Institut FEMTO-ST, Université de Franche-Comté 16 route de Gray 25030 BESANCON

France

Self-trapped beams for fabrication of optofluidic chips

M. Chauvet, L. Al Fares, F. Devaux, B. Guichardaz, S. Ballandras

Outline of the talk

- Context- Self-trapped beam technique

- Experimental demonstration- Fabrication and test of index sensor- Potential for integrated optics- Conclusions.

Analysis of liquid or gas properties using light

Available devices are often bulky

Great interest for fabrication of integrated devices

Context

Integrated optofluidic devices

Domain of Interest : biology, chemistry, biomedical, integrated optics…

Interests : - Portable devices- Fast response- Very small quantity of analyte.

Context

Optofluidic dye laser. From : Li, Z. Y., Zhang, Z. Y., Emery, T., Scherer, A. & Psaltis, D., Opt. Express 14, 696–701 (2006).

Optofluidic chip for cell manipulation. From : R. Osselame, Politecnico Di Milano.

Context

Challenges : combination of micro-channels & optical waveguides :

Fluidic channels with smooth walls Buried waveguides Optimization of waveguides/channels alignment

Integrating optical sensing into lab-on-a-chip systems, R. Osellame et. al. SPIE Newsroom, DOI: 10.1117/2.1200905.1597(1997)

Fabrication technique

Scanning beam technique

Fabrication technique

Self-trapped beams

Self-trapped beam writing technique

- Single step process- Self-induced singlemode waveguides - Low loss circular waveguides- Self-aligned trajectory

Requirements and characteristics

- Nonlinear focusing medium (Kerr, thermal, photopolymer, photorefractive..)

- Stable self-confinement of 2-D beams with saturable nonlinearity

- Ultimately spatial soliton can be formed

Studied configuration

nc

LiNbO3

FluidicChannel

Light-writtenwaveguides

Why LiNbO3 ?- Photonic material(transparent Vis-NIR, electro-optic, ..)- Properties(photorefractive, pyroelectric, ..)

Precision saw Disco DAD 321

Precision dicing/polishing is used to cut the sample and to inscribe the fluidic channel.

Samples fabrication

The optical waveguide is induced by photorefractive beam self-trapping controlled by the pyroelectric effect.

heated sampleAt room temperature

Optical set- up

Waveguides induction : principle

Experimental observation(15mm long crystal, ΔT= 20°C, P=200µW)

Safioui et. al., “Pyroliton: pyroelectric spatial soliton”,Opt.Express 17,2209 (2009).

Underlying physics

Normalized distributions

0

1

-1

0Intensity charges Electric field Refractive index

Pyroelectric field

Schematic distribution of charge

0

1pyro

r

E pε ε−= ∆T

42 /kV cm∆ ⇒ ≈pyT = 20°C E

Input Beam (FWHM≈10µm)

Waveguide crossing channel (θ=0)

ΔT=20°C

Properties of induced waveguide :- Singlemode- Low losses (0.9dB) - Quasi-permanent (lifetime > several months)

Parameters : λ = 532nm,

extraordinary polarization, P=

70µW

Without channel

With channel (θ=0)

Optical set-up

Determination of the refractive index of the liquid (nliq) by

transmission analysis

Laser diode640nm

Realization of an index sensor

Step 1 : waveguide induction- No liquid- Laser diode at high power- Sample temperature = 40°C

Step 2 : index measurement- Liquid present- Laser diode current below threshold- Sample at ambient temperature

Chauvet et. al., “Integrated optofluidic index sensor based on self-trapped beams in LiNbO3”,Appl. Phys. Lett, 101, 181104 (2012).

Tran

smis

sio

n v

aria

tio

Tran

smis

sion

var

iatio

n

t = 14 s, droplet of ethanol T

ran

smis

sion

va

riatio

nTest of index sensor

Evolution of the normalized sensor transmission versus time.

Sensor response with ethanol

Modeling of index sensor

Fresnel reflections : Beam diffraction : Total gap transmission :

(nLN)

(nliq

2 2

2 LN liqF

LN liq

n nT

n n=

+ 2 2

2 guide beamc

guide beam

W WT

W W=

+ c FT T T= ×2

02

1beam guideliq guide

LW W

n W

λπ

= + ÷ ÷

1 . 0 1 . 2 1 . 4 1 . 6 1 . 81 . 0 0

1 . 0 5

1 . 1 0

1 . 1 5

1 . 2 0

1 . 2 5

1 . 3 0

1 . 3 5

n L

T

Normalised Tc*TF

Normalised TF

1.360

1.230N

orm

aliz

ed t

ran

smis

sio

n

Refractive index nL of the liquid

Image of guided mode

Wguide= 6.8 µm

Theoretical response of sensor

Parameters : L = 200µm, λ0 = 640nm, nLN : Sellmeier equation.

Ethanol

1.202

n methanol =1.317 No

rmal

ized

tra

nsm

issi

on

Refractive index nL of the liquid

Measurement with methanol

exp 1.317 0.005 1.320theomethn n= ± =

Tilted channels (θ0)

nc

nLN

When incident angle θ approach the critical angle - Beam shift d - Beam distortion- Beam transmission T

Interest : high sensitivity sensors, innovative integrated optics ..

d L tan arcsin sinLN

c

n

nθ

= ÷ ÷ ÷

θ= 24°

Influence of channel tilt angle

Tra

nsm

issi

on

Incident angle (θ)°

θBr =23,6°

θCTM= 25,5°

θCTE=26,5°

Shi

ft d

(µ

m)

Incident angle (θ)°

Fabricated structures

Parameters : λ = 532nm, ordinary

polarization (TM), P = 20µW,

ΔT=20°C

Input beam FWHM≈12µm FWHM≈13µm

Experimental results T

rans

mis

sion

Incident angle (θ)°

θ = 24,6°

θ = 25°

Successful self-trapping

Unsuccessful self-trapping

Experimental results

2 3 . 6 2 3 . 8 2 4 . 0 2 4 . 2 2 4 . 4 2 4 . 6 2 4 . 8

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

T M

T E

0 5 1 0 1 5 2 0 2 50

2 0 0

4 0 0

6 0 0

8 0 0

A n g l e o f i n c i d e n c e

dm

Angle d’incidence (θ)°

Beam self-focusing is obtained for both TE and TM polarizations for incident angle close to critical angle

Potential for sensing devices

q =34°

q =32°

q =30°

q =28 ° λ=633nm

1.32 1.34 1.36 1.38nc

0.2

0.4

0.6

0.8

1.0

T Transmission vs nc

θ

L=200µm

W1=10µm

Fabrication : Waveguide written

with ref. liquid

Sensor sensitivity and span can be

adjusted with θ value

Use : Transmission highly dependent on

test liquids index

nc writing = 1.33

0 . 6 2 0 0 . 6 2 5 0 . 6 3 0 0 . 6 3 5 0 . 6 4 00 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

µ m T

nc =1,33nc=1,331 nc=1,329

λ writing =633nmnc writing = 1.33

Transmission fct λ

θ=34.5°

Potential for sensing devices/spectral filters

θ

Broadband spectrum

source

Optical spectrum analyser

Index sensor

Sensitivity can reach 10-6

Spectral filter Spectral width adjustable with θ Central wavelength tunable with fluid

410 /cdn nmdλ

−≈

Potential for integrated optical components

Material birefringencecan be used to design an integrated

polarization separator :

Inputarbitrary

polarization

Output 1 : vertical (TE) polarization Output 2 :

horizontal (TM) polarization

2 3 . 6 2 3 . 8 2 4 . 0 2 4 . 2 2 4 . 4 2 4 . 6 2 4 . 8

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

T M

T E

0 5 1 0 1 5 2 0 2 50

2 0 0

4 0 0

6 0 0

8 0 0

A n g l e o f i n c i d e n c e

dm

Beam shift ddepends on

polarization !

TMTE

Al Fares et. al., Self-trapped beams crossing tilted channels to induce guided polarization separators”,Appl. Phys. Lett, 103, 041111 (2013)

Fabrication of a polarization separator

The polarization separator is inscribed in a one step self-writting process.

Parameters : λ = 532nm, P= 20µW,θ = 23.9°ΔT=20°C

Injection TM Injection TE

Injection TE +TM

Injection TE +TM

Injection TE +TM

Input beam FWHM≈12µm

Test of polarization separator

Properties of component :

- Extinction ratio TE/TM > 20db- Transmission 68% for TM et 25% for TE

100µm

Input polarization

Detected intensity

TM TE TM+TE

Conclusions Ability of self-trapped beams to induce unique automatically adapted

buried waveguides crossing channels have been demonstrated

Integrated components based on LiNbO3 have been fabricated● Index sensor ● Integrated polarization separator

More elaborated devices taking advantage of self-aligned waveguides can be developed :

High sensitivity sensors, tunable filters, couplers to optical resonators

Use of other materials such as photopolymers (low cost and permanent structuring)

Perspectives