MODULATION OF BALLISTIC PHONON FLUXES IN Ge BY A He GAS FILM AT A SOLID/SUPERFLUID INTERFACE

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MODULATION OF BALLISTIC PHONON FLUXES IN Ge BY A He GAS FILM AT A

SOLID/SUPERFLUID INTERFACE

M. Greenstein, J. Wolfe

To cite this version:

M. Greenstein, J. Wolfe. MODULATION OF BALLISTIC PHONON FLUXES IN Ge BY A He GAS

FILM AT A SOLID/SUPERFLUID INTERFACE. Journal de Physique Colloques, 1981, 42 (C6),

pp.C6-274-C6-276. �10.1051/jphyscol:1981680�. �jpa-00221616�

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JOURNAL DE PHYSIQUE

CoZZoque C6, suppZe'ment au n012, Tome 42, de'cembre 1981

MODULATION OF BALLISTIC PHONON FLUXES IN Ge BY

A

He GAS FILM AT

A

SOLID/

SUPERFLUID INTERFACE

M. G r e e n s t e i n and J.P. Wolfe

Department of Physics and Materials Research Laboratory, University o f IZZinois a t Urbana-Champaign, U r b m , I L 61801, U. S. A.

Abstract.-A He gas bubble may b e formed a t t h e s u r f a c e of a sample which i s immersed i n a s u p e r f l u i d He c o o l i n g b a t h and s u b j e c t t o i n t e n s e o p t i c a l e x c i - t a t i o n . We have d i s c o v e r e d t h a t t h i s bubble o s c i l l a t e s a t 1-10 KHz, under s t e a d y s t a t e o p t i c a l e x c i t a t i o n , and t h i s l e a d s t o a modulation of t h e b a l - l i s t i c phonon f l u x i n t e n s i t y i n t o t h e sample.

Focused l a s e r e x c i t a t i o n i s commonly used t o g e n e r a t e f r e e c a r r i e r s and phonons i n c r y s t a l s a t low temperatures. For samples immersed i n s u p e r f l u i d He and under high o p t i c a l e x c i t a t i o n d e n s i t y

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100 w/cm2), a s m a l l He g a s f i l m o r bubble may b e formed a t t h e e x c i t a t i o n p o i n t . The e x i s t e n c e of He g a s f i l m s c r e a t e d by ohmic h e a t i n g h a s been p r e v i o u s l y n o t e d [ I ] . This paper d e a l s w i t h how t h e photo-

~ r o d u c e d gas bubble modifies t h e b a l l i s t i c phonon f l u x i n t o t h e sample. We have observed s i m i l a r bubbles on Ge, S i , A t , Au, b u t h e r e we c o n c e n t r a t e on pure c r y s t a l - l i n e Ge. We have used time r e s o l v e d l i g h t s c a t t e r i n g , phonon f l u x d e t e c t i o n , and i n f r a r e d luminescence imaging t o s t u d y t h i s He Eas bubble, i t s modulation of t h e b a l l i s t i c phonon f l u x and t h e e f f e c t of t h e bubble on t h e t r a n s p o r t of e l e c t r o n - h o l e d r o p l e t s i n Ge.

The most i n t r i g u i n g a s p e c t of t h e bubble i s i t s tendency t o o s c i l l a t e i n s i z e under s t e a d y s t a t e e x c i t a t i o n c o n d i t i o n s . The dynamic p r o p e r t i e s of t h e bubble may be exposed w i t h a simple time r e s o l v e d l i g h t s c a t t e r i n g technique. An ~ r + l a s e r

(51451) i s used t o c r e a t e t h e bubble, w h i l e t h e l i g h t from a low power He Ne probe l a s e r (6328i) i s s c a t t e r e d o f f of t h e bubble. The s c a t t e r e d l i g h t i s d e t e c t e d with a f a s t photodiode. The o s c i l l a t i o n of t h e bubble i s shown i n Fig. 1 a s a f u n c t i o n of i n c i d e n t Ar+ l a s e r Dower f o r focused (100pm) e x c i t a t i o n a t T = 1 . 7 4 K. Here, t h e ~ r + l a s e r i s chooned a t 1 KHz t o synchronize t h e bubble o s c i l l a t i o n s f o r d i s p l a y and t o show t h e r e l a t i v e magnitude of t h e o s c i l l a t o r y e f f e c t on t h e s c a t t e r e d l i g h t s i g n a l . I n f a c t , such o s c i l l a t i o n s a r e a l s o observed f o r s t e a d y s t a t e Ar+ e x c i - t a t i o n .

The maximum bubble diameter may be measured by scanning t h e He Ne probe l a s e r . The r e l a t i o n s h i p between o s c i l l a t i o n frequency and bubble diameter was o b t a i n e d a s a f u n c t i o n of Ar+ e x c i t a t i o n power, a s shown i n Fig. 2. The bubble frequency de- pends on diameter a s v D - ~ ' ~ ; t h e r e f o r e l a r g e r bubbles o s c i l l a t e more slowly.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981680

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TIME (ms) DIAMETER (mmf

Fig. 1 : Power dependence of t h e b u b b l e F i g . 2 : O s c i l l a t i o n f r e q u e n c y a s a func- o s c i l l a t i o n f r e q u e n c y a t T = 1.74 K. t i o n o f b u b b l e d i a m e t e r .

What is t h e c a u s e o f t h i s o s c i l l a t i o n ? The dependence o f

v

on D a r g u e s a g a i n s t s i m p l e c a p i l l a r y modes f o r which v % D - ~ ' ~ . To r e s o l v e t h i s i s s u e we have d e v i s e d a s t r o b o s c o p i c imaging t e c h n i q u e ( t o be r e p o r t e d e l s e w h e r e ) t h a t r e v e a l s a g r a d u a l growth and sudden c o l l a p s e o f t h e b u b b l e d i a m e t e r a s a f u n c t i o n of time.

We t h e r e f o r e conclude t h a t t h e o s c i l l a t i o n s a r i s e from a m e c h a n i c a l i n s t a b i l i t y of t h e b u b b l e i n t h e s u p e r f l u i d b a t h .

O p t i c a l e x c i t a t i o n of Ge a t low t e m p e r a t u r e s produces a b a l l i s t i c phonon f l u x which can b e d e t e c t e d w i t h a s u p e r c o n d u c t i n g b o l o m e t e r . To measure t h e e f f e c t of t h e bubble o n t h i s f l u x , two c o i n c i d e n t l a s e r beams are used: A cw ~ r + l a s e r c r e - a t e s t h e b u b b l e , and a Q-switched YAG: Nd l a s e r p r o v i d e s a r e f e r e n c e (57W, 250 n s ) h e a t p u l s e f o r comparison of i n t e n s i t y . The d e t e c t e d phonon s i g n a l from t h i s ex- periment i s shown i n Fig. 3. The i n s e t shows t h e h e a t p u l s e t h a t s e t s t h e i n t e n s i - t y s c a l e . The phonon f l u x i n t e n s i t y m o d u l a t i o n s caused by t h e b u b b l e a r e observed i n t h e t a i l o f t h e h e a t p u l s e . These m o d u l a t i o n s a r e about 1% o f t h e h e a t p u l s e i n t e n s i t y .

O p t i c a l e x c i t a t i o n o f Ge a t l i q u i d He t e m p e r a t u r e s a l s o p r o d u c e s a c l o u d of e l e c t r o n - h o l e d r o p l e t s . The s i z e and s h a p e of t h e d r o p l e t c l o u d i n Ge a r e d e t e r - mined by a "phonon wind" produced i n d i r e c t l y by o ~ t i c a l e x c i t a t i o n 121.

A m o d u l a t i o n o f t h e e l e c t r o n - h o l e d r o p l e t c l o u d s i z e by t h e b u b b l e i s o b s e r v e d i n a s p a t i a l p r o f i l e of t h e cloud. The c l o u d p r o f i l e i s r e c o r d e d by p a s s i n g a f o c u s e d i n f r a r e d image of t h e r e c o m b i n a t i o n luminescence (X = 1.751.1m) a c r o s s t h e s p e c t r o m e t e r s l i t . By d e f o c u s i n g t h e laser, t h e e x c i t a t i o n d e n s i t y can b e reduced below t h e 100W/cm o p t i c a l power d e n s i t y t h r e s h o l d f o r t h e h u b b l e . 2 A h y s t e r e s i s i s observed i n t h e power d e n s i t y r e q u i r e d f o r t h e b u b b l e t o come on and t o go o f f .

T h i s h y s t e r e s i s e f f e c t i s used t o i s o l a t e t h e e f f e c t of t h e b u b b l e on t h e c l o u d s i z e f o r a g i v e n o p t i c a l power. F i g u r e 4a shows f o r r e f e r e n c e a c l o u d pro- f i l e f o r f o c u s e d (100pm) e x c i t a t i o n . The remaining t r a c e s a r e f o r a

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JOURNAL DE PHYSIQUE

5 0 0 ~ e x c i t a t i o n s n o t diameter. Trace b ) i s o b t a i n e d i n t h e h y s t e r e s i s r e g i o n w i t h t h e bubble on. Trace c ) i s t h e n o b t a i n e d a f t e r momentarily b l o c k i n g t h e l a s e r t o remove t h e bubble. The cloud D e n e t r a t e s 1.4 times as f a r i n t o t h e s a m ~ l e w i t h t h e bubble p r e s e n t . The s o a t i a l l y i n t e g r a t e d t o t a l luminescence remains unchanged, i n d i c a t i n g t h a t t h i s i s t r u l y a s p a t i a l r e d i s t r i b u t i o n . We conclude t h a t t h e bubble reduces t h e phonon t r a n s m i s s i o n i n t o t h e b a t h , i n c r e a s i n g t h e f l u x r e f l e c t e d back i n t o t h e sample and t h u s i n c r e a s i n g t h e t o t a l phonon f l u x .

I n Fig. 4 , t r a c e s a ) , b ) , c ) were re- corded slowly w i t h a 0.15s time c o n s t a n t , s o no o s c i l l a t o r y phenomena was observed. I n Fig. 4d) we s e e t h a t t h e cloud shape does a c t u a l l y o s c i l l a t e i n time, synchronous w i t h t h e bubble. Here t h e cloud p r o f i l e i s d i g i - t i z e d i n one quick sweep w i t h a lOOys time c o n s t a n t . T h i s i s a novel demonstration of t h e fundamental r o l e t h a t b a l l i s t i c phonons p l a y i n t h e s p a t i a l d i s t r i b u t i o n of e l e c t r o n - h o l e d r o p l e t s .

This work was supuorted by t h e N a t i o n a l Science Foundation under Grant DMR-80-24000.

REFERENCES

1. W. Kappus and 0. E e i s , J. Appl. Phys.

g,

1947 (1972).

2. M. G r e e n s t e i n and J. P. Wolfe, Phys. Rev.

L e t t .

3,

715 (1978).

TIME (msl

Fig. 3 : Phonon f l u x modulation by by t h e bubble.

Fig. 4 : Modulation of t h e drop- l e t cloud shape by t h e bubble.