Organic Semiconductor Lasers

Organic Semiconductor Centre, SUPA

School of Physics and Astronomy,

University of St Andrews, UK

I.D.W. Samuel, Y. Yang, Y. Wang, G.A. Turnbull

Outline

Introduction

- Polymer lasers and photonics

- Very brief history

Distributed feedback lasers

Nitride LED pumped polymer laser

Explosive sensing

Organic Semiconductors

Conjugated molecules

Novel semiconductors

Easy to process

Can tune properties

Can emit light

Flexible

Plastic Photonics

Active Passive

Potential for Polymer Lasers +Amplifiers

Absorption and emission

separated - 4 level system

Strong absorption ~105 cm-1

- Enormous gain

Little concentration quenching

Broad spectra - broad bandwidth

Compatible with polymer fibre

transmission windows (500–

560 and ~660 nm),

Scope for low cost manufacture

Possibility of electrical pumping

400 500 600 700

3.2 3 2.8 2.6 2.4 2.2 2 1.8

C8H

17H

17C

8

n

O

O

n

n

Ph

oto

lum

ine

sce

nce

(a

rb.)

Wavelength (nm)

Energy (eV)

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

O

O

n

Ab

sorp

tion

(1

05/c

m)

Wavelength (nm)

Ph

oto

lum

ine

scen

ce (a

rb.)

Mirrors, MEH-PPV solution

Moses 1992

Q-switched Nd:YAG laser pump

Microcavity, PPV film

Tessler, Denton, Friend 1996

Regenerative amplifier pump

Early Semiconducting Polymer Lasers

Polymer laser resonator geometries

Solution processing- high quality waveguides

Sub-micron structures- laser microresonators

1-D DFB 2-D DFB /

photonic crystal random scatterers

Chemical Reviews 107 1272 (2007)

microring microring microcavity

Distributed Feedback Lasing

DFB lasing in an MEH-PPV film 1st order scattering provides output coupling 2nd order scattering provides distributed feedback

Air

MEH-PPV(100nm)

Silica

Pump laser: 500ps pulses at 500nm

540 560 580 600 620 640 6600

10

20

30

40

50

60

70

Wavelength (nm)

Ou

tpu

t E

nerg

y (

arb

)

Pump Energy

4.4J

1.8J

0.70J

DFB Laser Operation

0 1 2 3 4 50

20

40

60

Pump Energy (J)

Ou

tpu

t E

nerg

y (

arb

.)

Light scattered from counter-

propagating guided waves

interfere to favour oscillation

only at one band-edge

At high powers emission

spectrum is dominated by laser

mode and ASE

540 560 580 600 620 640 6600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Wavelength (nm)

Ou

tpu

t E

nerg

y (

arb

)

Pump Energy

0.70J

0.56J

0.35J

Elastomeric mould

Simple Fabrication of Polymer Nanostructures

Hot embossing

Solvent assisted micro-moulding

UV-nanoimprint lithography

Appl Phys Lett 81, 1955 (2002); Appl Phys Lett 82, 4023 (2003);

Polymer film

Solvent

Elastomeric mould

12

0 s

ec

on

ds

AFM image of 400 nm period, 90

nm deep eggbox corrugation

Towards Practical Polymer Lasers:

Shrinking Polymer (Pump) Lasers

1995 Regenerative amplifier ~2000 Q-switched Nd:YAG

2004 Microchip laser 2006 Diode pumped

Electrically Pumped Organic Lasers?

Main Challenges

High projected threshold current density (> 100s A/cm2)

Low carrier mobilities

Losses from metal contacts

Polaron and triplet absorption losses

http://en.wikipedia.org/wiki/Laser

Chemical Reviews 107 1272 (2007); Nature Photonics 3, 546 (2009)

Why?

Novel, tuneable, simple to fabricate, (flexible) lasers

Without need for expensive (and bulky) separate pump laser

Towards LED Pumped Polymer Lasers:

Hybrid Optoelectronics

Alternative approach: indirect electrical pumping

Take advantage of advances in nitride semiconductors

Inject charge and generate light in nitride LED

Gives all advantages of direct injection, without problems

Challenges

Incoherent source

Pulsed behaviour not known

• Laser peak: 568 nm

• Lasing threshold: 208 W/cm2

Low Threshold Polymer Laser:

Structure and Characteristics

10 15 20 25 30 35 400

10000

20000

30000

40000 Threshold energy : 27 nJ/pulse

Threshold intensity: 208 W/cm2

outp

ut at 568 n

m

pump energy (nJ)

Air

Copolymer

Silica

Pump

Fluorene copolymer

1-D Distributed feedback resonator,

350 nm period

LED-Pumping of Polymer Lasers

Pumping source: inorganic LED

Lumiled K2 emitter (LXK2-PR14-Q00)

Lasers: organic gain medium

1D DFB surface emitting laser

Fluorene copolymer

Device: compact

LED in contact with laser

Dichroic filter: reflect pump light

CYTOP: encapsulation

A Nitride LED Pumped Polymer Laser

Sharp laser peak starts to form at

152 A (233 W/cm2)

At 568 nm (TE mode): nonlinear increase

At 555 nm (TM mode): linear increase

HYPIX project

Hybrid organic semiconductor-GaN-CMOS smart pixel arrays

(St Andrews, Strathclyde, Edinburgh, Imperial College)

µLED

array

CMOS device

2 mm

InGaN LED InGaN LED

Si-CMOS backplane

Organic photonics

Solid State Polymer Amplifiers

560 580 600 620 640 660 6800

5

10

15

20

Red-F

Imperial

St AndrewsMEH-PPV

St Andrews

F8BT

St Andrews

Gain

(dB

)

Wavelength (nm)

Pump Signal

IN Signal

OUT

Grating coupled

Gain up to 21 db: signal

amplified 100 times

Wide wavelength range

Explosives Detection

Urgent need to detect explosive devices in war zones, airports

Metal detectors, ground penetrating radar to spot bombs

Sense vapours of explosives around bomb

Humanitarian demining

Explosive molecules

TNT

Semtex: RDX and PETN

Dynamite: trinitroglycerine

Fluorescent Explosives Detection

Many common explosives include TNT, DNT, DNB etc

Nitroaromatic compounds are strong electron acceptors

Introduction of nitroaromatic molecule leads to dissociation of exciton and quenches emission

Detect by fluorescence change Swager et al. Chem. Rev. 107, 1339 (2007) or

Lasing change Rose et al Nature 434, 877 (2005)

S0

S1

S0

S1

n

C8H

17C

8H

17

TNT

Fluorescence Sensing with Polyfluorene

Polyfluorene film exposed to ~10 ppb dinitrobenzene vapour in air

15% drop in fluorescence

Fluorescence recovers to original value when purged in nitrogen

400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

Flu

ore

scen

ce

wavelength (nm)

before exposure

after 2min exposuren

C8H

17C

8H

17

PFO

DNB

Laser output before exposure to ~10 ppb DNB (green),

after exposure (red), and after removal of DNB (white)

Explosive Vapour Sensor – Change of Slope Efficiency

Slope efficiency 3 times lower

Threshold 1.8 times higher

After 5 minutes exposure 5 10 15 20 25 30

0

10000

Eth: 6.8J

Eth: 6.8J

Eth: 12.5J

outp

ut (a

.u.)

energy (J)

Yang et al., Adv. Fun. Mat (2010); See also Rose et al Nature 434, 877 (2005)

Sensing with Polymer of Intrinsic Microporosity

Could a porous material give a faster response?

Microporosity of the polymer PIM-1 forms as a result of the

rigidity of the macromolecular chain and contorted structure

PIM-1

～0.65 nm

Collaboration with N.B. McKeown & K.J. Msayib, Cardiff

0 300 600 9000.0

0.2

0.4

0.6

0.8

1.0

No

rmalized

Laser

Em

issio

n (

a.u

.)

Time (s)

PFO

PIM-1

Comparison between PIM-1 and PFO sensors

0 300 600 9000.0

0.2

0.4

0.6

0.8

1.0

No

rmalized

PL

(a.u

.)

Time (s)

PFO

PIM-1

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

• PL sensing efficiency 15 min

PFO: 40%

PIM-1: 82%

• Laser sensing efficiency 1 min

PFO: 36%

PIM-1: 88%

-6 0 6 12 18 24 30

0.6

0.7

0.8

0.9

1.0

Laser sensing

PL sensing

No

rmalize

d E

mis

sio

n (

a.u

.)

Time (s)

• Laser exposed to 10 ppb

DNB vapour for 90 s

• Before exposure, laser

threshold:

535 W/cm2 (32 A/pulse)

• After exposure, laser

threshold:

711 W/cm 2 (50 A/pulse)

1.4 times higher

• Laser emission drops by

30% in 10 s (@ 61

A/pulse)

• Much higher sensitivity

compared to PL

LED Pumped Organic Laser Sensor

0 200 400 600 800 10000

50

100

150

200

Laser

Em

issio

n (

a.u

.)

LED Power Intensity (W/cm2)

0 200 400 600 800 10000

50

100

150

200

Laser

Em

issio

n (

a.u

.)

LED Power Intensity (W/cm2)

Polymer Laser Explosives Sensor

Potential for IED / landmine detection

Sensitivity to ppb nitroaromatic explosive vapours

Larger, faster response for lasers than PL

LED-pumped laser sensor demonstrated

Organic Semiconductor Lasers : Conclusion

Compact, tuneable visible lasers

Simple fabrication

Direct pumping by InGaN LED

Explosive sensing

Further reading Chemical Reviews, Nature Photonics