Thermally poled germanosilicate films with high second-order nonlinearity

CFC3 2005 Conference on Lasers & Electro-Optics (CLEO)

Thermally

poled

germanosilicate

films

with

high

second-order

nonlinearity

A.Ozcan, M. J. F.Digonnet, G. S. Kino

Edward L. GinztonLaboratory; Stanford University,Stanford, California94305 Phone:(650) 723-0251Fax:(650) 725-2533 Email: aozcan(distanfordedu

F.Ay, and A. Aydinli

DepartmentofPhysics, Bilkent University, 06800, Ankara,Turkey

Phone: 90(312)290-1972Email:ay@,fen.bilkent.edu.tr

Abstract: Accurate measurements of the second-order

nonlinearity profile

of

thermally poled

low-loss

gernanosilicate films grown on fused-silica substrates are

reported,

of interest as

potential electro-optic

devices. After

optimization,

wedemonstratearecord

high

nonlinear coefficient

d33

1.6

pm/V,

atwo-fold

improvement

over highestreported d33 value in fused silica that we attributetothe presence ofgermanium.

©2005 OpticalSociety ofAmerica

OCIS codes:(310.6860)Thinfilms, optical properties; (190.4400)Nonlinearoptics, materials 1.Introduction

Thermalpoling is one ofthemostreproducible andwidely used techniquestoinduceastablesecond-orderoptical nonlinearity in glass. Some of the important

potential

applications

of

thernally poled glass

include integrated

electro-optic

phase and

amplitude

modulators or

parametric

oscillators.[1]

Poled

glass exhibits

significant

advantages compared to existingnonlinear materials, including low loss, broad transmission bands, high optical

damage threshold, and compatibility with fiber technology. The largest reported peak nonlinear coefficient for thermally poled silicaglass is d33 z 0.8pm/V.[2] As a resultof this relatively weaknonlinearity, allpoled-glass

devicesreported to daterequirefairlyhigh voltages and/or longlengths.Forthistechnologytobecome

practical,

the

strength of theinducednonlinearity hastobeimproved.

Inthisletter,weshow that thenonlinear coefficient can be doubled by doping the glass with germanium. After

optimizingthematerialcompositionandpoling

conditions,

wereportarecordpeakd33 coefficientof -1.6pm/Vin

thermally poled germanosilicate films. These films are of interestprimarily for two important reasons: (1) their propagation loss can be verylow,[3] whichmakes themexcellent waveguidematerialswith a refractive index close tothatof silica, and (2)the addition of Ge tothe silicamatrix increases the third-order

optical susceptibility

J

3) of

the glass, which should result in an increase in the induced nonlinear

coefficient.

These properties make poled germanosilicatefilmsapromisingcandidate forfuturelow-lossintegrated electro-opticdevices.

Table1.Characteristics of different germanosilicate filmspoled in air at -5 kV and -280 'C.

Germaneflow Molefractionof X(G:SiO2 Refractiveindex Peakd33

SampleI rate GeO,7(%) X(3) at1064nn Thickness Poling time m/)

I Osccm 0 -1 1.469 4

gm

10mm 0.54 2 33 scem -20 1.54 1.497 4pm 5min 0.80 3 33sccm -20 1.54 1.497 4pm 10min 1.59 4 33sccm -20 1.54 1.497 4

jm

15min 1.00 5 33sccm -20 1.54 1.497 2pm 10min 1.02 6 50sccm -30 2.22 1.514 4pm 10min 0.78 7 90sccm -56 4.77 1.553 4pm 10min 0.81

2.Germanosilicatethin filmgrowthprocess

Germanosilicatefilms were

deposited

on fused-quartzsubstrates

(Infrasil,

25x25x0.15

mm) by

plasma-enhanced

chemicalvapor

deposition (PECVD) using

a

parallel-plate

reactor

(Plasmalab

8510C).

The filmsweregrownat350

°C

andapressureof 1 TorratanRFpowerof 10Wat 13.56MHz

applied

to the

plates.

Theprecursor gaseswere

CFC3 2005ConferenceonLasers &Electro-Optics(CLEO)

silane (2%Sil{4/N2), germane (2%GeH4/He), andnitrous oxide (N20).The flow rates of silane and nitrous oxide werekept constantat180and 225 sccm,respectively, while that of germane waschanged from run to run between0 and 90 sccm. The growth rate of the films was-40nnm/min.

Annealing is almost always required to reduce thepropagation loss ofthe optical waveguides that utilize CVD-grownsilicon-based layers asthe core. Recently, we have reported the lowestpropagation loss values for as-grown germanosilicate films without the need for thermal annealing.[3] Inorder tooptimnize the poling process for this material, we grew seven individual germanosilicate filmswithdifferentcharacteristics, as listed in Table 1. Based on ourprevious

work,[3]

the propagation losses of the as-grown waveguides were estimated to be lessthan 0.15 dB/cmat1550 mn.

i #4 I51M.2',; S4I Kk o t".2,

0. ri"E1. rt#$i

n.4 l. ~~~~ ~~~~~0.4~ A

a 2 s8 Islo is 14iS is O 2 4 8 8 i 200 t4 iS i o0 2 4 S 8 t0 12 14 18 1S

Depl:lVicn1.hs31n1cbaiu s(Pm Oap[hfnm lte Eim aukm (W DepM hirn Ehs anok suftea 8xn)

Fig. 1.Therecovered nonlinearity profiles of(a) samples #2, #3, and #4; (b) ) samples #1, #3, #6and#7; (c)samples #3and#5. 3.Thermal poling results

The grown germanosilicate-Infrasil structures were thermally poled using polished n-type silicon electrodes (the

positive electrode facingagainst thegermanosilicatelayer)in airat -5kVand 280 'C. Thepolingtimewasvaried between 5 and 15 min (see Table 1) to investigate its effect on the peak nonlinear coefficient and maximize this coefficient. The nonlinearity depth profile of each poled sample was then measuredby the Maker fringe-Fienup

technique.[2] Afundamental laser beam at 1064nm was focused onto the sample and the second-harmonic (SH) power generated within thepoled region was recorded as a function of the beam incidence

angle.[4]

In orderto avoidtotal internal reflection at the output face of the Infrasil substrate, and henceachieve high incidence angles (e.g.>85°) a pair of half-cylinders made of Infrasil wasclamped on both sides ofthe poled samples.[5] Theresulting Maker fringe (MF) curve was then correctedfirstforangle-dependent multiplereflectionsarising in the film from the refractive index mismatch between the film and the substrate, and second for Fresnel reflections at both the fundamental and the SH wavelengthsoccurring at the boundary betweenthe inputhalf-cylinder and the film andat the film-substrate boundary. Finally, the corrected MF curves were processed using an iterative Fouriertransforn

technique[2]

touniquely retrieve the induced nonlinearity depth profile d33(z),where z is the depth into the sample measured from the anode surface. For want of space,themeasured MF curves of the samples are not shown here. Therecoverednonlinearity profiles forthe seven poled samples of Table 1 are shown in Fig. 1. The results in Fig. 1 aregrouped according to the polingconditionsandfilmproperties. Figure

1(a)

showstheprofiles of samples #2, #3, and #4, all ofwhich have a

4-pm

thickgermanosilicate films grown at a 33-sccmgermane flow rate. Toidentifythe optimum polingtime,thesenominally identical sampleswerepoled under identicalconditions except for thepoling time,which was 5 min for sample #2, 10 min for sample #3, and 15 minfor sample#4. As showninFig.

1(a),

the profiles recovered for these three samples exhibit similar features, namely asharppeakcentered about 0.5

Am

below the anode, followed by a weak pedestal that is approximately constant to a depth of -9-12 jimand that gradually

decreases to zero at a depth of 13-16

gim.

This resultreveals that the optimum poling time for these germanosilicate-Infrasil structures at an applied E-field of -32.5 MV/m is -10 min, corresponding to sample #3. The peak d33 coefficient obtained under these poling conditions is as high as -1.6pm/V. To ourknowledge, this is the highest second-order optical nonlinear coefficient measured without any ambiguities in athermallypoled germanosilicate

glass. The peak d33 coefficients measured for poling times of 5 and 15 min are -0.8

pm/V

and -1.0 pm/V,

respectively. As physically expected, as the poling time is increased from 5 min to 15 min the depth of thepedestal gradually increases from -9

gm

to -12

,Lm

(see Fig. l(a)). Thetotal depth of the induced nonlinear region (-13-16

jim)

inFig.

1(a)

issignificantlynarrowerthan for bulk Infrasil samplesthermallypoled undersimilar

conditions,

for which the depth is typically -40

pm.[2]

Furthermore, unlike in thermally poled Infrasil, the d33(z) profile of the

CFC3 2005Conference on Lasers& Electro-Optics (CLEO)

poled

germanosilicate-Infrasil

structuresdoes not

change

sign. We believe that the reasonfor these differences is

that the

germanosilicate

film limits the diffusion of

positive

ions such as

H30

from the anode surface into the

sample,

which results in theformationof a narrower

depletion region

within the film and hence a shalloweroverall nonlinear

region.

A similar

blocking

behavior in

germanosilicate

films,

which also resulted in narrowernonlinear

widths,

has been

reported

by

others.

[6]

Afteroptimizingthe

poling

time,

we

investigated

theeffectof thegermaneflowrate ontheinduced

nonlinearity

profile.

Forthispurpose,we

poled

samples #1,

#3,

#6,

and#7,which all havea4-rim thick

germanosilicate

film but

were grown at different germane flow rates,

namely 0, 33,

50 and 90 sccm,

respectively,

so that their Ge concentrationsweredifferent

(see

Table

1).

Allfour

samples

were

poled

underidentical

conditions,

i.e.,

in airat-5 kV and 280 °C, for 10 min. The recovered

nonlinearity profiles

for these

samples

are shown in

Fig.

1(b).

The

profiles

exhibitthesame overall

shape

asthe

previous

samples (see Fig. 1(a)).

The

peak

d33

coefficients ofsamples #1, #6 and #7 are 0.54, 0.78 and 0.81 pm/V,

respectively.

This

investigation

shows that the

highest

peak d33

coefficient

(1.6

pm/V)

isachieved foragermaneflow of 33 sccm

(sample

#3). However,

a

higher

germaneflowrate

produces

a

higher

Geconcentrationand thusa

higher

,j3)

(seefourth column of Table

1,

wherethe listed23)values

were calculated fromthemeasured

dispersion

curves ofthe

samples),

sobasedonthisargumentalone we expect thatthe

highest peak d33

shouldoccur atthe

highest

flowrate.The factthatthe

peak

d33

coeffilcient is maximum in

the 33-sccm

sample

suggests that the built-in field

drops

at

higher

Ge flow rates. We

mostly

relate these observations to an increase in the film electrical

conductivity

as the Ge concentration

increased,

which has been

previously

confirmed.[7]

On the other

hand,

thebuilt-in field ofthe 33-sccm

sample

is

higher

than thatofthe

0-sccm

sample (pure

SiOi),

although

thelatter hasalowerelectrical

conductivity.

This

points

outthatthere shouldbe

an

optimum

electrical

conductivity

range foragiven setof

poling

conditions. This

hypothesis

is supported by the observationthatundersimilar

poling

conditions, Suprasil,

which contains much less

impurity

thanInfrasil and thus hasalower

conductivity,

develops

abuilt-in field

nearly

oneorder ofmagnitudelower thanInfrasil.[1]

Finally,

weinvestigatedtheeffect of the film thickness onthe induced

nonlinearity

profile. Forthispurpose, we

poled sample #5,

grownat a33-sccm germaneflowrate toafinalthickness of2

gm,

for 10min.

Figure

l(c) shows

the

nonlinearity profile

recovered forthis

sample.

For

comparison,

the

nonlinearity

profile

ofsample #3, whichwas grownatthesame flowrate and

poled

underidentical conditions but is thicker(4

pm),

is alsoshown inFig. 1(c). The two

samples

have very similar

profiles,

which was

expected

since

they

havethe same

composition

andwere

poled

under thesameconditions.However,the total

depth

ofthe

nonlinearity

is

larger

forsample#5

(A17

gm)

than for

sample

#3

('13

pm).

A

probable

explanation

for thisdifferenceis that thethinner

2-Am

germanosilicate film

in

sample

#5 acts asaweakerbarrierfor ion diffusionthanthe

4-pm

film in

sample

#3. Furthermore,webelievethat this

charge spreading

is atthe

origin

of the weaker

peak

d33

coefficient in sample#5 (1.02

pm/V

vs. 1.6

pm/V

in

sample #3).

4.

Summary

We have

reported

adetailed

study

of the

nonlinearity

profileofthermalpoledgermanosilicate films grown on

fused-silica substrates

by

PECVD. This

study

sheds a better

understanding

on the physics of thermal poling in

germanosilicate

films. Inferred

profiles

all exhibita

sharp

peak-0.5

gm

beneaththe anode surface,followed by a weaker

pedestal

of

roughly

constant

amplitude

downtoa

depth

of13-16pm. Comparedto

thermally

poled undoped

silica,

they

are shallower and do not

exhibit

a

sign

reversal, which indicates that the germanosilicate film

significantly

slows down the

injection

ofpositiveions from air into theglass.Afteroptimizing the Ge concentration,

thefilm

thickness,

andthe

poling

time,

weobtainedarecord

peak d33

coefficient of 1.6

pmn/V

inthe sample grown at

a33-sccm germane flowrate(-20

mole%

GeO2) and poled for 10min. Combinedwith the low

propagation

loss of these

films,

thisenhanced

nonlinearity

makespoledgermanosilicate filmsapromising candidate for planar

electro-optic

devices.

[1]Y.Quiquempois,P.Niay,M.Douay,andB.Poumellec,"Advances inpolingandpermanently induced phenomena in silica-basedglasses,'

CurrentOpinioninSolid State & Materials Science7,89-95(2003)

[2]A.Ozean,M.J. F.Digonnet,andG. S.Kino,"Iterativeprocessingofsecond-orderopticalnonlinearitydepth profiles," Opt. Express 12, 3367-3376(2004),http://www.opticsexpress.org/abstract.cfrn?URI=OPEX-12-15-3367

[3]F.Ay,A.Aydinli,andS.Agan"Low-lossas-growngermanosilicatelayers foroptical waveguides," Appl. Phys. Lett. 83, 4743-4745(2003)

[4]P. D.Maker,R.W.Terhune,M.Nisenhoff,andC. M.Savage,"Effectsofdispersionandfocusing on production of opticalharmonics,"Phys. Rev. Lett.8,21-22(1962)

[5]A.Ozcan, M. J. F.Digonnet,andG.S. Kino,"Cylinder-assistedMaker-fringe technique," Electron. Lett. 39, 1834-1836 (2003)

[6]D.Faccio,A.Busacca,D.W. J.Harwood,G.Bonfrate,V.Pruneri,and P.G.Kazansky, "Effect of core-cladding interface on thermal poling ofgermanosilicate optical waveguides,"Opt.Comm.196,187-190(2001)

[7]R.T.Crosswell,A.Reisman,D.L.Simpson,D.Temple,andC. K.Williams,"Planarizationprocessesand applications:III.As-deposited and annealed filmproperties,"J.Electrochem. Soc.147,1513-1524(2000)