THE DECREASE OF OPTOACOUSTIC SIGNALS INDUCED BY THE ABSORPTION OF MEDIUM INTENSITY LASER PULSES ON SOLIDS

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THE DECREASE OF OPTOACOUSTIC SIGNALS INDUCED BY THE ABSORPTION OF MEDIUM

INTENSITY LASER PULSES ON SOLIDS

J. Možina, J. Diaci

To cite this version:

J. Možina, J. Diaci. THE DECREASE OF OPTOACOUSTIC SIGNALS INDUCED BY THE AB-

SORPTION OF MEDIUM INTENSITY LASER PULSES ON SOLIDS. Journal de Physique Collo-

ques, 1983, 44 (C6), pp.C6-73-C6-77. �10.1051/jphyscol:1983611�. �jpa-00223170�

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J O U R N A L DE PHYSIQUE

Colloque C6, suppldment au nOIO, Tome 44, octobre 1983 page C6- 73

THE DECREASE OF OPTOACOUSTIC SIGNALS INDUCED BY THE ABSORPTION OF MEDIUM INTENSITY LASER PULSES ON SOL1 DS

J. Mozina and J . Diaci

Faculty of Mechanical Engineering, University o f Ljubljana, 61000 Ljubljana, POB 394, Yugoslavia

~ 6 s u m 6 - Les mesures simultanges d e s signaux optoacoustiques dans l e s o l i d e abporbant e t dans l e gaz qui l ' e n t o u r e s o n t accomplies pour e t u d i e r l e s m o d i f i c a t i o n s d e l a s u r f a c e caus6es p a r l ' a b s o r p t i o n d e s quelques impulsions i n i t i a l e s du l a s e r de f l u e n c e O,4 J/cm2 e t de dure'e 20 ns.

Abstract - Simultaneous measurements of o p t o a c o u s t i c s i g n a l s i n t h e absorbing s o l i d and i n t h e surrounding g a s have been performed i n o r d e r t o s t u d y s u r f a c e m o d i f i c a t i o n s on v a r i o u s s o l i d s , caused by t e a b s o r p t i o n of t h e f i r s t few l a s e r p u l s e s 9

of f l u e n c e 0.4 J/cm and d u r a t i o n 20 ns.

I n t r o d u c t i o n - Using l a s e r a s t h e e x c i t a t i o n source i n pulsed opto- a c o u s t i c spectroscopy, it may r e a d i l y happen t h a t t h e incoming l i g h t beam i n t e n s i t y exceeds t h e damage t h r e s h o l d of t h e i r r a d i a t e d ma- t e r i a l , t h u s l e a d i n g t o t h e c o n s i d e r a b l e i n c r e a s e of t h e optoacous- t i c s i g n a l / I / . It i s w e l l known however, t h a t t h e r e e x i s t s e v e r a l o t h e r s u r f a c e m o d i f i c a t i o n s induced by l a s e r p u l s e i n t e n s i t i e s w e l l below t h e v i s i b l e damage t h r e s h o l d , which a r e d i f f i c u l t p e r c e i v a b l e by o t h e r methods /2/. The aim of t h i s a r t i c l e i s t o demonstrate t h e p o s s i b i l i t y of i n d i c a t i n g t h e s e m o d i f i c a t i o n s by observing t h e chan- ges of

OA

response induced by t h e a b s o r p t i o n of t h e f i r s t few l a s e r s h o t s i n v a r i o u s s o l i d s u r f a c e s . I n o r d e r t o g e t more exhaustive d a t a on t h e complex source mechanisms, i t i s convenient t o measure simul- t a n e o u s l y t h e t r a n s i e n t

OA

waves i n t h e absorbing s o l i d and i n t h e surrounding g a s / 3/ .

A

a r a t u s - I n t h e " f r e e " a i r geometry experimental arrangement

6k-m a Q-switched Nd-YAG l a s e r was used t o produce s i n g l e pul- s e s w i t h energy of 0.1 J , d u r a t i o n 20 n s and diameter of 5 21 mm.

This provided t h e f l u e n c e of approximately 0.4 J/crn2, which i s below t h e damage threshod f o r most m e t a l s

/2/.

T r a n s i e n t

OA

waves i n t h e s o l i d were d e t e c t e d by t h e 200 kHz resonant a c o u s t i c emission PZT t r a n s d u c e r bonded t o t h e specimens - p l a t e s of d i f f e r e n t dimensions.

The

OA

waves i n t h e surrounding g a s were d e t e c t e d by B&K 4133 micro- phone placed i n f r o n t of t h e absorbing s u r f a c e . S i g n a l s of b o t h k i n d s were amplified and f e d t o a d i g i t a l t r a n s i e n t r e c o r d e r t r i g g e r e d by a f a s t photodiode. The d i g i t i z e d d a t a was t h e n s t o r e d v i a p r o c e s s computer f o r l a t e r a n a l y s i s .

Eaaeriments - Various metals and g l a s s e s were chosen f o r t h e experi- ments. For each m a t e r i a l s e v e r a l s e r i e s of l a s e r s h o t s were p e r f o r - med on as-received, on grinded and on p o l i s h e d and c a r e f u l l y cleaned

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983611

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C6-74 JOURNAL DE PHYSIQUE

s u r f a c e s . Except on as-received s u r f a c e s no v i s i b l e d e t e r i o r a t i o n of t h e i r r a d i a t e d specimens could be observed.

S e v e r a l couples of t r a n s i e n t o p t o a c o u s t i c responses d e t e c t e d by microphone and PZT t r a n s d u c e r simultaneously a r e shown i n Fig. 2.

The waveform of t h e microphone s i g n a l i s found t o be p r a c t i c a l l y c o n s t a n t i n a broad amplitude i n t e r v a l from 2 mV (AR t h i n f i l m ) t o

5

V ( A l

p l a t e ) . Since t h e d u r a t i o n of t h e l a s e r p u l s e and correspond- i n g a c o u s t i c p u l s e i n a gas i s v e r y s h o r t i n comparison w i t h t h e resonance p e r i o d of t h e microphone, t h e waveform i s mainly d e t e r - mined by t h e impulse response f u n c t i o n of t h e microphone. By ana- logy t o t h e simple b a l l i s t i c pendulum t h e amplitude of t h e micro- phone s i g n a l i s assumed t o be p r o p o r t i o n a l t o t h e l i n e a r wave momen- tum c a r r i e d by t h e o p t o a c o u s t i c wave i n a gas.

The s t r u c t u r e of t h e PZT s i g n a l i s c o n s i d e r a b l y more complex. It i s i n f l u e n c e d not o n l y by r e s o n a n t n a t u r e of t h e t r a n s d u c e r , b u t a l s o by t h e e x c i t a t i o n of d i f f e r e n t wave modes ( l o n g i t u d i n a l , s h e a r , s u r f a c e ) and by numerous r e f l e x i o n s and mode conversions of t h e o r i g i n a l o p t o a c o u s t i c wave i n s i d e t h e s o l i d specimen. PZT responses on d i f f e r e n t specimens show g r e a t d i f f e r e n c e s i n waveform s t r u c t u r e , while t h e i r amplitudes cover r e l a t i v e l y narrow i n t e r v a l fromJmV t o lomV a t t h e t r a n s d u c e r output.

Regarding t h e above r e s t r i c t i o n s it i s v e r y d i f f i c u l t t o compare t h e r e s u l t s of s i n g l e s h o t experiments on v a r i o u s specimens. The s i t u - a t i o n i s b e t t e r , however, i f s u c c e s s i v e o p t o a c o u s t i c responses a r e observed d u r i n g m u l t i p l e l a s e r s h o t s on t h e same s u r f a c e a r e a of t h e specmen. Since t h e s e n s i t i v i t y of t h e experimental system i s rema- i n e d c o n s t a n t i n t h i s c a s e , any change of t h e o p t o a c o u s t i c response could be used a s t h e evidence of l a s e r induced s u r f a c e m o d i f i c a t i - ons.

Common c h a r a c t e r i s t i s t o a l l experiments i n which l a s e r p u l s e s of 2

t h e f l u e n c e 0.4 J/cm were used, was t h e s i g n i f i c a n t d e c r e a s e of t h e simultaneously measured o p t o a c o u s t i c response i n gas and s o l i d a f t e r t h e first few l a s e r s h o t s . For t h e most p a r t t h e amplitude of t h e second response was found t o

be

l e s s t h a n l o % of t h e corresponding i n i t i a l value. Shown i n Fig. 3 a r e t h e simultaneous o p t o a c o u s t i c responses a s obtained by t h e f i r s t f o u r l a s e r s h o t s on grinded s t e e l s u r f a c e . Comparable w i t h t h e decrease of t h e amplitude of t h e micro- phone s i g n a l which i s n o t changing i t s waveform, i s only t h e decre- ase of t h e f i r s t peak i n a complex b u r s t s i g n a l of PZT transducer.

Time i n t e r v a l s between s u c c e s s i v e r e f l e c t i o n s of this wave confirm t h a t it corresponds t o t h e l o n g i t u d i n a l component of t h e optoacous- t i c wave. It i s e v i d e n t t h a t t h e l o n g i t u d i n a l component i s much more s e n s i t i v e t o t h e l a s e r induced s u r f a c e m o d i f i c a t i o n s t h a n t h e r e s t of t h e o p t o a c o u s t i c s i g n a l , which corresponds t o t h e s h e a r and sur- f a c e waves.

S e v e r a l mechanisms, such a s s u r f a c e c l e a n i n , adsorbed t h i n l i q u i d l a y e r evaporation, annealing o r some o t h e r ? t o u s unknown) phenome- na could l e a d t o s u r f a c e m o d i f i c a t i o n s r e l a t e d t o t h e changes of o p t o a c o u s t i c response. D i f f e r e n t behaviour of l o n g i t u d i n a l and s h e a r o p t o a c o u s t i c wave component c o a be u s e f u l i n seeking adequa- t e t h e o r e t i c a l e x p l a n a t i o n of t h e above d e s c r i b e d phenomena.

Discussion - I n summary,simultaneous measurements of t h e " f r e e n a i r

and s o l i d sample o p t o a c o u s t i c response have been c a r r i e d out on

v a r i o u s substances,Considerable d e c r e a s e of microphone s i g n a l and

t h e changes of PZT response have been found t o be c l o s e l y r e l a t e d

t o t h e s u r f a c e a l t e r a t i o n s induced by l a s e r p u l s e s of f l u e n c e

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below t h e s i n g l e - s h o t s u r f a c e damage t h r e s h o l d . The e x a c t mechanism of t h e observed phenomena h a s n o t been determined.

Though uncomplete, t h e experiments i n d i c a t e t h e u t i l i t y of s i m u l t a - neous d e t e c t i o n of t h e " f r e e " a i r and s o l i d sample o p t o a c o u s t i c s i g n a l s f o r t h e s t u d y of l a s e r induced s u r f a c e m o d i f i c a t i o n s . A d d i t i o n a l e x p e r i m e n t a l and t h e o r e t i c a l t e c h n i q u e s should be used t o e x t r a c t more d e t a i l e d i n f o r m a t i o n about t h e complex o p t o a c o u s t i c source mechanism. W e b e l i e v e t h a t t h i s would open s e v e r a l i n t e r e s t - i n g a p p l i c a t i o n s of t h e above d e s c r i b e d t e c h n i q u e i n s u r f a c e clean- n e s s monitoring and l a s e r - i n d u c e d damage t h r e s h o l d d e t e r m i n a t i o n . References

1.ROSENCWAIG

A.

e t a l . , A p p l . O p t . a (1980) 4133.

2.MUSAL H.M., i n L a s e r Induced Damage i n O p t i c a l M a t e r i a l s , Begnett e t al,,Eds.,

NBS

SP 568 (1980) 159.

3.MOZINA

J. e t a l . , i n Proc.Conf.Acoust~Emission

&

P h o t o a c o u s t i c Spectroscopy,R.W.Stephens

Ed., I O A ,

London (1981).

Fig.1 - Shematic of t h e experimental set-up f o r simultaneous

o b s e r v a t i o n of t h e " f r e e " a i r and s o l i d sample

opt o a c o u s t i c r e s p o n s e

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JOURNAL DE PHYSIQUE

Fig.2 - Simultaneous " f r e e " a i r and s o l i d sample o p t o a c o u s t i c

responses on v a r i o u s specimens (a-A1 plate,b-common

g l a s s , c - o p t i c a l t h i n f i l m on absorbing g l a s s s u b s t r a t e ,

d-Ti02 t h i n film on non-absorbing g l a s s s u b s t r a t e )

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+4mV

r

I i

f i r s t

second

f o u r t h