nanocomposite at room temperature by Fisons SC500 sputter coater. The gold film together with the nanocomposite on the photoresist was lifted off by dissolving the photoresist in acetone. Finally, the chip was

placed on a ceramic board with gold bonding pads. The electrodes on the SLM chip were wire-bonded to the ceramic board for electrical connection.

Figure 4. Fabrication process of the nanocomposite based spatial light modulator. (a) TiW, Au deposition; (b) TiW, Au patterning; (c)Photoresist deposition; (d) Nanocomposite spin coating and Au film sputtering; (e) Lift-off of Au and nanocomposite layer, wire-bonding. 3. Results and discussion 3.1 Particle dispersion

Inhomogeneous dispersion due to pronounced agglomeration tendency of nanoparticles is a common phenomenon for synthesizing a polymer nanocomposite [25]. However, a good dispersion is necessary to obtain a nanocomposite with optimized properties. Two techniques have been applied to suppress the agglomeration. Firstly, high-energy ball-milling was employed during mixing. It provided high impact and shear energy to break agglomerates. Secondly, polyether-modified silicone, which acts as a good dispersant for dispersing TiO2 into silicones, was used for homogenous dispersion and avoiding the re-agglomeration. The dispersant has branched polyethylene chains which can bond to the -OH group on TiO2 surface. The reaction formula is given in figure 5. The TiO2-PDMS dispersion with or without using the dispersant are compared, and shown in figure 6. In this test, both mixtures have been left at room temperature for 24 hours after ball milling. The suspension without dispersant is found to quickly settle to the bottom of the bottle. In contrast, the one with dispersant shows no phase separation even after 24 hours. The stable and homogenized dispersion indicates that the polyether-modified silicone dispersant is bonded to the TiO2 particle surface.

Figure 5. Chemical reaction between polyether-modified silicone dispersant and TiO2 nanoparticles.

Figure 6. Comparison between 5 wt% TiO2 in PDMS solution without (left) and with (right) the dispersant.

The size distribution of the dispersed particles in PDMS solution is characterized by particle size analyzer and SEM. The results are given in figure 7 and figure 8, respectively. As shown in figure 7, by using the polyether-modified silicone dispersant, the particle size decreases from around 1000 nm to 500 nm, indicating the function of the dispersant for suppressing the particle agglomeration. The SEM image in figure 8 shows agreement with the results in figure 7, that the particles exist in the PDMS as size around 500 nm. It is very difficult to further reduce the agglomerates size even with high energy ball milling, because of the strong agglomeration tendency of nanoparticles.

Figure 7. Particle size distribution of nanoparticles in PDMS solution with and without the dispersant, measured by particle size analyzer.

Figure 8. SEM image of TiO2 nanoparticles. 3.2 Electro-mechanical properties

The dependence of nanocomposite elastic modulus on TiO2 particle concentration is plotted in figure 9. The elastic modulus decreases from 600 kPa to 487 kPa, when the particle concentration increases from 0 wt% to 3 wt%. As more particles added into PDMS, elastic modulus increases to 872 kPa at 30 wt% concentration. The influence of particles to the elastic modulus is attributed to two factors: softening effects due to the interfering of cross-linking (major effect for TiO2 concentration lower than 5 wt%) and hardening effects due to the particle-polymer interaction (major effect for TiO2 concentration higher than 5 wt%). At 5 wt% particle concentration, the two effects are balanced with each other, and the elastic modulus is more or less the same as unloaded PDMS elastomer. The classical rule of mixture is frequently used to describe the relationship between elastic modulus and particle concentration. It predicts a linear increase of elastic modulus with volume fraction of particles. This is obviously not the case as it is observed in figure 9. The reason is that the classical rule of mixture describes a rather ideal situation. When nanoparticles are mixed with polymer, the polymer-particles interaction may result in the formation of an interfacial region with mechanical properties different to the bulk polymer. This effect is not taken into account in the classical rule of mixture. The change of elastic modulus with regard to particle concentration has been also investigated by other research groups [1, 12]. In these works, the elastic modulus increased continuously with TiO2 concentration. The deterioration of the cross-linking process by particles was not mentioned. However, this work shows that the interfering of cross-linking is inevitable. At particles concentration above 5 wt%, strong particle-polymer interaction results in a reinforcement of the polymer network [26], i.e. particles acting as effective chemical “cross-links”. Chemical cross-linking leads to a decrease in mobility and increase the elastic modulus of nanocomposite.

Figure 9. Elastic modulus of nanocomposites with varied TiO2 concentrations.

The effect of response time on particle concentration is

given in figure 10. The PDMS without any particles has response time of 9 μs. It increases to 11.5 μs at 2 wt% particle concentration and decrease to 3 μs at 30 wt% particle concentration. At low particle concentration, the cross-linking process is interfered by the particles. The polymer chains are less cross-linked and therefore there are high mobility chains existing in polymer network which can have large-scale chain relaxation. The result is prolonged response time. When the polymer network is enhanced by the polymer cross-linker and the particle-polymer interaction at high particle concentration, the chain motion is suppressed. The decreased chain mobility leads to a decreased response time. The relationship between elastic modulus and response time can be fitted to “Voigt-Kelvin model”. In brief, the elastic modulus is inversely proportional to the response time [27]. However, at 5 wt%, though the elastic modulus of nanocomposite (5 wt%) is more or less the same as neat PDMS (0 wt%), the response time improves from 9 µs to 6 µs. Possible explanation for the improvement of response time is due to the reduced moveable entanglements. When particles are added into polymer matrix, the particle-polymer interactions freeze the surrounding chain entanglements and reduce the amount of moveable entanglements. Since the movement of entanglements is a rather slow process, the reduced amount of entanglements improves the response time.

Figure 10. The response time of nanocomposites with varied TiO2 concentrations.

The dependence of the dielectric constant on particle

concentration is shown in figure 11. A sharp increase of the dielectric constant from 2.97 to 3.85 is observed when the concentration of TiO2 particles increases from 0 wt% to 5 wt%. Thereafter, the dielectric constant gradually increased to 4.4 with particles concentration reaching to 30 wt%. The increase in dielectric constant is attributed to the high dielectric constant of TiO2. For a modulator based on the deformation of electroactive materials, the increased dielectric constant gives an advantage of increased electrostatic force, therefore reduce the requirement of the driving voltage. The measured dielectric constant of nanocomposite is compared with theoretical values calculated from several mixing rules: Maxwell-Wagner equation [28, 29], Sillars equation [30], Looyenga equation [31], and Lichtenecker equation [32]. The results are plotted in figure 11. As seen from figure 11, none of the theoretical model predicts the dielectric constant change of nanocomposite well. The reason could be due to: firstly, these mixing rules hold for the dielectric constant at high frequency range, while in this work the dielectric constant has been measured at 100 Hz. At low frequency, the Maxwell-Wagner relaxation process [33] arises and influences the dielectric constant. Secondly, the mixing rules assume that the particles have perfectly sphere shape, and have not taken account of the particles real geometry.

Figure 11. Experimental and theoretical dielectric constant of nanocomposites with varied TiO2 concentrations. 3.3 Optical properties Light transmittance spectra for the nanocomposites films (after curing) with different particle concentrations is plotted in figure 12. The transmittance maintains around 80% at visible wavelength for TiO2 concentration below 3 wt%. It decreases very fast when the TiO2 concentration cross 10 wt%. At the UV range (300 nm – 400 nm), the light loss is dramatic for all nanocomposite films. This is because of the absorption of TiO2 fillers at this wavelength range. The band gap for TiO2 (anatase) is 3.2 eV, corresponding to the absorption edges 388 nm [34]. When TiO2 is illuminated by light with energy higher than its band gaps, i.e. with

wavelength shorter than the absorption edge, electrons will jump from valance band to conduction band by absorbing the energy from photons. When it comes to the visible wavelength (400nm – 800 nm), the light absorption of TiO2 is negligible, since the electrons do not have enough energy to jump to the conduction band. The light loss in visible wavelength is mostly due to the Mie scattering when light illuminated on TiO2 particles which has a size comparable with wavelength. In figure 13, a photograph of nanocomposite films prepared by spin coating is given. A high level of transparency is observed when the particle concentration is lower than 5 wt%.

Figure 12. The transmittance spectra of 10 µm nanocomposite films with varied TiO2 concentrations.

Figure 13. Comparison of 10 µm nanocomposite with varied TiO2 concentrations on glass plates. a)1 wt% TiO2; b) 5wt% TiO2; c)10 wt% TiO2. To conclude the investigation, the properties of nanocomposite can be tuned by modifying TiO2 concentration in PDMS host material. The choice of particle concentration depends on the application demands. If a fast response speed is the top consideration, a nanocomposite with high particle concentration is a good candidate. 3 µs response speed is obtained with 30 wt% TiO2 in PDMS. If a low driving voltage is mostly desired, a nanocomposite with 3 wt% TiO2 particles is a wise choice, since this nanocomposite has the smallest elastic modulus. For a material with smaller elastic modulus, the strain is bigger under the same driving voltage. However, the cost is the prolonged response speed. In some optical applications, where a high transparency is necessary, the nanocomposite should have particle concentration lower than 5 wt%. From the electro-mechanical point of view, the optimal point or

balanced point is found at 5 wt%. Comparing with the pure PDMS, nanocomposite with 5 wt% TiO2 has faster response time. The driving voltage is also reduced, attributed to the smaller elastic modulus, and the increased dielectric constant. 3.4 Application in SLM The nanocomposite with 5 wt% TiO2 concentration is applied in MEMS spatial light modulators as the electroactive material. The elastic modulus of nanocomposite is 567 kPa, which is slightly smaller than the elastic modulus of pure PDMS. The response time and dielectric constant are 6 µs and 3.85, respectively. In this SLM, light is reflected at the gold film which is deposit on the surface of nanocomposite, therefore a high transmittance is not necessary. The schematic drawing of the SLM is given in figure 14a. The working principle has been described in previous publication [35]. In brief, when no voltage is applied, the surface of nanocomposite is flat. Incoming light is reflected at the gold film on nanocomposite surface. When the driving voltage is applied through the top gold electrode and bottom interdigital electrodes, a sinusoidal surface grating is formed on nanocomposite, diffracting incoming light into many higher orders. By selectively picking/blocking these orders, the intensity of the light is modulated. Performances of the SLM depend on both material properties and structure dimensions. The following design parameters of the SLM are obtained from finite element method simulations in COMSOL Multiphysics targeting maximum diffraction efficiency of gratings: nanocomposite thickness 4 μm; period of interdigital electrodes in a range of 50 μm to 100 μm.

Figure 14. SLM chip with nanocomposite utilized as EAP to create the gratings. a) Schematic of the SLM. b) The fabricated SLM chip. The top gold electrode also works as a reflective mirror.

Surface topographies of the SLM are measured by an optical interferometer (Wyko NT9100) when the gratings are non-actuated and actuated, as shown in figure 15. Sinusoidal gratings at the actuated SLM are observed. However, the surface of nanocomposite in a non-actuated SLM is not extremely flat. The relatively rough surface is

because of the TiO2 nanoparticles inside the polymer host. The surface roughness Ra is measured as 15 nm. As the value much smaller than the wavelength of the incident light, the nanocomposite surface still works as a reflective mirror. In order to ensure the diffraction efficiency, 200V voltage is applied via the top and bottom electrodes. The peak-to-peak deformation of gratings in figure 15b is measured as 180 nm. The first order diffraction efficiency is calculated by Jq

2(m/2), where Jq is the Bessel function of the qth order, and m is a parameter which are determined by the SLM structure and polymer relief depth. In the case of SLM with surface reflection, m is equal to 4dπ/λ, where d the the grating relief depth and λ is the wavelength of incident light. The calculated first order diffraction efficiency is 33% for the light with 633 nm wavelength. Typically, a driven electric field up to 150 V/μm is required to actuate EAP films [7]. The low driving voltage requirement in this work (200 V on 4 μm EAP film) is benefited from low elastic modulus and high dielectric constant. The response time of the device is the same as the response time of the nanocomposite, which is 6 μs. This indicated that the time lag due to RC circuit in the device is negligible. Comparing with other EAP based SLMs, which have response time in the range of millisecond [36], 250 microseconds [3], 9 microseconds [8], the response time in this work is improved thanks to the modified PDMS network with nanoparticles. The fast response speed gives many advantages in its practical applications. For example, the motion-blur of fast moving objects in action scenes is eliminated in display applications.

Figure 15. The profile of nanocomposite gratings. a) When no voltage is applied, the surface is flat (Ra = 15 nm). b) When voltage is applied, the grating profile is observed. The dimension in z-direction is exaggerated for observation purpose. 5. Summary and conclusion TiO2 nanoparticles with concentration up to 30 wt% were dispersed into PDMS host material, forming nanocomposites. The agglomeration of nanoparticles was suppressed and TiO2 particles are uniformly dispersed in PDMS with sizes below 500 nm. The nanocomposite with optimized electro-mechanical properties were found at 5 wt% particle concentration. The elastic modulus, response time, dielectric constant of the nanocomposite was 567 kPa, 6 μs and 3.85, respectively. Nanocomposite with 3 µs response time was obtained by dispersing 30 wt% nanoparticles into PDMS. Nanocomposite with 5 wt% particle concentration

was utilized in a MEMS spatial light modulator. Comparing with traditional technology, where pure polymer is used as the electroactive material in SLMs, the requirement for driving voltage is reduced to 200 V and the response speed is improved to 6 µs.

Acknowledgements

The authors are grateful to Dr. M.N. Akram, Dr. L. Henriksen and Dr. V. Kartashov for fruitful discussions. This research work is funded by the Research Council of Norway under grant BIA-286 project number 182667 and poLight AS. References [1] Carpi F and Rossi D D 2005 Improvement of electromechanical

actuating performance of a silicone dielectric elastomer by dispersion of titanium dioxide power, IEEE T Dielect El In. 12 835-843.

[2] Liu Y, Shi L, Liu L, Zhang Z, Leng J 2008 Inflated dielectric elastomer actuator for eyeball's movements: fabrication, analysis and experiments, Proc. SPIE 6927 69271A.

[3] Ouyang G, Wang K, Henriksen L, Akram M N and Chen X Y 2010 A novel tunable grating fabricated with viscoelastic polymer (PDMS) and conductive polymer (PEDOT), Sensor. Actuat. A – Phys. 158 313-9.

[4] Michel S, Dürager C, Zobel M, Fink E 2007 Electroactive polymers as a novel actuator technology for lighter-than-air vehicles, Proc. SPIE 6524 65241Q.

[5] Jung K, Nam H, Lee Y, Choi H 2004 Micro inchworm robot actuated by artificial muscle actuator based on nonprestrained dielectric elastomer, Proc. SPIE 5385, 357-67.

[6] Koo I M, Jung K, Koo J C, Nam J D, Lee Y K, Choi H R 2008 Development of Soft-Actuator-Based Wearable Tactile Display, IEEE Trans. Robotics 24 3 549-58.

[7] Halloran A O, Malley F O and McHugh P 2008 A review on dielectric elastomer actuators, technology, applications and challenges J. Appl. Phys. 104 071101-11.

[8] Ouyang G, Wang K, Akram M N, Henriksen L and Chen X Y 2011 Optimization of PDMS network for a fast response and sensitive actuation material applied in a MEMS spatial light modulator Appl. Phys. A – Mater. 103 381-8.

[9] Lotz P, Matysek M, Lechner P, Hamann M and Schlaak H F 2008 Dielectric elastomer actuators using improved thin film processing and nanosized particles Proc. SPIE 6927 692723.

[10] Zhang Z, Liu L, Fan J, Yu K, Liu Y, Shi L and Leng J 2008 New silicone dielectric elastomers with a high dielectric constant Proc. SPIE 6926 692610.

[11] Gallone G, Carpi F, Rossi D D, Levita G and Marchetti A 2007 Dielectric constant enhancement in a silicone elastomer filled with lead magnesium niobate–lead titanate Mater. Sci. Eng. C 27 110–6.

[12] Zhang X Q, Wissler M, Jaehne B, Broennimann R and Kovacs G 2004 Effects of crosslinking, prestrain and dielectric filler on the electromechanical response of a new silicone and comparison with acrylic elastomer Proc. SPIE 5385 78-86.

[13] Krakovský I, Romijn T, and Posthuma de Boer A 1999 A few remarks on the electrostriction of elastomers, J. Appl. Phys. 85 1 628-9.

[14] Liaw W C, Chen K P 2007 Preparation and characterization of poly(imide siloxane) (PIS)/titania(TiO2) hybrid nanocomposites by sol–gel processes, Eur. Polym. J., 43 6 2265-78.

http://spie.org/x648.xml?product_id=747288&origin_id=x648

[15] Yang T I, Kofinas P 2007 Dielectric properties of polymer nanoparticle composites, Polymer, 48 3 791-798.

[16] Silva V P, Paschoalino M P, Gonçalves M C, Felisbert M I, Jardim W F, Yoshida I V P 2008 Silicone rubbers filled with TiO2: Characterization and photocatalytic activity, Mater. Chem. Phys., 113 15 395-400.

[17] Iketani K, Sun R D, Toki M, Hirota K, Yamaguchi O 2003 Sol-gel-derived TiO2/poly (dimethylsiloxane) hybrid films and their photocatalytic activities, J. Phys. Chem. Solids, 64 3 507-13.

[18] Nasu A, Otsubo Y 2006 Rheology and UV protection properties of suspensions of fine titanium dioxides in a silicone oil, J. Colloid Interf. Sci., 296 15 558-64.

[19] Nasu A, Otsubo Y 2008 Effects of polymeric dispersants on the rheology and UV-protecting properties of complex suspensions of titanium dioxides and zinc oxides, Colloid Surfaces A, 326 1 92-7.

[20] Szabo J P, Hiltz J A, Cameron C G, and Underhill R S 2003 Elastomeric composites with high dielectric constant for use in Maxwell stress actuators, Proc. SPIE 5051 180-90.

[21] Kartashov V, Henriksen L, Ulvensøen J H, Svardal B, Svortdal T, Berglind R and Hedin G 2009 Image improvement in a laser projection display with a spatial light modulator with a deformable polymer J. Soc. Inf. Disp. 17 581-7.

[22] Esteves A C C, Brokken-Zijp J, Laven J, Huinink H P, Reuvers N J W, Van M P and With G D 2009 Influence of cross-linker concentration on the cross-linking of PDMS and the network structures formed Polymer 50 3955–66.

[23] Rong Y, Chen H Z, Wu G, Wang M 2005 Preparation and characterization of titanium dioxide nanoparticle/polystyrene composites via radical polymerization, Mater. Chem. Phys., 91 15 370-4.

[24] Jang I B, Sung J H, Choi H J and Chin I, J 2005 Synthesis and characterization of TiO2/polystyrene hybrid nanoparticles via admicellar polymerization, Mater. Sci., 40 11 3021-4.

[25] Carpi F, Gallone G, Galantini F and Rossi D D 2008 Silicone-

Poly(hexylthiophene) Blends as Elastomers with Enhanced Electro- mechanical Transduction Properties Adv. Funct Mater. 18 235-42.

[26] Dreiss C A, Cosgrove T, Benton N J, Kilburn D, Alam M A, Schmidt R G and Gordon G V 2007 Effect of crosslinking on the mobility of PDMS filled with polysilicate nanoparticles: Positron lifetime, rheology and NMR relaxation studies Polymer 48 4419-28.

[27] Cowie J M G and Arrighi V 2007 Polymers:Chemistry and Physics of modern Materials (Oxford: Taylor & Francis, Inc).

[28] Maxwell J C 1982 Electricity and Magnetism (Oxford: Clarendon Press) p 452.

[29] Wagner K W 1924 Die Isolierstoffe der Elektrotechnik Vol. 1, ed Schering H (Berlin: Springer Press).

[30] Sillars R 1937 The Properties of a Dielectric Containing Semi-Conducting Particles of Various Shapes J. Inst. Elect. Eng. 80 378-94.

[31] Looyenga H 1965 Dielectric Constants of Heterogeneous Mixtures Physica 31 401-6.

[32] Lichtenecker K and Rother K 1931 Die Herleitung des Logarithmischen Mischungsgesetzes aus Allgemeinen Prinzipien der Station aren Stromung Phys. Zeitschr. 32 255-60.

[33] van Beek L K H 1967 Progress in dielectrics vol. 7, ed J B Birks (London: Heywood Books) p 69.

[34] Mo S D, Ching W Y 1995 Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite Phys Rev B 51, 13023-32.

[35] Ouyang G, Wang K, Akram M N and Chen X Y 2009 Fabrication and characterization of polymer based spatial light modulators Proc. SPIE 7426 742601.

[36] Sakarya S, Vdovin G, Sarro P M 2003 Spatial light modulators on micromachined reflective membranes on viscoelastic layers Sensor. Actuat. A – Phys 108 271-5.

http://www.sciencedirect.com/science/article/pii/S0079670002000151#bBIB1


54

Article IV

G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y. Chen, "A novel tunable

grating fabricated with viscoelastic polymer (PDMS) and conductive polymer

(PEDOT)," Sensors and Actuators A: Physical, Vol. 158, Issue. 2, pp. 313-319, 2010.

Author's personal copy

Sensors and Actuators A 158 (2010) 313–319

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

A novel tunable grating fabricated with viscoelastic polymer (PDMS)and conductive polymer (PEDOT)

G. Ouyanga,∗, K. Wanga, L. Henriksenb, M.N. Akrama, X.Y. Chena

a Vestfold University College, Horten 3179, Norwayb PoLight AS, Horten 3179, Norway

a r t i c l e i n f o

Article history:Received 16 November 2009Received in revised form 3 February 2010Accepted 9 February 2010Available online 17 February 2010

Keywords:MEMSConductive polymerPolydimethylsiloxaneTunable gratingResponse time

a b s t r a c t

A tunable grating using viscoelastic PDMS (polydimethylsiloxane) and conductive polymer PEDOT (poly3,4-ethylenedioxythiophene): PSS (polystyrenesulfonate) was designed and the fabrication process wasdeveloped. A sandwich structure (PEDOT: PSS/PDMS/Gold electrode) was utilized. By applying voltagebetween the PEDOT: PSS top film and the bottom interdigital gold electrode, the initial plane surface ofpolymers deforms into a sinusoidal grating. The PDMS material properties were tuned by modifying thenetwork structure to obtain good actuation of the polymer, including large relief depth and fast responsespeed. The grating profile on the surface was observed with the maximum relief depth of 200 nm. Theresponse time of the modulator can reach 250 �s.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

A spatial light modulator (SLM) is a device which is used tomodify the optical wavefront properties, such as intensity, phase,and polarization, etc. Many different configurations of SLMs havebeen investigated during the past decade and applied in projectiondisplays, printing and optical communications [1,2]. The gratinglight valve (GLV) [3], which was developed at Stanford University,is a spatial phase modulator made by an array of tiny movableribbons mounted on a silicon substrate. The digital MicromirrorDevice (DMD) [4], which was invented by Texas Instruments, usesa two-dimensional array of tilting micromirrors. The spatial Opti-cal Modulator (SOM) [5], invented by Samsung, uses an array ofmovable ribbons made of polarized lead zirconium titanate (PLZT).These successful modulators are primarily based on silicon tech-nology, and complicated processing steps are required for thefabrication.

Organic materials, such as polymers, are an alternate for SLMs.Polymers have many desirable features, such as lightweight, easyfabrication and low production cost [6]. Polymethylmethacrylate(PMMA) [7], P(VDF-TrFE) terpolymer [8], PDMS [9–14] based mod-ulators have been investigated and reported. In polymer basedSLMs, gratings are used to modify the light and they are commonly

∗ Corresponding author at: RI, Vestfold University College, Raveien 197, N-3184Borre, Norway. Tel.: +47 33037934; fax: +47 33031100.

E-mail address: [email protected] (G. Ouyang).

generated by a sandwich structure (flexible metal film/polymeractuation layer/fixed electrodes). In such a structure, the flexiblemetal film is utilized as both the top electrode and the reflector,and it deforms with the polymer actuation layer underneath. Thisquite often leads to a stability problem. A wrinkle-like surface andcracks were observed on the flexible metal film [9,12,15] due tothe large coefficient of thermal expansion (CTE) mismatch and thepoor adhesion between the polymer and the metal layer.

In this work, a tunable grating is designed and fabricated usingconductive polymer PEDOT: PSS as the flexible electrode instead ofmetals to improve the surface quality and the device stability. Withminor electrodes modification, this tunable grating can be utilizedas a spatial light modulator for high-resolution display. The choiceof PEDOT: PSS is made based on its high stability and compatibil-ity with other polymer materials such as PDMS. Moreover, PEDOT:PSS is a cheap material and gives opportunity to manufacture byscreen printing [16], inject printing, spin coating [17], roll-to-rollcoating [18], and chemical vapor deposition [19]. PDMS is chosenas the actuation material since it is soft, inert, and highly resis-tant towards oxidation and chemical attack [20]. Compared withother polymer based tunable gratings and SLMs, our device hasseveral advantages. First, thanks for the small CTE mismatch andthe good adhesion between PEDOT: PSS and PDMS, a smooth sur-face was obtained and the stability of the modulator was improved.Second, the fabrication method is simple and straight forward.All the processing is carried out at room temperature. No sophis-ticated equipments are required for the fabrication. At last, thedevice is light weight, low cost and compatible with laser display

0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.sna.2010.02.006


314 G. Ouyang et al. / Sensors and Actuators A 158 (2010) 313–319

Fig. 1. Schematic drawing of the designed tunable grating.

system. Using the techniques described in this paper, one canreadily manufacture a spatial light modulator for high-resolutiondisplay applications

2. Structure design and working principle

The structure of the modulator is shown in Fig. 1. A thin PDMSfilm was sandwiched between a PEDOT: PSS film and rigid bot-tom electrodes. Gold is chosen as the bottom electrodes for itshigh conductivity and reflectivity. It is compatible with the sub-sequent wire-bonding process. The shape of the bottom electrodesdetermines the eventual surface grating profile of PDMS. For simpli-fication, interdigital bottom electrodes were used. Different fromother polymer based modulators [7–15] in which incoming lightis reflected at the top metal layer, light is reflected at the bottomelectrodes underneath the polymer layers in our modulator.

The working principle of the modulator is illustrated in Fig. 2.If no voltage is applied between the top conductive polymer layer

Fig. 2. Operation principle of the tunable grating. (a) Without driving voltage, onlythe 0th order of light reflects from bottom gold electrodes. (b) With driving voltage,grating is formed on polymer surface and the incident light diffracts into high orders.

Fig. 3. Relationship between the diffraction efficiency of the qth order and the phasemodulation parameter m.

and the bottom electrodes, the polymer surface is flat. Since bothPEDOT: PSS and PDMS are transparent, incoming light goes throughthese layers with the same optical path and reflects at the underly-ing gold electrode. The reflected light is mainly in the 0th diffractionorder. If the driving voltage is applied, the polymer surface deformsinto a sinusoidal shape. When incoming light penetrates the trans-parent layers, which has varied thickness at different location, aspatially varying optical path of the light beam is obtained. Thepolymer layers act like a phase grating and diffract light into higherdiffraction orders.

When light passes through a grating area with dimension L × Wand electrodes spatial frequency f in x direction (dimensions areindicated in Fig. 1 and Fig. 2), the amplitude transmission functioncan be expressed by Eq. (1). The light intensity distribution at thefar-field output plane (x0, y0) is determined by Eq. (2) [21].

T(x, y) = exp[

jm

2sin(2�fx)

]rect

(x

L

)rect

(y

W

)(1)

I(x0, y0) =(

LW

�z

)2q=+∞∑q=−∞

Jq2(

m

2

)sin c2

×[

(x0 − qf�z)L�z

]sinc2

[y0W

�z

] (2)

where Jq is the Bessel function of first kind order q, � is the wave-length, z is the distance from the grating to the output plane, andm is the phase modulation parameter, which is the function of thepeak-to-peak relief depth d and the refractive index n of PDMS.The diffraction efficiency of the qth order can be obtained by thesquared Bessel function Jq2(m/2). In this case, light is reflected atthe bottom electrodes and the phase modulation parameter m iscalculated as [22]:

m = 2d(n − 1)2�/�

By controlling the relief depth d with applied voltage, the phasemodulation parameter m can be adjusted. Hence the amount oflight diffracted into the 0th and higher orders can be manipulated.The relationship between diffraction efficiency of the qth order andphase modulation parameter m is shown in Fig. 3. For light modula-tion efficiency, the grating depth is expected to be as big as possiblewith certain driving voltage. In this tunable grating, a relatively bigrelief depth is achieved by optimizing the structure dimension andthe viscoelastic properties of the actuation layer, PDMS. The opti-mized dimension of each layer is determined based on simulationsand experiments, which are: PDMS thickness = 3.5 �m, PEDOT: PSSthickness = 100 nm, linewidth of the gold electrodes = 30 �m andspacing between the electrodes = 3 �m.


G. Ouyang et al. / Sensors and Actuators A 158 (2010) 313–319 315

3. Experiments

3.1. PDMS synthesis

Commercial PDMS-based elastomer (e.g. Sylgard) has been usedas the electroactive material for optical modulator applications[9–12]. However, the experimental results from different researchgroups are inconsistent due to the unspecified chemical composi-tions. In this work, the PDMS is synthesized to satisfy the demandof deformability and fast response, by mixing vinyl-terminatedmonomer, cross-linker (PDMS that contains hydrides-sites in poly-mer chain), catalyst and certain amount of solvent. All the mixingand curing process were performed at room temperature. Young’smodulus and response time are characterized using home-builtmeasurement systems. The details of the measurement set up arereferred to [23].

3.2. Conductive polymers PEDOT: PSS characterization

CLEVIOSTM PEDOT: PSS is an aqueous dispersion of the con-ductive polymer PEDOT doped with PSS. The optical and electricalproperties of the conductive polymer were characterized beforeapplying it to the tunable grating. The transmission and sheet resis-tance were investigated for different thickness of PEDOT: PSS films.The transmission of PEDOT: PSS in UV light (385–405 nm) wasmeasured using a UV power meter and the sheet resistance is char-acterized by four-probe method.

3.3. Device fabrication

A tunable grating was fabricated on a glass substrate. First, a100 nm thick titanium–tungsten layer, which works as an adhe-sion layer between gold and glass, was sputtered on the glasssubstrate. Afterwards, a 150 nm thick gold layer was sputteredon a titanium–tungsten layer. These two layers were then pat-terned as interdigital electrodes by reactive ion etching (RIE) andwet etching, respectively. Afterwards, a 3.5 �m thick PDMS layerwas deposited by spin coating to obtain a smooth PDMS sur-face. Finally, a 100 nm thick PEDOT: PSS film was spin coated onPDMS.

Since PDMS is an originally hydrophobic material exhibitinga static contact angel of 110◦ [20], a surface treatment of PDMSto make it hydrophilic is necessary for the subsequent PEDOT:PSS deposition. Oxygen plasma treatment is commonly appliedto change the wettability of polymers [24,25]. However, it wasreported in Ref. [20] that plasma treated PDMS surface gradu-ally changed back to hydrophobic as hydrophobic groups slowlydiffuse towards the surface and replace polar groups existingon the plasma treated surface. This aging effect can be sloweddown using SF6 plasma followed by O2 plasma processes [26].Thus SF6, Ar, O2, plasmas were tried out with several sets ofprocessing parameters. The wettability was evaluated by mea-suring the contact angle between water drops and the modifiedsurface, and the morphology after the plasma treatment was eval-uated.

A prototype of the tunable grating has been fabricated as shownin Fig. 4. As a preliminary test, this modulator has interdigital bot-tom electrodes and it works for one pixel. Hence there are only twosignal voltages which are applied through the left and right bottomelectrodes, respectively. A SLM with many individually controlledelectrodes for high-resolution display (e.g. 1080 pixels for HDTV)can be readily fabricated using the same techniques except differ-ent bottom electrodes. This part of work is still going on and willbe reported in the future.

Fig. 4. A prototype of the tunable grating. The center of the chip is cov-ered by PDMS and PEDOT: PSS. The dimension of the PDMS layer are14 mm/10 mm/3.5 �m (Length/Width/Thickness) while the dimensions of PEDOT:PSS film are 14 mm/10 mm/0.1 �m (Length/Width/Thickness). The underlying inter-digital electrode is zoomed in and shown in the bottom-right corner.

3.4. Device characterization

The electric field induced grating profile on the PEDOT surfacewas characterized by an optical interferometer (Wyko NT9100),and the diffraction efficiency of the modulator was calculated basedupon the measured relief depth. The transmission of the modulatorwas obtained by comparing the incident and reflected light. A HeNelaser with 633 nm wavelength was illuminated at the modulator.The intensity of the incident and reflected light was measured usinga photo detector (Photomultiplier Tube).

Theoretical analysis of the static electro-mechanical property ofthe device was carried out using a finite elements software COMSOLMultiphysics 3.4, and the simulation results are compared with theexperiments.

The dynamic electro-mechanical properties of the tunable grat-ing were characterized in terms of the response time. It wasmeasured by a home-built measurement system. As shown in Fig. 5,a HeNe laser beam with 633 nm wavelength was directed onto thegratings. When no driving voltage was applied, only the 0th orderof light was reflected from the bottom gold electrode and thenblocked by a Schlieren stop. When driving voltage was applied,a grating appeared on the polymer surface, diffracting light intohigher orders. These higher orders were detected by a photo detec-tor (Photomultiplier Tube).

4. Results and discussions

4.1. Material properties

The Young’s modulus of the synthesized PDMS is measured as�s. The mea-

Fig. 5. Schematic drawing of the experimental setup for response time measure-ments.

600 kPa. The response time is characterized to be 10



Fig. 6. Transmission (*) and sheet resistance (�) with regard to PEDOT: PSS thick-ness.

surement results prove that the synthesized PDMS has both gooddeformability and fast response speed.

The transmission of PEODT: PSS in UV–visible wavelength isgiven in the product specification [27] that more than 80% trans-mission can be obtained for 200 nm PEDOT film. The transmissionof PEDOT: PSS in UV light (385–405 nm) was measured using a UVpower meter to verify the datasheet. The result is shown in Fig. 6.

With the film thickness below 250 nm, the transmission was inthe range of 80–95%. This measurement results fit the data givenin Ref. [27]. The sheet resistance of the PEDOT: PSS films wereobtained using four-probe method as shown in Fig. 6. As expected,thinner film has better transmission but bigger resistance. In orderto obtain the efficiency of the device, it is important to find theoptimal balance between the transmission and resistance of thetransparent conductive polymer. In this work, 100 nm PEDOT: PSSfilms were utilized since they have relatively high transmission andsmall resistance.

4.2. Surface morphology

In SLMs, the surface morphology has significant effects on thelight modulation. Usually, a smooth surface is required so that the

light scattered into random direction can be kept at the minimum.After PDMS spin coating, a smooth surface was obtained with sur-face roughness of approximately 7 nm, which was measured by anoptical interferometer. However, the plasma processing, which isnecessary before PEDOT: PSS deposition, may change the surfacemorphology of PDMS. Varied plasma sources were tried to changethe wettability of PDMS without surface deterioration. The resultsare shown in Fig. 7. SF6 plasma created column-like structures dueto the selective etching. These structures have height around sev-eral hundred nanometers and this phenomenon was also reportedin Ref. [28]. Ar plasma gave smoother surface but slightly increasedthe surface roughness due to the physical impinging. A wrinkle-likesurface was observed after 5 min O2 plasma treatment, which maybe due to the elevated temperature caused by the long time plasmatreatment. The O2 plasma treatment time was then optimized as60 s which dramatically increased the surface wettability of PDMS(contact angle drops from 110◦ to 3◦) without noticeable surfacedeterioration. After PEDOT: PSS deposition, the surface roughnessof the device was further decreased to 3 nm. Meanwhile, the exper-iments indicated that if the PEDOT: PSS layer was deposited rightafter the plasma treatment on PDMS surface, it bonded to PDMStightly. The adhesion between PDMS and PEDOT: PSS remainedgood after two months. The reason for the eliminated aging effectcan be explained as follows: once the conductive polymer is bondedto the polar groups on PDMS surface, it stops the diffusion of polargroups towards bulk PDMS. In this work, PEDOT: PSS solution wasspin coated on PDMS surface right after the oxygen plasma pro-cessing, and then baked in an oven at 60 ◦C for 1 h to evaporatethe solvent in PEDOT: PSS. Thus the aging effect of PDMS surfacewettability was avoided.

4.3. Static characterization of the device

When driving voltage was applied between the top conductivepolymer layer and the bottom electrodes, electric field was gener-ated in the PDMS layer and led to gratings on PDMS surface. Thegrating profile on PEDOT: PSS surface under 250 V driving volt-age was measured by an interferometer and shown in Fig. 8. Thevalue of the peak-to-peak deformation was approximately 200 nm.

Fig. 7. Surface morphology of plasma treated PDMS film, measured by an interferometer. The surface structures are exaggerated. (a) SF6 plasma treated PDMS. The surfaceroughness is Ra = 250 nm. (b) Ar plasma treated PDMS. The surface roughness is Ra = 15 nm. (c) 5 min O2 plasma treated PDMS. The surface roughness is Ra = 35 nm. (d) 60 sO2 plasma treated PDMS. The surface roughness is Ra = 7 nm.



Fig. 8. Grating profile on PEDOT: PSS surface under 250 V driving voltage measuredby an interferometer. The relief depth is exaggerated.

The relief depth shows good uniformity across the whole device.From Eq. (2), the diffraction efficiency of the first order, definedas the amount of light diffracted in the first diffraction order, wascalculated as 27%.

The optical transmission of the modulator was measured as 78%at 633 nm wavelength. It is not as good as the SLMs with a contin-uously reflective surface, but it is considered acceptable. The lightloss is mainly due to the absorption in the polymer layers, the reflec-tion in the media interface, and the light loss in the small spacingbetween bottom electrodes.

According to Refs. [30,31], the refractive index of PEDOT: PSSand PDMS is 1.51 and 1.47, respectively. This refractive index vari-ation can influence the characteristic of the diffracted light. Basedon the Snell’s law, the angular change at the PEDOT/PDMS inter-face is 0.01(radians). Hence the diffraction angle is mainly definedby the grating period and the relief depth of PDMS. Small amount oflight will also reflected at the interface due to the refractive indexchange. From Eq. (3), where R is the reflectance, n1 and n2 are therefractive index of PEDOT: PSS and PDMS, respectively, the reflectedlight is calculated as 0.01% of the total incident light. In this case,the angular change and the reflection, which are originated fromthe refractive index variation, are considered negligible.

R = (n1 − n2)2/(n1 + n2)2 (3)

The theoretical analysis of the electric field induced deforma-tion was described in detail in Ref. [29], where the relief depth ofthe gratings was given as a function of the electric potential, thespatial frequency of bottom electrodes, the viscoelastic propertyand the thickness of the actuation layer. A 3D model was con-structed by coupling electrical domain and mechanical domain inCOMSOL Multiphysics 3.4. The electric field and the relief depthwere calculated by solving the Maxwell’s equations and the Hook’slaw respectively [32]. The simulation results are shown as Fig. 9,in which the grating profile is clear. To compare the experimentalresults with the simulation results, the relief depth of the PDMS

Fig. 9. Simulation results of surface grating from COMSOL. The black outline indi-cates the structure before deformation. The gray grade represents the magnitude ofthe electric potential.

Fig. 10. Experimental results (�) and simulation results (�) of PDMS relief depthwith regard to the driving voltage.

surface with regard to the driving voltage was plotted. As shownin Fig. 10, the experiment results show fair agreement with thesimulation results.

4.4. Dynamic characterization of the device

The results of the measured response time of the tunable gratingare shown in Fig. 11. In this measurement, a 200 V, 1 kHz rectan-gular wave was applied. The rise/fall time, which is defined as thetime required for the PDMS deformation to rise/fall from 10% (or90%) to 90% (or 10%) of its peak value, was measured as 240 �s and250 �s respectively.

The response speed of the device are determined both by thetime needed for polymer deformation, which is determined by itsviscoelastic property, and by the time constant of the RC circuitwhere the resistance is mainly originated from the conductive poly-mer and capacitance is from the PDMS layer. The response speed ofthe polymer based modulator is usually limited by the viscoelasticproperty of the polymer as the time constant of the RC circuit isrelatively small. In this work, the PDMS was synthesized to have ashort response time, which is measured as 10 �s, while the retar-dation time of the RC circuit, which is calculated as � = RC, is 230 �s.In this calculation, the relative permittivity of PDMS is 2.5 [33]. Theresistance of the wire bonds, the gold electrodes, the connecting

Fig. 11. Dynamic characterization of the device under 200 V, 1 kHz driving voltage.



cables and the impedance of the generator were neglected. Com-paring the RC retardation time with the measured rise time (240 �s)and fall time (250 �s), the response time of our modulator is mainlydetermined by the RC circuit. Therefore, to improve the responsespeed of the device, the best way is tuning PEDOT: PSS with highconductivity. It has been reported that the conductivity of PEDOT:PSS films can be dramatically increased by adding additives suchas glycerol [18,34,35]. In Ref. [35], the PEDOT: PSS was modified tohave conductivity approximately 750 S/cm. If this kind of PEDOT:PSS is used in our tunable grating, the time constant in electricaldomain will dramatically decrease to 148 ns and the response timecan reach 10 �s, which is limited by the intrinsic response time ofthe PDMS film.

Driving voltage of 200 V and 1 kHz rectangular wave was appliedto the tunable grating for 8 h. The test showed that the modulatorworked well after millions times of actuation. No breakdown orreduction in the relief depth was observed during this test. Eighthours continuously laser exposure did not bring any degradation ofthe modulator. The modulator works well after two months storedat room temperature.

5. Summary and conclusions

In conclusion, a tunable grating using conductive polymerPEDOT: PSS and viscoelastic PDMS was designed, fabricated andcharacterized. In contrast to other polymer/metal based modula-tors, our modulator uses PEDOT: PSS as the top electrode insteadof metals in order to improve the surface quality and the stability.A smooth surface of PEDOT: PSS was obtained with roughness ofapproximately 3 nm, and it showed good adhesion to the underly-ing PDMS layer. To ensure a fast response time of the modulator,PDMS with 10 �s response time was successfully synthesized. Thetunable grating was characterized with good static and dynamicperformance. Under 250 V 1 kHz driving voltage, a maximum200 nm peak-to-peak relief depth on PDMS surface was obtained,giving 27% first order diffraction efficiency. The rise and the falltime of the modulator are 240 �s and 250 �s, respectively. Simu-lation model for the static electro-mechanical performance of thetunable grating was built. The simulation in COMSOL shows fairagreement with the experimental results with respect to the reliefdepth.

The technique reported in this paper can be easily applied tomake a SLM for high-resolution display. Since the device shows nodegradation after long time laser exposure, it can be adapted forline-scan laser display with low production cost.

Acknowledgements

This research is supported by the Norwegian Research Council(NFR). The authors wish to express their gratitude to PoLight ASfor providing their facilities and Dr. V. Kartashov for the fruitfuldiscussions.

References

[1] O. Solgaard, F.S.A. Sandejas, M. bloom, A deformable grating optical modulator,Optics Letters 17 (1992) 688–690.

[2] T. Wada, A. Morimoto, Electro-optic devices with polarization reversal, Journalof the Korean Physical Society 46 (2005) 29–32.

[3] D.M. Bloon, The grating light valve: revolutionizing display technology, Pro-ceedings of SPIE 3013 (1997) 165–171.

[4] L.J. Hornbeck, Deformable-mirror spatial light modulator, Proceedings of SPIE1150 (1989) 86–102.

[5] S.K. Yun, J.H. Song, I.J. Yao, Spatial optical modulator (SOM): high-densitydiffractive laser projection display, Proceedings of SPIE 6487 (2007) 648710.

[6] R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, S. Chiba, High-field deforma-tion of elastomeric dielectrics for actuators, Materials Science and EngineeringC 11 (2000) 89–100.

[7] D. Cristea, P. Obreja, M. Kusko, E. Manea, R. Rebigan, Polymer micromachiningfor micro- and nanophotonics, Materials Science and Engineering C 26 (2006)1049–1055.

[8] D.Y. Jeong, E. Hong, Spatial light modulator usingpoly(vinylidenefluoride–trifluoroethylene–chlorofluoroethylene) Terpolymerwith a high electrostrictive strain, Journal of the Korean Physical Society 48(2005) 229–232.

[9] G. Ouyang, K. Wang, M.N. Akram, X. Chen, Fabrication and characterizationof polymer based spatial light modulators, Proceedings of SPIE 7426 (2009)742604.

[10] S. Sakarya, G. Vdovin, P.M. Sarro, Spatial light modulators based on microma-chined reflective membranes on viscoelastic layers, Sensors and Actuators A:Physical 108 (2003) 271–275.

[11] H. Kuck, W. Doleschal, A. Gehner, W. Grundke, R. Melcher, J. Paufler, R. Selt-mann, G. Zimmer, Deformable micromirror devices and phase-modulatinghigh-resolution light valves, Sensors and Actuators A: Physical 54 (1996)536–541.

[12] S. Sakarya, G. Vdovin, P.M. Sarro, Technology of reflective membranes for spatiallight modulators, Sensors and Actuators A: Physical 97 (2002) 468–472.

[13] M. Aschwanden, A. Stemmer, Polymeric, electrically tunable diffraction gratingbased on artificial muscles, Optics Letters 31 (2006) 2610–2612.

[14] M. Aschwanden, M. Beck, A. Stemmer, Diffractive transmission grating tunedby dielectric elastomer actuator, IEEE Photonics Technology Letters 19 (2007),NO. 14.

[15] K. Wang, G. Ouyang, L. Henriksen, M.N. Akram, X. Chen, Surface and elecrtrome-chanical properties of polydimethylsiloxane films prepared by modifiedmolding technique, Proceedings of SPIE 7381 (2009) 73810C.

[16] K. Ren, S. Liu, M. Lin, Y. Wang, Q.M. Zhang, A compact electroactive polymeractuator suitable for refreshable Braille display, Sensors and Actuators A: Phys-ical 143 (2007) 335–342.

[17] C. Yiming, W. Leeyih, S. Weifang, Polymer solar cells with poly (3,4-ethylenedioxythiophene) as transparent anode, Organic Electronics 9 (2008)968–973.

[18] D. Aurore, B. Anne, B. Julien, N. Charles, PEDOT: PSS coating on specialty papers:process optimization and effects of surface properties on electrical perfor-mances, Progress in Organic Coatings 63 (2008) 87–91.

[19] L.P. John, L.L. Jodie, Z.S. Nicole, I.G. sung, H.T. Paula, G.K. Karen, Electrochemicalinvestigation of PEDOT films deposited via CVD for electrochromic applications,Synthetic Metals 157 (2007) 894–898.

[20] M.E. Vlachopoulou, P.S. Petrou, S.E. Kakabakos, A. Tserepi, E. Gogolides, Hight-aspect-ration plasma-induced nanotextured poly(dimethylsiloxane) surfacewith enhanced protein adsorption capacity, Journal of Vacuum Science andTechnology B 26 (2008) 2543–2548.

[21] J.W. Goodman, Introduction to Fourier Optics, McGraw-Hill, New York, 1968.[22] P. Roth, New optical configuration for large-screen HDTV projector, Displays

16 (1995) 9–12.[23] L.Henriksen, V.Kartashov, European Patent Application EP1960819 A1 (2008).[24] M.T. Khorasani, H. Mirzadeh, S. Irani, Plasma surface modification of poly(l-

lactic acid) and poly (lactic-co-glycolic acid) films for improvement of nervecells adhesion, Radiation Physics and Chemistry 77 (2008) 280–287.

[25] S.K. Øiseth, A. Krozer, B. Kasemo, J. Lausmaa, Surface modification of spin coatedhigh-density polyethtylene films by argon and oxygen glow discharge plasmatreatments, Applied Surface Science 202 (2002) 92–103.

[26] M.E. Vlachopoulou, P.S. Petrou, S.E. Kakabakos, A. Tserepi, K. Beltsios, E.Gogolides, Effect of surface nanostructuring of PDMS on wetting properties,hydrophobic recovery and protein adsorption, Microelectronic Engineering 86(2008) 1321–1324.

[27] H.C.Starck, User’s manual for conductive polymer CLEVIOSTM P, Number PD-6002, Issue 2-05, 09 (2006).

[28] M.E. Vlachopoulou, A. Tserepi, K. Beltsios, G. Boulousis, E. Gogolides, Nanos-tructuring of PDMS surfaces: dependence on casting solvents, MicroelectronicEngineering 84 (2007) 1476–1479.

[29] R. Tepe, Theoretical analysis of an electrically addressed viscoelastic spatiallight modulator, Optical Society of America 4 (1987) 1273–1281.

[30] W. Li, R.A. Jones, S.C. Allen, J.C. Heikenfeld, A.J. Steckl, Maximizing Alq3OLED internal and external efficiencies: charge balanced device structure andcolor conversion outcoupling lenses, Journal of Display Technology 2 (2006)143–152.

[31] D.A. Cahng-Yen, R.K. Eich, B.K. Gale, A Monolithic PDMS waveguide systemfabricated using soft-lithography techniques, Journal of Lightwave Technology23 (2005) 2088–2093.

[32] COMSOL AB, COMSOL Multiphysics Model Library: 3D Cantilever Beam, (2007).[33] J.E. Mark, Polymer Data Handbook, Second ed., Oxford University, 2009.[34] S.L. Lai, M.Y. Chan, M.K. Fung, C.S. Lee, S.T. Lee, Concentration effect of glyc-

erol on the conductivity of PEDOT film and the device performance, MaterialsScience and Engineering B 104 (2003) 26–30.

[35] Y.H. Ha, N. Nikolov, S.K. Pollack, J. Mastrangelo, B.D. Martin, R. Shashidhar,Toward a transparent, highly conductive poly(3,4-ethylenedioxythiophene),Advanced Functional Materials 14 (2004) 615–622.

Biographies

G. Ouyang has a bachelor degree of Electronic Engineering from Harbin Institute ofTechnology, China. He graduated in Micro- and Nano-technology at the Vestfold



University College in 2008. He is currently a Ph.D student in Micro- and Nano-technology at the Vestfold University College. His scientific activities are focus onMOMES devices and polymer materials.

K. Wang (M) joined the Institute of Microsystem Technology in 2007 as an asso-ciate professor. He received his M.Sc. degree in Inorganic Chemistry in 1992from Northwest University, China and PhD in materials physics in 1995 fromthe Institute of Physics, Chinese Academy of Sciences (CAS). His research inter-ests are mainly focused on nano/thin film materials for magnetic, biological andenvironmental applications, MEMS fabrication and micro-bolometer packagingetc.

L. Henriksen graduated with a Ph.D. in Chemical Engineering from the Nor-wegian University of Science and Technology in 2002. He has professionalexperience from R&D within polymer science and has 10 years of experi-ence from development and industrialization of advanced polymer materialsand their application in optical components. He is currently the CTO ofPoLight AS, a Norwegian company developing miniaturized tunable lenses formicrocameras.

M.N. Akram was born in Pakistan on 25 December, 1971. He received B.Sc. in Elec-trical Engineering from University of Engineering and Technology, Lahore, Pakistanin 1994. In May 2000, he did his M.Sc in Electrical Engineering from King FahdUniversity of Petroleum and Minerals, Dhahran, Saudi Arabia. He did his Ph.D atThe Royal Institute of Technology, Department of Microelectronics and InformationTechnology, Photonics and Microwave Laboratory, Stockholm, Sweden in 2005. Cur-rently he is employed as associate professor at University College Vestfold Instituteof Microsystems technology, Horten, Norway. His research interests are integrated

optics, semiconductor laser modeling and growth, coherent and incoherent imagingoptics.

X.Y. Chen, born 1962, received his Bachelor degree in Semiconductor Physics fromNorthwest University, Xian, China in 1983, his M.Sc in Electrical Engineering fromXian Jiaotong University, Xian, China in 1988, and his Ph.D. in semiconductor mate-rials and devices from Eindhoven University of Technology, the Netherlands in1997. After finishing his Ph.D. degree, he joined Microelectronics Lab at School ofEngineering Science, Simon Fraser University, Canada, where he worked on sili-con microelectronic device and III-V DFB laser diodes. From 1997 he worked withPhysics Department at University of Tromso, Norway, as associate professor, andcontinued as professor until 2002. During that time, his research topics were micro-electronic devices and opto-electronics. In 2002, he took his one year sabbaticalleave in NSF Center for Advanced Manufacturing and Packaging of Microwave,Optical and Digital Electronics, University of Colorado, Boulder, USA, where heworked on MEMS designing, processing and packaging. He joined Microsystems andNano-technology, SINTEF ICT, Norway at end of 2002 where he started as a seniorresearch scientist. In 2004 Xuyuan Chen was appointed as Professor of Microsys-tems Technology at Vestfold University College (VUC), Norway. In part time, hejointed SAH MEMS Research Center at Xiamen University China as a distinguishprofessor in 2005, where he served as Executive Associate Director of SAH MEMSResearch Center from 2005 to 2008. He continued the research topic on Micro- andNano-technology, MEMS design and fabrication, such as, RF MEMS and THz MEMSdevices, MEMS micropower, MOEMS and bioMEMS. He is author or co-author ofabout 100 scientific publications in international journals and conferences, invitedtalks and overview papers in the area of semiconductor technology and microsys-tems.


55

Article V

G. Ouyang, Z. Tong, M. N. Akram, K. Wang, V. Kartashov, X. Yan, and X. Chen,

"Speckle reduction using a motionless diffractive optical element," Optics Letters,

Vol. 35, Issue. 17, pp. 2852-2854, 2010.

Speckle reduction using a motionlessdiffractive optical element

Guangmin Ouyang,1 Zhaomin Tong,1 M. Nadeem Akram,1 Kaiying Wang,1

Vladimir Kartashov,2 Xin Yan,3 and Xuyuan Chen1,*1Faculty of Science and Engineering, Vestfold University College, N-3103 Tonsberg, Norway

2poLight AS, N-3192 Horten, Norway3Pen-Tung Sah Micro-Nano Technology Research Center, Xiamen University, China

*Corresponding author: [email protected]

Received May 21, 2010; revised July 13, 2010; accepted July 31, 2010;posted August 4, 2010 (Doc. ID 128823); published August 19, 2010

Speckle reduction by moving diffuser has been previously studied in display systems with coherent light sources,such as lasers. In this Letter, we propose a motionless diffractive optical element (DOE) for speckle reduction. TheDOE was designed based on finite-element method simulations, fabricated using micromachining technology, andcharacterized for despeckle efficiency. Experiments using a DOE with two gratings have indicated that the specklewas suppressed to 50%, which shows fair agreement with theoretical analysis. With somemodification of this DOE,the speckle noise can be reduced to 10% according to the theory. © 2010 Optical Society of AmericaOCIS codes: 030.6140, 110.6150, 130.3990.

Speckle is induced by the light scattering and interfer-ence of coherent radiation from a screen with roughnessof optical wavelength. The fundamental theory of speckleformation, its statistical properties, and despeckle meth-ods are described thoroughly in [1]. Considerable effortshave been made to minimize the speckle in display sys-tems with coherent sources. The efforts use, to list a few,a moving diffuser placed at the image plane [2], dynamicHadamard phase patterns inside each detector resolutionpixel at the intermediate image plane [3], partially coher-ent beams [4], and a stationary phase plate based on theBarker or another binary phase code at the intermediateimage plane [5]. When a mechanically vibrating elementis used, such as those in [2,3], a motor is needed to pro-vide fast vibration or rotation. This can be difficult forpractical implementation and decreases the system’s re-liability. When a laser array is used, as in [4], the illumi-nation optics is complicated and the cost increases. Themethods based on binary phase code [5] are only applic-able in line-scan projectors.A micro-optoelectromechanical-system-based motion-

less despeckle modulator, which consists of a dynamicdiffractive optical element (DOE) and a static diffuser,is proposed in this work. It is applicable in a laser imagingsystem with a two-dimensional display chip, such as adigital micromirror device. An example of its implemen-tation in a laser full frame projector is shown in Fig. 1.Compared with other techniques, the reliability is im-proved and the noise is minimized, since no mechanicalvibration is needed for the modulator. Moreover, as fab-ricated with micromachining technologies, this modula-tor is compact; therefore, it is flexible for systemimplementation. Last but not least, the dynamic gratingsare made of deformable polymer material, such as poly-dimethylsiloxane (PDMS), which is low cost, lightweight,reliable, and easily processed. According to the experi-ments, it can stand 80 °C, 80% humidity, and long-timelaser exposure without any degradation [6], providingthe capability to work in harsh environments.The speckle contrast ratio can be suppressed to C ¼

1=M1=2 by sequentially creating a number of (M) uncor-

related speckle patterns and adding them together duringthe integration time of the detector, which is around30 ms for human eyes [1]. In this work, different specklepatterns were created using a dynamic DOE togetherwith a static diffuser. To explain the working principle,a dynamic DOE including three electrically controlledgratings is shown in Fig. 2. Each grating can be switched“on” and “off” independently by applying voltage be-tween the indium–tin–oxide (ITO) electrode and the op-posite gold interdigital electrodes. In the “off” state, thepolymer (PDMS) surface is flat and the incident light isreflected at the angle of the total internal reflection (TIR),while in the “on” state a phase grating in the form of per-iodical relief is generated on the PDMS surface, splittinglight into several diffraction orders. When one or severalgratings are actuated, the incoming light is reflected se-quentially from the areas of the polymer surface over thegold electrodes and is split into a multitude of diffractionorders. There are totally M ¼ 2m diffraction patterns fora DOE with m gratings, corresponding to different com-binations of actuated gratings. Each diffraction pattern isscattered by the diffuser and then homogenized by thelight pipe. Speckle fields created on the screen by thesediffraction patterns are fully independent if there are nooverlapping areas between different diffraction patternson the diffuser. In this case, the speckle contrast can bereduced to

Fig. 1. General structure of the projection display with thelaser light source and the motionless despeckle modulatorfor speckle reduction.

2852 OPTICS LETTERS / Vol. 35, No. 17 / September 1, 2010

0146-9592/10/172852-03$15.00/0 © 2010 Optical Society of America

C ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiM þ K � 1

MK

r; ð1Þ

where K is the number of resolution elements of theprojection lens lying within one resolution element ofthe eye [1]. However, the despeckle efficiency will bereduced if there is overlapping of spots of different dif-fraction patterns [7]. An efficient way to minimize theoverlapping is to use gratings with varied orientation an-gle and spatial frequency, which was done in this work bydesigning interdigital gold electrodes with varied pat-terns, as shown in Fig. 2.The dynamic DOE has been fabricated with microma-

chining technology. First, gold layers were patterned inthe form of interdigital signal electrodes on a glass sub-strate. Second, a flexible polymer, PDMS, was depositedon the prism that had a thin ITO layer on the surface, bycompression molding [8]. Last, the prism was bonded tothe glass substrate with spacers in between to set the airgap. Performance of the dynamic DOE is greatly depen-dent on the properties of the PDMS, which are the re-sponse speed and the elastic modulus. PDMS with 10 μsresponse speed and 550 kPa elastic modulus was synthe-sized by modifying its intrinsic network structure as de-scribed in [9]. The following design parameters of theDOE for practical implementation were obtained fromfinite element method simulations in COMSOL to ensurethe diffraction efficiency: PDMS thickness, 30 μm; airgap, 5 μm; period of gold electrodes in a range of 50 μmto 100 μm.A DOE with two gratings was fabricated and tested for

speckle reduction in the measurement setup, as shown inFig. 3. A laser beam illuminated the DOE with 45° inci-dent angle, which was slightly larger than TIR anglesfor the polymer/air and the glass/air interfaces. By apply-ing 50 V square-wave ac voltage with frequency of 100 Hzto the signal electrodes (Au interdigital electrodes) and

200 V dc voltage to the bias electrodes (ITO), four dif-fraction patterns appeared sequentially on the firstdiffuser, corresponding to M ¼ 22 states (Fig. 4). The dif-fraction spots were scattered by the first diffuser, whichhas a �10° divergent angle, and then passed through a175-mm-long hexagonal light pipe, which was used tohomogenize the intensity of the incoming light. In the ex-periments, the light pipe was placed adjacent to the firstdiffuser to avoid light loss. The second diffuser was illu-minated by the homogenized light after the light pipe andimaged by the CCD camera approximately 600 mm away.The CCD camera was equipped with an imaging lens offocal length f ¼ 75 mm and f -number ¼ 16 and operatedin the linear regime. The integration time of the camerawas set to 1=32 s. The speckle contrast was calculated asC ¼ σI=Imean, where σI was the standard deviation andImean was the mean value of light intensity [1] all overthe CCD sensor area (640 × 480 pixels). The speckle fieldregistered by the CCD camera when all gratings wereswitched off is shown in Fig. 5(a), and the speckle con-trast was 0.73, while the speckle field when four differentdiffraction patterns (Fig. 4) appeared during the expo-sure time (1=32 s) is shown in Fig. 5(b) with C ¼ 0:37.Therefore, the speckle contrast reduction wasð0:37=0:73Þ × 100% ≈ 51%. The original speckle contrastof 0.73 was close to the expected value, 1=21=2, as itshould be when the narrowband laser is scattered by adepolarizing screen [1]. According to simulations of

Fig. 2. Schematic drawing of the dynamic DOE. Picture atbottom shows the orientation of electrodes in each grating.The dimensions are not to scale.

Fig. 3. Experiment setup for speckle measurement.

Fig. 4. Comparison of multiple diffraction patterns from ex-periments (up) and ZEMAX simulations (down). The largernumber of diffraction orders in the experiment compared toZEMAX is because the gratings inside the dynamic DOE werenot perfectly sinusoidal.

Fig. 5. Images and sampling of intensity fluctuation of (a) theoriginal speckle and (b) the reduced speckle. The CCD level 256corresponds to the brightest spot in the speckle pattern, while 0corresponds to the darkest spot in the speckle pattern.

September 1, 2010 / Vol. 35, No. 17 / OPTICS LETTERS 2853

the designed DOE, there was no overlapping among anyof the four diffraction patterns, while the experiments de-monstrated, too, that the overlapping was negligible. Inthis case, Eq. (1) can be used. We assumed that K inEq. (1) is infinitely larger, because C was calculated overthe entire area of the CCD sensor with 640 × 480 pixels;one pixel had the approximate size of the projection lensresolution element. Then the speckle contrast C ¼1=M1=2, which gives C ¼ 0:5, or 50% for M ¼ 22. Thus,the experimental result shows fair agreement with thetheory.The proposed dynamic DOE provides an idea of using

a motionless spatial light modulator to reduce thespeckle noise, and it has been proved by preliminarymeasurements with 50% speckle suppression. Higherspeckle reduction can be achieved by using a DOE withmore gratings. Calculations based on [7] and simulationsin ZEMAX have shown that the speckle contrast can bereduced to 25% for the dynamic DOE with four gratings[10]. However, one cannot arbitrarily increase the num-ber of gratings to get higher speckle suppression, sincethe overlapping of different diffraction patterns will in-crease, as well. To further reduce the speckle, one canuse a laser beam with a smaller diameter or introducea lens between the dynamic DOE and the static diffuser.This will provide smaller diffraction spots and, therefore,less overlapping. Further speckle suppression can beachieved by using gratings with dynamic periods. In thatcase, the grating period is varied during the integrationtime. The speckle noise can be suppressed down to10% for the dynamic DOE with four gratings, with each

grating having three grating periods during the integra-tion time [10].

The authors are grateful to Dr. L. Henriksen for fruitfuldiscussions. This research work is funded by the Re-search Council of Norway under grant BIA-286 projectnumber 182667.

References

1. J. W. Goodman, Speckle Phenomena in Optics: Theory andApplications (Roberts, 2007).

2. S. Lowenthal and D. Joyeux, J. Opt. Soc. Am. 61, 847 (1971).3. J. I. Trisnadi, Opt. Lett. 29, 11 (2004).4. S. An, A. Lapchuk, V. Yurlov, J. Song, H. Park, J. Jang, W.

Shin, S. Karagpoltsev, and S. Yun, Opt. Express 17, 92(2009).

5. M. N. Akram, V. Kartashov, and Z. Tong, Opt. Lett. 35,444 (2010).

6. V. Kartashov, L. Henriksen, J. H. Ulvensøen, B. Svardal, T.Svortdal, R. Berglind, and G. Hedin, J. Soc. Inf. Disp. 17,581 (2009).

7. V. Kartashov and M. N. Akram, “Speckle suppression inprojection displays by using a motionless changing diffu-ser,” to be presented at Speckle 2010, Florianopolis, Brazil,September 13–15, 2010.

8. G. Ouyang, K. Wang, M. N. Akram, and X. Chen, Proc. SPIE7426, 742604 (2009).

9. G. Ouyang, K. Wang, L. Henriksen, M. N. Akram, and X. Y.Chen, Sens. Actuators A 158, 313 (2010).

10. Z. M. Tong, G. M. Ouyang, W. H. Gao, M. N. Akram, V.Kartashov, K. Y. Wang, and X. Y. Chen, “Simulation of laserspeckle reduction by using an array of diffraction gratings,”to be presented at Speckle 2010, Florianopolis, Brazil,September 13–15, 2010.

2854 OPTICS LETTERS / Vol. 35, No. 17 / September 1, 2010


56

Article VI

G. Ouyang, M. N. Akram, K. Wang, Z. Tong and X. Y. Chen, “Laser speckle

reduction based on angular diversity induced by piezoelectric benders,” Submitted to

IEEE photonics letters, 2012.

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

1

Abstract—Speckle reduction techniques have been investigated

in display systems with coherent light sources. We propose a simple but efficient method to reduce speckle via two fast vibrating piezoelectric benders. The concept is modulating laser beams to have angle diversity and reducing speckle by temporal averaging in the integration time of the detector. Experiments demonstrate that the speckle contrast is suppressed down to 0.06. Both free space and imaging geometry are considered. In order to fairly evaluate the speckle reduction, the optical configuration as well as the camera settings, which largely affect the speckle contrast, is discussed.

Index Terms— Angular diversity, Laser imaging, Projection display, Speckle.

I. INTRODUCTION n comparison with conventional lamps, the use of lasers in projection displays gives unrivaled imaging quality with

extensive color coverage. However, one major technique obstacle, which limits its applications, is the speckle phenomenon. Speckle is induced by the light scattering and interference of coherent radiation from a screen with roughness in the scale of optical wavelength. The presence of speckle masks the image information and therefore its reduction is highly desirable in projection displays. The fundamental theory of speckle formation, its statistical properties and de-speckle methods are described thoroughly in [1]. Considerable efforts have been made to minimize the speckle noise. A moving diffuser is placed at the image plane [2] and dynamic Hadamard phase patterns are employed at the intermediate image plane [3] for speckle reduction. However, a motor is needed in these cases, to provide fast vibration or rotation. This can be difficult for practical implementation and decreases the system reliability. Partially coherent laser beams are applied in laser projections [4]. The illumination optics is complicated. A stationary phase plate based on Barker code located at the intermediate image plane [5] is only applicable

Manuscript received February 07, 2001. This research work is funded by

the Research Council of Norway under grant BIA-286, project number 182667.

Guangmin Ouyang, Muhammad Nadeem Akram, Kaiying Wang, Zhaomin Tong and Xuyuan Chen are all from Micro and Nano Systems Technology, Vestfold University College, N3103, Tonsberg, Norway. (e-mail: [email protected], [email protected], Kaiying.Wang @hive.no, [email protected], [email protected] )

in line-scan projectors. A polymer dynamic diffractive optical element [6] is not suitable for high power lasers.

Two piezoelectric benders have been employed in this work for speckle reduction. The incoming laser beam is steered by the two piezoelectric benders. The moving beam is collected by a condenser lens, and falls on a transmitting diffuser at different angles. Speckle reduction is achieved by angular diversity of laser beams. Experiments are performed both for free space propagation geometry and imaging geometry. The speckle reduction technique proposed in this work is reliable, low cost and efficient. The integration in laser TVs or projectors is simple and straight forward. It is a good candidate in the case when high laser power is requested, since the piezoelectric ceramic is reliable under high temperature.

II. EXPERIMENTAL SETUP The experimental setup to implement speckle reduction in the free space propagation is shown in Fig. 1. In free space geometry, no imaging lens is mounted on the charged coupled-device (CCD). He-Ne laser is employed with power of 4 mW, wavelength of 633 nm, and beam spot size of 0.5 mm. The laser beam has a fixed polarization and passes through two polarizers. Polarizer 1 is rotated to adjust the beam intensity, and polarizer 2 maintains a fixed polarization of the transmitted beam. The piezoelectric benders are based on lead zirconate titanate material and are purchased from Noliac AS. The dimension is 50 mm×7.8 mm×0.7 mm (L×W×H). The surface of piezoelectric benders is deposited with thin gold film, which works as reflective mirrors. Two piezoelectric benders are placed perpendicularly to each other and both of them are fixed at one end. The maximum deflection range of the benders is characterized as ±1 mm at ±100 V 167 Hz square wave voltage. Laser beam is reflected at two benders and it is steered in both X and Y directions, creating a 2D scanning. The moving laser beam is collected by the condenser lens with focal length 25 mm, and falls on a transmitting diffuser with different angles. The diffuser is very rough on the optical wavelength scale to ensure a sufficient phase modification. After passing through the diffuser, the beam propagates in free space and falls on the CCD sensor. The CCD has a resolution of 640 × 480 pixels, and each pixel has a dimension of 5.6 μm × 5.6 μm. In order to demonstrate the application of the piezoelectric benders for speckle reduction in full frame laser display, an imaging geometry is constructed as shown in Fig. 2. Unlike

Laser Speckle Reduction Based on Angular Diversity Induced by Piezoelectric Benders

Guangmin Ouyang, Muhammad Nadeem Akram, Kaiying Wang, Zhaomin Tong and Xuyuan Chen

I



mailto:Kaiying.Wang%[email protected]

mailto:Kaiying.Wang%[email protected]




2

free space geometry, after passing through diffuser 1, the scattered light is collected and homogenized by a 175 mm long hexagonal light pipe. Diffuser 2 is placed at the output of the light pipe, to further homogenize the beam. The object, which is a transparent plastic sheet printed with a letter “S”, is placed right after diffuser 2. The CCD camera is equipped with an imaging lens of focal length 50 mm. Lens extenders are employed to adjust the distance between imaging lens and the CCD sensor, which is f1 in Fig. 2.

Fig. 1. Speckle contrast measurement setup of free space geometry.

Fig. 2. Speckle contrast measurement setup of imaging geometry.

III. RESULTS AND DISCUSSION When the piezoelectric benders are actuated, laser beam is steered. The moving beam is focused at a spot on diffuser 1 by the condenser lens. The spatial position of the laser spots on diffuser 1 remains the same while the illumination angle is changed. The laser spots with different illumination angle generate different speckle patterns, which are averaged by the CCD camera during its integrating time. The speckle is evaluated by speckle contrast (SC) which is defined as the standard deviation divided by the mean value of light intensity in the speckle pattern [1]. Many measurement parameters will influence the value of SC. In order to fairly evaluate the speckle reduction, it is important to perform the calibration and find the suitable measurement parameters. The calibration process is based on three criterions: 1). The speckle size detected in CCD sensor should be big enough compared with the CCD pixel size. According to [1], the ratio between the speckle size and CCD pixel size affects the value of SC, by: where k is equal to Ac/Am, Ac is the “coherent area” or “speckle size” and Am is the CCD pixel size. By plotting parameter k with regard to SC, it is seen that k should be at least above 10, to avoid the speckle reduction introduced by the spatial averaging of CCD pixel. 2). The CCD camera should be operated in the linear regime. The detected light intensity

distribution should be neither over-saturated nor under-saturated. 3). In free space geometry, ideally the original SC is equal to 1 without any de-speckle method applied. In practice, however, the SC should be 1/20.5 (0.71) even without the motion of piezoelectric benders, due to the narrowband laser scattered at a depolarizing screen [1]. In free space geometry, the center-to-center spacing of adjacent dark spots or of adjacent light spots in speckle pattern is given by λz/D [7], where λ is the laser wavelength, z is the propagation distance between diffuser 1 and CCD sensor, D is the diameter of the scattering spot. This is the one dimensional “width of speckle”. A reasonable approximation is that Ac = (λz/D)2 [1]. In this case, λ is 633 nm, z is 50 mm and D is 0.5 mm. Therefore the speckle size at the CCD sensor is 63.3 × 63.3 μm, which is big enough to avoid the spatial averaging due to the CCD pixel. To ensure that the CCD camera is operated in the linear regime, the camera parameters are set as: Gamma=100, Brightness=0, Gain=550, Exposure time=1/34 sec (to simulate the averaging time of human eyes). The light intensity is adjusted by the polarizer 1 to avoid over-saturation and under-saturation of the detected speckle in CCD. In this calibrated system, when laser illuminated at the diffuser, which works as a depolarizing screen, the SC is 0.71. The captured speckle image with and without the actuation of the piezoelectric benders are shown in Fig. 3 (a) and (b), respectively. As expected, without the actuating of benders, the speckle is significant. As benders actuated, the speckle is averaged by angular diversity of beams and the image becomes smooth. The minimal SC is 0.06, which is obtained when piezoelectric benders are driven under ±100 V, 167 Hz square wave voltage. From Fig. 3 (b), the speckle pattern almost can not be perceived.

Fig. 3. Speckle reduction in free space geometry: (a) original speckle SC=0.71; (b) reduced speckle SC=0.06. Speckle reduction in imaging geometry with both fine and coarse speckle: (c) original speckle SC=0.87; (d) reduced speckle SC=0.16. Speckle reduction in imaging geometry with only coarse speckle: (e) original speckle SC=0.71; reduced speckle SC=0.09.

kkkerfkSC exp1


3

For imaging geometry, in order to determine the measurement parameters, the first two criterions as in free space geometry are employed. The third criterion is not applicable in this case, due to “compound speckle” phenomena [1]. In imaging geometry, when light passes through diffuser 1, and falls on a finite-sized diffuser 2, there can be a compounding of speckle statics. The speckle generated from diffuser 1 has bigger size, or “coarse speckle”; while the speckle generated from diffuser 2 has smaller size, or “fine speckle”. The SC of compound speckle can be bigger than 1 [1]. Due to the dimension difference of the coarse and fine speckle, it is difficult to measure and evaluate them in a single setup. The optical configuration has to be adjusted in order to observer one or the other. The fine speckle size is calculated by [(1+M)×λ×F#]2 [7], where M is the magnification of the optical configuration, which is f1/f2 as shown in Fig. 2. In order to obtain sufficiently large fine speckle size, F# of the imaging lens is set as the maximum value 16. The lens is mounted 150 mm away from the CCD sensor and placed 75 mm away from diffuser 2. Hence, the speckle size is 30.4 μm × 30.4 μm, which is big enough to avoid the spatial averaging from CCD. The camera parameters and light intensity are fixed the same as in free space geometry, to ensure that the CCD camera is operated in the linear regime and have comparable results. The captured speckle image with and without the actuation of the piezoelectric benders are shown in Fig. 3 (c) (d). The SC is reduced from 0.87 to 0.16 by using the piezoelectric benders. The original SC is 0.87 rather than 0.71, which is due to the compound speckle phenomena. In this case, both the fine and coarse speckle is detected by the CCD sensor. However, the coarse speckle is big and difficult to distinguish from Fig. 3(c). The size of coarse speckle is calculated by two steps. From diffuser 1, the scattered beam propagates in the light pipe with multiple reflections. This is free space propagation and the speckle size is (λz/D)2. The speckle pattern at the output of light pipe is magnified into the CCD sensor by magnification M. Therefore the final coarse speckle size is [(λz/D) × M]2, where z is the propagation distance within the light pipe. We assume that z is three times longer than the length of light pipe. To observe the coarse speckle clearly, the optical configuration is adjusted to: f1=60 mm, f2=250 mm, F#=1.4. Hence, the coarse speckle size is 159.5 μm × 159.5 μm. Please note that the coarse speckle size calculation is just an approximation because the light propagated inside light pipe in a complicated manner and it is difficult to predict the light path. Moreover, unlike the free space propagation, in light pipe propagation the speckle pattern is folded many times by multiple reflections from the side walls and this may affect the speckle size. The coarse speckle is shown in Fig. 3(e). It is clear that the coarse speckle is much bigger than the fine speckle, since the letter “S” is the same as in Fig. 3(c). The height of the letter “S” is approximately 0.5 mm, indicating that the calculated coarse speckle size 159.5 μm × 159.5 μm agrees with the experiments. In this measurement setup, the

fine speckle size is [(1+M)×λ×F#]2 = 1.1 μm×1.1 μm. The speckle size is much smaller than the CCD pixel size therefore it is not perceived by CCD sensor. The coarse speckle is reduced from 0.72 to 0.09 by piezoelectric benders in imaging geometry. From Fig. 3 (e) (f), the masked letter “S” becomes quite distinct after speckle reduction. In practice the f1/f2 ratio is usually much smaller than 1, since f1 is normally the distance between the eye lens and retina. Therefore, the fine speckle is difficult to be observed due to the small size. In conclusion, by using two fast vibrating piezoelectric benders, the speckle contrast is efficiently reduced from 0.71 to 0.06 and 0.09, in free space geometry and imaging geometry, respectively. It demonstrated that the proposed method can be implemented into full frame laser display for speckle reduction. The method is simple to construct with low cost and shows good reliability and stability. It is especially suitable for large display screen where a high power laser is needed.

REFERENCES [1] J. W. Goodman, Speckle phenomena in optics: theory and applications.

Roberts & Company Publishers, 2007. [2] S. Lowenthal, and D. Joyeux, “Speckle removal by slowly moving

diffuser associated with a motionless diffuser,” J. Opt. Soc. Am. A., vol. 61, pp. 847-851, 1971.

[3] J. I. Trisnadi, “Hadamard speckle contrast reduction,” Opt. Lett., vol. 29, pp. 11-13, 2004.

[4] S. An, A. Lapchuk, V. Yurlov, J. Song, H. Park, J. Jang, W. Shin, S. Karagpoltsev, and S. Yun, “Speckle suppression in scanning laser display using several partially coherent beams,” Opt. Express, vol. 17, pp. 92-103, 2009.

[5] M. N. Akram, V. Kartashov, and Z. Tong, “Speckle reduction in line-scan laser projectors using binary phase codes”, Opt. Lett., vol. 35, pp. 444-446, 2010.

[6] G. Ouyang, Z. Tong, M. N. Akram, K. Wang, V. Kartashov, X. Yan, and X. Chen, “Speckle reduction using a motionless diffractive optical element”, Opt. Lett., vol. 35, issue. 17, pp. 2852-2854, 2010.

[7] G. Cloud, “Optical Methods in Experimental Mechanics”, Exp. Techniques, vol. 31, issue. 3, pp. 19–22, 2007.

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