2nd Conference on Centro Fermi’s Projects, Rome 19-20 April 2012

OPTICAL MICRORESONATORS & BIOPHOTONIC SENSORS PROJECT

Simone BerneschiCentro Fermi Grants

CNR, Institute of Applied Physics “Nello Carrara”

Project Coordinator:Stefano Pelli

CNR, Institute of Applied Physics “Nello Carrara”

OUTLINE

• Motivations;• Objectives;• WGM microresonators: a brief overview; • Applications & Results;

NL effects;biosensors;

• Conclusions

MOTIVATIONS

• Light – matter interaction increases in the presence of small objects;

« …smaller objects in nature are not just scaled replicas of similar big objects and in fact they have improved properties…»

Galileo «Dialogue Concerning Two New Sciences» (1638)

• High Q microcavities, with strong spatial localization of the field, well respond to this principle and receive an even greater interest in many fundamental processes in photonics (e.g.: QED & NL processes; biosensing….)

OBJECTIVES

Investigating Whispering Gallery Modes (WGMs) microcavities for:

• Developing highly sensitive, label free biosensors (early diagnosis); (microsphere/microbubble)

• Developing all-optical switch by means of NL polymeric coating; (microsphere)

• Studing possible integration solutions with optical planar devices. (millidisk)

Lord Rayleigh (1842 – 1919)

• The Whispering Gallery phenomenon was initially described by Lord Rayleigh based on observations in St. Paul’s Cathedral in London;

L. Rayleigh, “The Problem of the Whispering Gallery,” Philosophical Magazine 20, 1001–1004 (1910).

Whispering Gallery under the cupola of the St. Paul’s Cathedral in London

WGMs RESONATORS

• a whisper spoken close to the wall can be heard all the way along the gallery, 42 m to the other side, thus the term “whispering gallery”

• Microdisks

• Microspheres

• light can be resonantly guided by total internal reflection, along an equatorial plane, with long cavity lifetime and strong spatial confinement;

WGMs RESONATORS• Microbubble

Field radial component for the fundamental mode

Field azimuthal component

(periodical function)

Evanescent field tail

Field polar component for the fundamental mode (spherical Legendre function)

Maxwell + boundary conditions:

WGMs RESONATORS

Efficient and robust coupling of the light to the cavity requires:

phase matching and mode overlap!

Approaches for efficient evanescent coupling of light into the microspheres:

Prism Tapered fiber

Surface waveguide Hybrid fiber-prism

Q factor measurement: experimental setup

• From WGM spectral linewidth dν• Q=ν/dν

MonitorTunable

LD

camera

camera

piezo

Scope

Modulator

Mux

Vis. LD

Detector

dn

dn=300 KHzDn=1.5 GHz

WGMs RESONATORS

loss/cycleEnergy cavity) theinto stored(Energy 2

Q

Fiber Tip

electrodes

• A cleaved tip of the fiber is inserted between two metal electrodes;

D = 2R = 150 – 350 μmdepending on the number of shots

WGMs RESONATORSSiO2 microspheres by fusion splicing

• Arc discharges partially melts the fiber tip;

• Surface tension forces produce the spherical shape.

• Partial melting + surface tension effect cannot be applied to crystals.

• Polishing procedure by using a home-made lapping station. The almost spherical profile of the edges is obtained through a rotational stage whose pivot point can be finely adjusted.

Polishing protocol:

• Grinding phase steps (abrasive disk);

• Fine polishing phase (diamond suspensions);

WGMs RESONATORSCrystalline microdisk by polishing

CRYSTALLINE MICRODISK INTEGRATION

320 360 400

2.8

3.0

3.2

3.4

n=1.5 MHz,Q=1.3 x 108

Tran

smis

sion

(a.u

.)

Detuning (MHz)

G.Nunzi Conti et al., Opt. Express, 19, 3651 (2011)

Q = 1.3 108

WGMs RESONATORS

The system is all in guided integrated optics architecture (LiNbO3) !

APPLICATIONS

NL EFFECTS IN COATED MICROSPHERES

pump

probe

Motivation: optical switch based on electronic Kerr effect (n = n0 + n2 I) on spherical WGMR coated by a nonlinear medium;

Large resonance shift obtained on low time scales (10-12 s) using intensities well below the damage thresholds of the polymers.

All-optical switching for a probe signal Iprobe by a resonant pump beam Ipump which change the coating refractive index and hence the resonance position.

PUMP-PROBE Configuration:

Coated microspheres

Dipping Wet layer formation

Solvent evaporation

Polymer: liquid crystal polyfluorene (λpeak = 379 nm; n2 Re ((3)) = 2 10-10 cm2/W; β Im ((3)) = = 2 10-7 cm/W)

Solution: 0.1 mg/ml of polymer in toluene

NL EFFECTS IN COATED MICROSPHERES

Q factor from spectral linewidth Uncoated microsphere

Q = 1.5 108

2500 2750 3000 3250 3500 37503.0

3.5

4.0

4.5

5.0

Tran

smitt

ance

[a.u

.]

Detuning [MHz]

Coated microsphere

Q = 5 106

Coating thickness < 100 nm

NL EFFECTS IN COATED MICROSPHERES

(@ 1550 nm)

NL EFFECTS IN COATED MICROSPHERES

An optically induced shift of WGM of up to 250 MHz is obtained in the CW pump regime, which is nearly an order of magnitude smaller as compared to the pulsed probe regime.

Such a difference of the values of the shift induced optically by the power of the pump radiation is an indicator of the nonlinear-optical mechanism of the shift.

S. Soria et al. Opt. Express (2011)

WavelengthSweep

Generator

DFBLaser Power

Detector

R

R ’

• from the resonance condition: nn

rr

res

res

WGMs are morphological dependent:

any change in its surrounding environment (i.e. refractive index) or on its surface (due to some chemical and/or biochemical bonding) causes a shift of the resonances and reduces the Q factor value.

By measuring this shift, it is possible to obtain the refractive index change and/or the concentration of the analyte.

SiO2 MICROSPHERESAS OPTICAL BIOSENSORS

Aptamers: are RNA or DNA molecules (ca. 30 to 100 nucleotides) that recognize specific ligands and that are selected in vitro from vast populations of random sequences [so named in 1990 by Ellington and Szostak].

They exhibit:

- comparabile affinity and specificity

- more reproducibility and higher stability

- reversible denaturation and ease of modification

SiO2 MICROSPHERES FORPROTEIN APTASENSORS

Functionalization procedurea) Activationb) Silanizationc) Thrombin Binding Aptamer (TBA) immobilizationd) Passivation (mercapto-ethanol 1mM 1h)

Dithiol-TBA5'-GGTTGGTGTGGTTGG- 3'

10M in carbonate buffer 0.5M pH9

for 2 hours at 60 rpm

c)

Mercaptopropyl-trimethoxy silane

1% v/v toluenefor 10 minutes at 60°C

b)

100 m

OHOH

OH

OH

OH

OH

OHOH

OH

OH

OH

OH

OH

OH OH

OH

OH

Piranha treatment:H2SO4: H2O2

4 : 1for 3 minutes

a)

SiO2 MICROSPHERES FORPROTEIN APTASENSORS

Q factor measurement

Q = 4.0 107

bare microsphere

silanized microsphere

Q = 4.0 106

Thrombin binding microsphere

Q = 8.0 105

(in aqueous environment)

(in buffer solution)

(@ 773 nm)

SiO2 MICROSPHERES FORPROTEIN APTASENSORS

L. Pasquardini et al. , J. of Biophotonics (2012)

Uniform distribution of the filmCoating thickness < 100 nm

Thrombin: coagulation factorIt involves many pathological diseases like:Aatherosclerosis;marker for some cancer;

Set – Up measurement Detected Proteins:

VEGF (Vascular Endothelial Growth Factor): regolatorfor angiogenesis;

SiO2 MICROSPHERES FORPROTEIN APTASENSORS

Binding measurements showed that derivatized glass microspheres can act as efficient aptasensors in complex matrices: buffer and no filtered human serum.

Measure conditions:Thrombin concentration of 0,3mg/ml in non filtered 10% diluted human serum

SiO2 MICROSPHERES FORPROTEIN APTASENSORS

0 200 400 600 800 1000 1200 1400

0

1

2

3

4

5

6

Freq

uenc

y sh

ift [G

Hz]

Time [s]

stabilityVEGF165

a)

b)

L. Pasquardini et al. , J. of Biophotonics (2012)

VEGF165 concentration of 0,3mg/ml in buffer

Modulator

Laser

Systems based on Bulk Microresonators

The optical microcavity (microsphere) & coupling system (taper fiber) are immersed in a liquid medium (fluidic cell)

Problems: possible instability on the resonance position due to the induced perturbations by the liquid environment on the coupling system. No integrated solution.

Systems based on Hollow Microresonators

Modulator

Laser

Advantage: Possibility to test liquid or gas flows inside the microbubble without disturb the microfiber alignment.Integrated solution.

The fluidics is integrated inside the device (microbubble) & coupling system (tapered fiber) is external to the fluidics

FROM MICROSPHERES TOMICROBUBBLES (MB)

WHAT IS AN OPTICAL MB:THE BASIC IDEA

Similarly to the snake which has swallowed an elephant, an optical microbubble is a resonant microcavity structure, obtained starting from a microcapillary preform (the snake in the corresponding picture) by means of a particular fabrication process which locally increases the radial dimension of the hollow microtube (the elephant) along the axial direction.

M. Sumetsky et al., Opt. Lett. 35, p. 898 (2010)

Antoine De Saint-Exupéry Le Petit Prince (“The Little Prince”) - 1945

OPTICAL MB FABRICATION: A NEW PROCEDURE

Uniform heating of the pressurized capillary is obtained by rotation of the U shaped holder around the capillary.

A pair of electrical wires connects the electrodes to the splicer

The electrodes were moved outside the splicer and placed in a U shaped holder able to rotate by 360° by means of a step by step motor.

Modified Fusion Splicer

Q factor measurement

ParametersPostnova

Z-DI160481

UFEcapillary

Outer CapillaryDiameter (µm)

280 122

Capillary WallThickness (µm)

20 21

MBR Outer Diameter (µm)

380 240

MBR WallThickness (µm)

4 6

Postnova Microbubble

Contact - Critical coupling condition

No Contact – undercoupling condition

OPTICAL MICROBUBBLERESONATORS

S. Berneschi et al., Opt. Lett. (2011)

A peristaltic pump is connected to the microbubble

Router = 190 μm w = 4 μm

Different water – ethanol

solutions:

(4:1, 4:2, 4:3) in volume

Postnova Microbubble

OPTICAL MICROBUBBLE:REFRACTOMETRIC TEST

Sensibility:

0.5 nm/RIU

Detection Limit: 10 -6 RIU

S. Berneschi et al., Opt. Lett. (2011)

CONCLUSIONS & PERSPECTIVES

• Demonstration of optical microsphere aptasensors for protein detection (in human serum) take the detection to the limit;

• Possibility to obtain high Q WGM resonators in different materials and with different fabrication process;

• Possibility to integrate optical WGMRs in planar structures (LiNbO3 millidisk) add-drop filters & optoelectronics oscillators in RF systems;

• Demonstration of all – optical switch by NL coated microspheres add-drop configuration;

• Demonstration of optical microbubble resonators possibility to use this structures for biosensing;

RELATED PROJECTS &

COLLABORATIONSAramos Project EDAOptoelectronics Oscillators

Naomi National ProjectBiosensors (protein essays)

FBK (Fondazione Bruno Kessler), Trento; Ospedale di Careggi (Firenze).

CNRS, LAAS & Univ. deToulouse, France

Short term mobility program CNR

Collaboration with different european Research Institutes & Universities (Moscow, Budapest, Trento,..)