Raman study of planar optical waveguides in LiNbO3 and LiTaO3

Journal of

MOLECULAR STRUCTURE

ELSEVIER Journal of Molecular Structure 348 (1995) 289-292 RAMAN STUDY OF PLANAR OPTICAL WAVEGUIDES IN LiNbO 3

and

LiTag

Savatinova

I.

Institute of 72 Tzarigradsko

Solid State Chaussee

Proton-exchange

(PE)

opt ical

waveguides

Studying

post-exchange

of

properties

guide

followed. of

LiNbO

has

with

spectroscopy show

layers

“paraelectric-like”

a

used

L iTa

(APE)

decreasing

Academy Bulgaria

been

and

3

annealed

the

of

PE and

high

degree

evolution

concentration

to

APE

planar

crystals. the

proton

Sciences,

form

single

3

used

of

to

waveguides,

is

characteristics

PE

1.

technology in

Raman

vibrational

Bulgarian 1784 Sofia.

Physics, blvd.,

is the

investigate The

layers.

of

spectra

disorder

and

a

form.

INTRODUCTION Planar

waveguides

exchange

are

attractive

[l]

because

(PE).

optoelectronics technology

and

the

large

designing

and

fabricating

solution

of

some

instabilities, exists

[4]. between

nonlinear probe

refractive

is

well

the

and known

such

protonated

12.31.

structures

it

(H:LiTa03

However the

refractive in

index

electro-optic

a definite

correlation effect

would and

and

need

electro-optic

Therefore,

layers

change as

that

proton

low-temperature

reduction

scattering,

[5].

by opt its

simple

index

,

made

3

integrated

PE waveguide

losses

Raman

properties the

for

problems It

L iTa

and

3

of

of

optical

coefficients

LiNbO

in

be

and

desirable

H:LiNb03)

by

to Raman

spectroscopy. 2.

EXPERIMENT Li

and in

H NbO, and Li H Tao3 waveguides i-x x 1-x x X-cut congruent single crystals by

benzoic

layers order

acid

were to

melt

subjected

stabilize

for to

their

l-8

hrs

at

have

immersing 225-240°C.

subsiquent

annealing

properties

and

0022-2860/95/$09.50 0 1995 Elsevier Science B.V SSDI 0022-2860(95)08645-5

All rights reserved

been

to

formed the The

up reduce

to

in

Z-

substrates exchanged

T =420°C 0 the losses.

in

290 As

a

rule,

the

extraordinary change

refractive

Ane(0)s0.13

(A=633

nm),

zero.

propagate modes

in Z-

were

in the

integrated

was

slit

were

guiding

An

deecreases is

with

the

In

layer,

The

can

guided

commonly

used

lasers

focusing

the

by Ar and Kr of

a double

monochromator.

by

order

to

using

x(zy)z

localize

the

Raman scattering In

the

of

T <320°C.

After

the

dicreasing

of

at

(at

320°C)

highest is

An ,

the

The

representative

at

most

m=O

guide

anomalous

acount:

although

with

annealing,

An

gradually

1 ighte confinement the

for

y(zz)x

an

T %320°C!,

therefore

e

and

LiTaO 3'

x [2P.

E and Ai

light

from

case

at

best

spectrum

TE modes

method

of

inreases

3

practically

TM or

An on T should be taken into 8 0 H concentration, x, monotonously decreases

the

H:LiTaO

respectively.

coup1 ing

activated

registered.

dependence

3.

excited

geometry.

the

prism

index

for

being

only

step-like

surface

An*%O. 02 Ano

substrates,

the

on the

phonons

scattering modes

were

trace

of

change

a

optics.

Raman spectra type

by

with

and

3

anisotropy,

and X-cut

selected

waveguide

index

inherent

have

distribution

H:LiNbO

ordinary

this

waveguides

index

for

the

Due to

into

as-exchanged

corresponding

these

waveguides.

WAVEGUIDERAMANSPECTRA According

optic

the

the

the

[51,

coefficients

from

lowest-frequency

type

1

of

virgin

spectra

overlap

of

is the

annealings

when

scattering

strengths

and

the

in

different

appears

which

could

be used

spaced the

for

the

at

in Fig. bands

absent

is

indication

of

the

Li-H

the

to

high,

bulk

the

1. Also

an

low-temperature

changes in

different

leading

At

A strong

- from 22 spectra.

A common feature

lowest-frequency other

comes

r

IR)

obtained given.

electro-

LiTa03

Raman (and

bands.

direction.

completely

rq2 and

to

concentration

The

reduced.

and

is of

the

and

presented

(bulk)

proton of

proceed

is

lh are

to

LiNb03

in the

broadening

closely the

the

APE LiNb03

crystal

are

a

of for

cm-’

band

275

of

E vibration

temperatures

spectrum

rt3

contribution

Ai vibration

Raman spectra

annealing the

dominant

r33 and

lowest-frequency

A

of

to

bands

in

,the

band

at

pure

the at

254

spectra 70

crystal.

substitution.

cm-* This A

291

broad to

spectral

grow

Since

in

intensity

this

strong the

is

could

be

broad

band

the

of

supposed. at

of

cm-’

the

Nb-0

g

phase the

wave

could

not

to

the

g

qua1 ity

of

in

waveguides. L iTa

E

l.Raman

due

the A

spectra

vibrations,

a

800

300 WAVE

400 NUXBER.

scattering

(Ai)

of

I300

700

600 so0 CM-1

H:LiNbOJ

6

X-cut series

of

APE shown

are

3

stretching

protons.

could

obtain

cracks

of

incorporated

seen

Ai-spec

H:LiTaOJ

be

not

is

0

100

Figure

tra

line

.

crystal.

Good

of

bulk

B

for

isomorphous

to

amount

cm-’

The

690

So

layer

pure

large

632

octahedral

LiNbOa.

be

the

of

paraelectric

-guide

a

region

characteristic

the

around

at

distortion

niobium

is

structure

Fig. 2. The band at 70 --i rather cm . although weak,

can

still

in

be

seen

the

bulk

spectrum.

Approaching

Ta%3 20°C

(from

sides ure bulk

of

the

temperat-

scale).some bands

both of

the

200

(especially

600

400 frequency,

cm- 1

Figure 2.Raman spectra (El of PE (a) and APE H:LiTa03 at Ta: b-2653 c-295, d-320, e-350, f-400 and g-420 C, h-bulk. the

lowest-frequency

ones

592

cm-‘)

decrease

the

intensity

last

two

gradually of

becoming

the very

bands

at at

broad.

142,

206

but

in

intensity.

70,

380

and

also

those In

665

the

cm-’

at

315

same grows,

and

time, the

292 4.

DISCUSSION Two

types

of

changes

PE LiNbOa

and

lines

appearencz

and

Raman

LiTaO

of of

of

the

be

observed

intensity

:

scattering

reminescent

can

in

the

reduction

of

lattice-disorder highly

some

spectra

of

of

the

bulk

induced

broad

bands.

layers

(large

An*)

protonated

high-temperature

Raman

Raman

scattering

of

is

LiNbO 3

and

L iTaO

of

the

near

broad

their

bands

at

H:LiTaO 3 suggests in

direction

consists reveal

that

the the

the

positions

of

data

polar used

a structure

the

one-tenth From

the

disorder

close

the

bulk

other

in

exists in

the

part

the

scattering

into

selection

to of

the

r33 that

less

[4,81.

doubt

that

and

form

towards

deterioration

or

rules

ferroelectric

H:LiNb03

shown

exchanged

their 171

has

of

the

The

and

vibrations

all

coefficient

a high

layers. broad

idea

HNb03 one.

lead

no

in

which

III.

shifted

have

is

This

the of

cm-’

octahedra

materials.

exactly

cubic

even

380

niobium

devices

there

the

breakdown

the

and

appearence

analysis

measurements

value

side,

the

RHEED study

to

waveguide

the

slightly

should

Direct

most

of

are our

of

occupy

The

H:LiNb03

structural

not

situation

properties. in

do

Also

A paraelectric

the

[61.

of

state

but

sites.

cm-‘in

distortion

of

protons

states

690

non-polar

Li-atoms

paraelectric indicated

about

a strong

of with

paraelectric

spectral

r33

can

degree

disorder

be of

causes can

take

maxima.

ACKNOWLEDGEMENTS The

work

Bulgarian

has

been

Foundation

done

under

of

Science.

the

Contract

no.

F-57

with

the

REFERENCES 1. 2. 3. 4. 5 6. 7 8.

C.E. Rice, J. Solid St. Chem., 64 (19861 188. C.Ziling, L.Pokrovskii, N.Terpugov, I. Savatinova, M. Kuneva S.Tonchev, M.N.Armenise, V.Passaro:J.Appl.Phys.73 (199313125 I.Savatinova, S.Tonchev, M.Kuneva, Appl. Phys.AS6 (1993) 81. M.Rottschalk, A.Rasch, W.Karthe, J.Opt.Commun., 9 (1988) 19. . I.P. Kaminow and W.D. Johnston, Phys. Rev., 160 (1967) 519. Yu.K.Voron’ko, A.B.Kudryavtsev, V.V.Osiko, A.A.Sobol, E.V. Sorokin, Sov. Phys. Solid State 29 (1987) 771. I.Savatinova, S.Tonchev, T.Popov, J.Opt. Commun. in K.Nanev, print. I.Savatinova, S.Tonchev, T.Popov, M.Armenise, I?. Liarokapis. SPIE Conference EO Laser’94, Los Angeles, USA (1994).