BACKWARD WAVE PHONON ECHOES AT 17 GHz IN SINGLE CRYSTALS OF LiTaO3, LiNbO3 AND Bi12SiO20

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BACKWARD WAVE PHONON ECHOES AT 17 GHz IN SINGLE CRYSTALS OF LiTaO3, LiNbO3 AND

Bi12SiO20

D. Meredith, J. Pritchard, J. Wigmore

To cite this version:

D. Meredith, J. Pritchard, J. Wigmore. BACKWARD WAVE PHONON ECHOES AT 17 GHz IN

SINGLE CRYSTALS OF LiTaO3, LiNbO3 AND Bi12SiO20. Journal de Physique Colloques, 1981,

42 (C6), pp.C6-870-C6-872. �10.1051/jphyscol:19816258�. �jpa-00221346�

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JOURNAL DE PHYSIQUE

CoZZoque

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12,

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4 2 ,

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1901 page ^C6-870

BACKWARD WAVE PHONON ECHOES A T

1 7

GHz I N S I N G L E CRYSTALS OF L i T a O j , L i N b 0 3 AND B i 1 2 S i 0 2 0

D . J . M e r e d i t h , J.A. P r i t c h a r d and J . K . Wigmore

University

of

Lamaster,

U . K .

A b s t r a c t . - From measurements of t h e power and t i m e dependence o f U-U 2 p u l s e e c h o e s a t 17 GHz, we i n f e r r e d t h a t , a t low powers, a n i n t r i n s i c n o n - l i n e a r i t y y s 2 ~ ' was r e s p o n s i b l e f o r t h e i r g e n e r a t i o n i n LiTaOB and LiNbOg. However, this model c o u l d n o t e x p l a i n e i t h e r t h e h i g h power d a t a i n t h e s e m a t e r i a l s , o r any o f t h e r e s u l t s i n B i 1 2 S i 0 2 0 -

I t i s now w e l l e s t a b l i s h e d t h a t t h e r e a r e s e v e r a l d i f f e r e n t m i c r o s c o p i c mechan- i s m s which a r e a l l c a p a b l e of p r o d u c i n g phonon e c h o e s , depending on t h e n a t u r e of t h e m a t e r i a l and t h e form o f t h e specimen. Thus, i t h a s been s u g g e s t e d t h a t , i n macro- s c o p i c s i n g l e c r y s t a l s o f non-centrosymmetric m a t e r i a l s , t h e n o n - l i n e a r i t y r e s p o n s i b l e f o r backward wave phonon e c h o e s c o u l d b e an i n t r i n s i c f e a t u r e o f t h e c r y s t a l l a t t i c e

o r i t c o u l d b e due t o d e f e c t c e n t r e s t h a t a r e s t r o n g l y c o u p l e d t o t h e phonons 2

.

I n an a t t e m p t t o u n d e r s t a n d b o t h t h e d e t a i l s o f t h e s e mechanisms and t h e c o n d i t i o n s under which t h e y might b e i m p o r t a n t , we have been s t u d y i n g U-w backward wave e c h o e s i n l i t h i u m t a n t a l a t e ( L i T a 0 3 ) , l i t h i u m n i o b a t e (LiNb03) and bismuth s i l i c o n o x i d e (Bi1 ~ S i 0 ~ ~ ) .

I n t h e i n t r i n s i c t h e o r y of t h e o r i g i n o f such e c h o e s L , phonons and p h o t o n s , b o t h a t t h e same a n g u l a r f r e q u e n c y , w , a r e c o u p l e d t h r o u g h t h e term y s ' ~ ~ i n t h e e x p a n s i o n o f t h e c r y s t a l f r e e e n e r g y a s powers o f strain,^, and e l e c t r i c f i e l d , E. The i n t e n - s i t y , I of t h e e c h o i s found t o be p r o p o r t i o n a l t o y2w21 I ~ , where Y i s t h e c o u p l i n g

e ' u m

p a r a m e t e r , I U t h e i n t e n s i t y o f t h e f o r w a r d t r a v e l l i n g u l t r a s o n i c wave, and I t h e m i n t e n s i t y of t h e (microwave) photon p u l s e . I3ajak3 l a t e r showed t h a t t h i s t y p e o f u-U phonon e c h o i s a s p e c i a l c a s e of t h e more g e n e r a l i n t e r a c t i o n i n v o l v i n g 2 phonons and N p h o t o n s , c o u p l e d weakly t h r o u g h a n o n - l i n e a r i t y of t h e p u r e c r y s t a l l a t t i c e .

By c o n t r a s t , S h i r e n e t a12 s u g g e s t e d t h a t t h e p r e s e n c e i n t h e c r y s t a l of s t r o n g l y c o u p l e d i m p u r i t i e s m i g h t g i v e r i s e t o n o n - l i n e a r d i s p e r s i o n and a b s o r p t i o n which c o u l d b e e q u a l l y e f f e c t i v e i n c o u p l i n g phonons a n d p h o t o n s . The dependence o f t h e e c h o i n t e n s i t y on I and I would n o t be t h e s i m p l e form p r e d i c t e d by F e d d e r s and Lu, b u t

m

would be a c o m p l i c a t e d e x p r e s s i o n depending on t h e p r e c i s e c h a r a c t e r i s t i c s of t h e d e f e c t . I n a d d i t i o n , a t h r e e - p u l s e , o r " s t o r a g e " e c h o , might b e o b s e r v e d .

I n o u r e x p e r i m e n t s t h e specimen was c o n t a i n e d wholly w i t h i n t h e microwave c a v i t y , which was immersed d i r e c t l y i n l i q u i d helium a t 1.25 K t o minimise a c o u s t i c a t t e n u a - t i o n i n t h e sample. The two e x c i t i n g p u l s e s had l e n g t h s t y p i c a l l y o f 200 n s e c and a s e p a r a t i o n o f 1-10 p s e c . Microwave powers up t o 1 k W c o u l d be o b t a i n e d from a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816258

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Echo 0 i n t e n s i t y

(dB)

0 5 10

microwave power (dB)

F i g u r e 1. Dependence o f t h e e c h o i n t e n s i t y o n t h e power o f the e x c i t i n g p u l s e s , ( a ) LiTaO3 ( P Z ) , ( b ) LiNb03 ( P I ) ,

( c ) B i 1 2 S i O z 0 ( P 2 )

-

Echo 0

i n t e n s i t y (dB)

0 5 10 m i c r o s e c o n d s

F i g u r e 2. Dependence o f t h e e c h o i n t e n s i t y on the t i m e s e p a r a t i o n o f t h e two e x c i t i n g p u l s e s ,

( a ) LiTaO3, ( b ) B i i z S i O z o -

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C6-872 JOURNAL DE PHYSIQUE

L i t t o n 6 2 4 t r a v e l l i n g wave t u b e a m p l i f i e r , and a s e c o n d , low n o i s e TWTA (MO v a l v e TWJ30) p r e a m p l i f i e d t h e e c h o b e f o r e i t s d e t e c t i o n w i t h a s e l f o s c i l l a t i n g Gunn d i o d e m i x e r .

The i n t e n s i t y o f t h e e c h o was measured a s a f u n c t i o n o f t h e power l e v e l s o f t h e two e x c i t i n g p u l s e s , which d e t e r m i n e d I and I m

.

T y p i c a l r e s u l t s a r e shown i n f i g u r e 1. At low powers and w i t h e x c i t i n g p u l s e s n o l o n g e r t h a n a b o u t 200 n s e c , LiTa03 f i t t e d t h e i n t r i n s i c model v e r y w e l l . Below a s l i g h t f l a t t e n i n g a t h i g h powers, t h e dependence on Im was c l o s e l y q u a d r a t i c , and t h a t on I ( n o t shown) l i n e a r .

u

I n a d d i t i o n , t h e p u l s e s h a p e o f t h e e c h o was t h e p r e d i c t e d t r a p e z o i d . We e s t i m a t e d the c o u p l i n g c o n s t a n t y t o b e a b o u t

lo-'

F

6';

more d e t a i l s a r e p u b l i s h e d e l s e - where.

4

LiNb03 behaved s i m i l a r l y , b u t w i t h a c o n s i d e r a b l y b r o a d e r r e g i o n of c u r v a t u r e . I n d e e d , t h e dependence o f I on I h a r d l y r e a c h e d t h e l i n e a r regime i n t h e r a n g e o f powers t h a t were a c c e s s i b l e ( f i g u r e 1 ) . The o r i g i n of t h e c u r v a t u r e is n o t under- s t o o d s i n c e such power l e v e l s s h o u l d s t i l l b e i n t h e s m a l l s i g n a l l i m i t c o n s i d e r e d by F e d d e r s and Lu. I t i s p o s s i b l e t h a t some a d d i t i o n a l c h a n n e l which c o u p l e s

o

p h o t o n s and w phonons and removes e n e r g y from t h e w e c h o , c o u l d e x i s t w i t h i n B a j a k ' s model.

The e c h o o b t a i n e d i n B i l Z S i O had a t o t a l l y d i f f e r e n t power dependence 2 0

( f i g u r e 1) which c o u l d n o t b e f i t t e d t o any s i m p l e f u n c t i o n a l form. The dependence of t h e e c h o on t h e p u l s e s e p a r a t i o n a l s o showed anomalous b e h a v i o u r ( f i g u r e 2 ) . U n l i k e t h o s e f o r LiTaOB and LiNbOs, t h e c u r v e d i d n o t f i t a s i n g l e e x p o n e n t i a l b u t a p p e a r e d t o c o n s i s t of a r a p l d decay superimposed on a much more g r a d u a l s l o p e . S i n c e , even q u a l i t a t i v e l y , Bi12Si02,, b e h a v e s i n s o d i f f e r e n t a manner t o LiTa03 and LiNb03, it i s t e m p t i n g t o s u g g e s t t h a t o u r sample i s b e h a v i n g e x t r i n s i c a l l y . However,no 3 p u l s e e c h o h a s y e t b e e n o b s e r v e d .

W e a r e g r a t e f u l t o t h e S c i e n c e R e s e a r c h C o u n c i l (London) f o r s u p p o r t .

1. P.A.Fedders and E.Y.C.Lu, A p p . P h y s . L e t t e r s

2

502 (1973)

2 . N.S.Shiren, W-Arnold, and T.G.Kazyaka, P h y s . R e v . L e t t e r s

39

239 (1977) 3 . I . L . B a j a k , P h y s . R e v . G 2405 (1978)

4. D.J.Meredith, J - A - P r i t c h a r d and J.K.Wigmore ( t o b e p u b l i s h e d )