Applications de l’exaltation de l’effet électro- optique ...cmdo.cnrs.fr › IMG › pdf › Bernal_JNCO2007.pdf · PCs in lithium niobate • 2D photonic crystal on a lithium

JNCO-Grenoble 03/06/2007Équipe nano-optique, DOPMD

Applications de l’exaltation de l’effet électro-optique dans un cristal photonique en

niobate de lithium induit par le ralentissement de la lumière

M.-P. Bernal1, M. Roussey1, J. Amet1, F.I. Baida1, G.W. Burr1,2

1 Institut FEMTO-ST, Département d’Optique, 16 Route de Gray, 25000 Besançon (France)

2 IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 (USA)


Summaryo Motivationo Device parameterso Defect fabrication studieso Optical characterisationo Theoretical study of the slow lighto Modulationo Superprisms in LNo Conclusions and perspectives


Motivation

Lithium Lithium NiobateNiobate : a : a veryvery interestinginteresting dielectricdielectricmaterialmaterial for for thethe PC fabricationPC fabrication

LN PCs can reduce the size of classical modulators

LN PC devices open the path to multi-tunable devices since it is

- electro-optic- piézoelectric- acousto-optic- non-linear


PCs in lithium niobate• 2D photonic crystal on a lithium niobate waveguide• Waveguide fabrication by annealed proton exchange (APE)

The center of the guided mode is at 1.4μm from the surface.Hole depth > 1.5 microns !!!!!

Photoniccrystal

APE Waveguide

Electrodes

YZ

X

Electrodes

1 0-1-2-3-4-5-6-7

0.10.090.080.070.060.050.040.010.020.01

-4 -3 -2 -1 0 1 2 3 4

1.4μm

E(V/m)

Width (μm)Mode simulation for λ = 1.55 μm


PC fabrication: FIB

F. Lacour et al., Optical Materials, 27, p.1421-1425 (2005).

Depths of 2 μm on sub-micron structures

Advantage:- High aspect-ratio

Disadvantage:- Conical shape holes- Time consuming


Proposed structure

Structure

Nor

mal

ized

tran

smis

sion

FDTDcalculation

4µmSEM image

10mm

sample

Photography of the complete device

a=766nm

r=207nmn=2.143

n=1

15 ro

ws of

holes

a 2r


Fab. defects: misalignment

Perfect lattice :15 x 15 holesa = 766nmr = 207 nm

Slightly non-perfect :We introduce a smallrandom displacementΔxmax = Δymax = 40 nm

Tran

smis

sion

Wavelength

Figure 1

Tran

smis

sion

900 1000 1100 1200 1300 1400 1500 1600 1700 18000.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Figure 2

Wavelength

a

r

a +/- x Δ

a +/- y Δ


Fab. Defects: hole conicity

We can observe on these theoretical transmission spectra that for a depth hole (cylinders) of 2.5µm, we obtain similar transmission profile than in the experimental results.


Fab. Defects: hole conicity3D-Simulations: Hole conicity and hole depthStudy for λ = 1.42 μm Depths: 2, 4, 6, 8 μm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

•Cylindrical holes forbid light, most light is reflected back•Conical holes deviate light inside the LN wafer


Fab. Defects: hole conicity3D-Simulations: Hole conicity and hole depthStudy for λ = 1.57 μm Depths: 2, 4, 6, 8 μm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

0

-5

-10-5 0 5 10 15 20 25 3530

µm

µm

Cylindrical holes let light go through the PCConical holes: Cones behave like prisms, PBG location is changed


Optical characterisation

Laser 1064nm PC fibre(20m)

OSA

INPUT

Spectrum Analyser

WaveGuide+

Photoniccrystal

1100 1200 1300 1400 1500 1600-60

-55

-50

-45

-40

-35

-30

-25

Opt

ical

pow

er (d

Bm

)Wavelength (nm)

Source

Optical spectrum


SNOM characterisation

Computer

Control Elec.

Detector

Exit fiber SNOM head

Laser

Fiber tip

PC

Tunningfork

Device

Voltage generator

Injectingfiber

XYZ stages


ResultsTransmission spectrum Gap shift vs. applied voltage

• U=80V Δλ=200nm

• Shift 300 times bigger than theoretical predictions corresponding to Δn = 0.3

• The gap moves in the opposite direction when the voltage sign is inverted (EO effect)

12dB

1286nm

M. Roussey, et al., Appl. Phys. Lett., 89, 241110 (2006).


Outside the gap Inside the gap

λ = 1286nm

This is a qualitative description of the photonic effect

k ku.a. u.a.


Outside the gap Inside the gap

Input of thecrystal

Output of thecrystal

Commercial SNOM (SMENA), Pulled dielectric tipCollection Mode


Theoretical analysis: slow light

Where is the LOCAL FIELD FACTOR

Ernn ×××−=Δ 333

21

Efrnn ××××−=Δ 333

3

21

333)(33

322 frrf PCBULKPC ×=⇔×= ><>< χχ

Pockels equation for theBULK lithium niobate

MODIFIED Pockels equation

The nanostructuration changes the material non linearity

For lithium niobate only the second order is taken into account

f


Theoretical analysis: slow light

PCg

BULKg

vv

f = ∫=PC BULK

loc

PCloc dydz

EE

NSf 1

PWE FDTD

Δn = 0.34

Efrnn ××××−=Δ3

333

21


Modulation

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

Am

plitu

de (m

V)

Temps (µs)0,000 0,005 0,010 0,015 0,020

-30

-20

-10

0

10

20

30

Ampl

itude

(mV)

Time (µs)

10000 100000 1000000 1E7 1E8 1E9-80

-70

-60

-50

-40

-30

Frequency

1GHz100MHz10MHz1MHz100kHz10kHz

Gai

nA B

A B

C

C


The superprism effect in PCs

Group velocity : Vg = k . ω Phase velocity : Vφ = ω / IkI

Isotropic uniform materialVg and k have the same direction and depend on the light direction propagation and wavelength.

Photonic crystal

Nanostructuration (periodic variations): P. Yeh [1] has shown that in PC, Ve=Vg (energy velocity Ve). Vg andk can have differents directions.

Increase of the dependance of the light direction propagation and wavelength.

A material can act upon the light propagation by affecting the group velocity and/or the phase velocity.

[1] P. Yeh «Electromagnetic propagtion in birefingent layered media », JOSA, 1979.


Light direction propagation in the PC for different wavelengths (λ=1500nm and λ=1600nm) for n=2.143 and θinc=23°.

Analysis of CFDS (PWE)

λ=1500 nm θPC=-70°

λ=1600 nm θPC=10°

λ dependence

λ=1500 nm λ=1600 nm


Light direction propagation in the PC for differents wavelength (λ=1500nm and λ=1600nm) for n=2.143 and θinc=23°.

λ dependence


Δn (Classical Pockels)Geometry for largest superprism effect if the conventional Pockels effect is considered in order to calculate the induced modification on the refractive index; the TM polarization and the telecommunication wavelength (λ=1550nm) are considered in both scenarios.

At 1550 nm, for a modification of 1% of the effective refractiveIndex of LN the Δϑvg is about 10°. This corresponds to an external applied electric field of 150 V/μm

PWE method FDTD method


Δn: local field factorWe are looking for a geometry that allows beam steering keeping high local field factor

At 1550 nm, for a modification of 1%of the effective refractive index of LN the Δϑvg is about 7°.This corresponds to an external appliedelectric field of 0.5 V/μm.

J. Amet, M.-P. Bernal, D. Van Labeke, Journal of microscopy, To appear (2007)


Conclusions

• Fabrication of lithium niobate photonic crystals• Study of fabrication imperfections: delicate issue in PC fabrication on classical waveguides.• Experimental demonstration of a PC LN intensity modulator via an enhanced electro-optic effect (300 times bigger than classicalelectro-optic effect in LN!!).• Theoretical analysis show that this enhancement is due to slow light propagation.

Perspectives• Fabrication of high speed modulator > 40 GHz• Fabrication on performant PC superprism devices based on thecombination of ultrarefraction and slow light.


Acknowledgements

ACI « COBIAN » N° 137INTERREG III (« CRISLAR »)

BlueGene IBMMIMENTO : FEMTO-ST (R. Salut, G. Ulliac)

Documents

Applications de l’exaltation de l’effet électro- optique ...cmdo.cnrs.fr › IMG › pdf › Bernal_JNCO2007.pdf · PCs in lithium niobate • 2D photonic crystal on a lithium