INSTABILITY OF LIQUID METAL SURFACES UNDER INTENSE INFRARED IRRADIATION

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INSTABILITY OF LIQUID METAL SURFACES UNDER INTENSE INFRARED IRRADIATION

F. Keilmann

To cite this version:

F. Keilmann. INSTABILITY OF LIQUID METAL SURFACES UNDER INTENSE IN- FRARED IRRADIATION. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-77-C5-78.

�10.1051/jphyscol:1983511�. �jpa-00223091�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6rnent au nOIO, Tome 44, octobre 1983 page C5-77

I N S T A B I L I T Y OF L I Q U I D METAL SURFACES UNDER INTENSE INFRARED IRRADIATION

F

.

^Keilmann

Max-PZanck-Institut fur Festkb'rperforschung, 7000 Stuttgart 80, F.R.G.

R'esumG - Les surfaces de m@taux li q u i d e s (Hg, I n , Sn, Al, Pb) d6veloppent une s t r u c t u r e pPriodique ondulGe, caus6e par une i r r a d i a t i o n i n f r a - r o u g e de haute i n t e n s i t ' e . Cet e f f e t e s t peut- Gtre i m p o r t a n t pour l ' i n t e r a c t i o n des l a s e r s avec l e s m'etaux e t plasmas.

A b s t r a c t - L i q u i d metal surfaces (Hg, I n , Sn, A l , Pb) become p e r i o d i c a l l y corrugated from t h e a c t i o n o f h i g h i n t e n s i t y i n f r a r e d r a d i a t i o n . The c o r r u g a t i o n spacing equals the i n c i d e n t wavelength i f normal incidence i s used. The c o r r u g a t i o n i s shown t o r e s o n a n t l y couple t h e i n c i d e n t f i e l d t o surface plasmons. The l a t t e r , i n t u r n , gives r i s e t o s p a t i a l l y p e r i o d i c h e a t i n g and evaporation o f the surface m a t e r i a l which r e s u l t s i n s t i m u l a t e d growth o f t h e c o r r u g a t i o n . A s i m i l a r feedback process works f o r a r b i t r a r y angle o f incidence. A t a l a t e s t a t e o f t h e i n s t a b i l i t y a t r a n s i t i o n occurs from t h e i n i t i a l l y small (Fresnel) t o a h i g h absorption. This p o i n t may be o f considerable i n t e r e s t t o l a s e r processing o f metals and plasmas.

The basic i n t e r a c t i o n mechanism i s sketched i n F i g . 1: The i n c i d e n t l a s e r wave L decays p a r a m e t r i c a l l y i n t o two e x c i t a t i o n s , SP

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t h e surface plasmon and SD - t h e s u r f a c e deformation. The l a t t e r i s a surface Rayleigh o r c a p i l l a r y wave, w i t h e s s e n t i a l l y zero frequency. It provides momentum matching f o r t h e s c a t t e r i n g o f L i n t o SP.

F i g . 1

Q u a n t i t a t i v e experiments were performed / I / w i t h l i q u i d Hg, b u t o t h e r molten metals l i k e I n , Sn, A1 and Pb showed t h e same q u a l i t a t i v e behaviour. I n s h o r t , a s i n g l e C02 l a s e r ("pump") p u l s e o f 150 ns d u r a t i o n i s used (Fig.2). D i f f r a c t i o n p a t t e r n s a r e photographed u s i n g a v i s i b l e probe l a s e r o f 3 ns d u r a t i o n . These prove t h a t t h e surface c o r r u g a t i o n p e r i o d i s s t r i c t l y equal t o t h e wavelength

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983511

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C5-78 JOURNAL DE PHYSIQUE

of t h e i n c i d e n t l a s e r (10.6 pm), w h i l e t h e o r i e n t a t i o n o f t h e r i p p l e l i n e s i s m o s t l y perpendicular t o t h e d r i v i n g e l e c t r i c f i e l d .

Fig. 2 - Experimental setup t o observe t h e s u r f a c e c o r r u g a t i o n on l i q u i d metals.

The c o r r u g a t i o n i s generated by a v e r t i c a l l y i n c i d e n t high-power C02 l a s e r beam.

The growth o f t h e c o r r u g a t i o n s can best be observed by u s i n g a c.w. v i s i b l e probe l a s e r and f a s t d e t e c t o r s . ble cannot d e t e c t any r i p p l i n g i f t h e pump fluenTe i s below 2.5 ~/cm2, w h i l e a t 5 ~ / c m 2 we o b t a i n s c a t t e r i n g s i g n a l s f o u r o r d e r s o f magnitude above noise. I n t h e l a t t e r case, t h e c o r r u g a t i o n depth reaches approximately 0.2 pm a t t h e end o f the l a s e r pulse. The growth o f t h e c o r r u g a t i o n

depth i s found t o be approximately exponential w i t h a c h a r a c t e r i s t i c t i m e constant o f 60 ns a t an i n c i d e n t i n t e n s i t y o f 3.107 ~ l c m z .

The c o r r u g a t i o n s a r e found t o gr w s t e a d i l y u n t i l a depth o f t h e o r d e r o f 1 pin i s reached. Then two i n t e r e s , t j f o t h i n g s happen. F i r s t , the r i p p l e p r o f i l e becomes nonsinusoidal. Second, a t o n l y s l i g h t l y h i g h e r i r r a d i a t i o n "dose", t h e surface m a t e r i a l explodes. Strong plasma l i g h t emission s i g n a l s t h e occurence o f a threshold-1 i ke t r a n s i t i o n from an i n i t i a l l y h i g h metal r e f l e c t i v i t y , as given by F r e s n e l ' s formul'ae, t o a v i r t u a l l y complete a b s o r p t i o n given by resonant

" g r a t i n g c o u p l i n g " /2/ o f t h e i n c i d e n t l a s e r r a d i a t i o n i n t o s u r f a c e plasmons.

The importance o f t h i s i n s t a b J l i t y t o m a t e r i a l s processing thus may m a i n l y r e s t on t h e f a c t t h a t i t provides e f f i c i e n t c o u p l i n g o f a l a s e r beam t o a h i g h l y r e f l e c t i n g t a r g e t . Note t h a t t h e b a s i c mechanism works f o r a r b i t r a r y angle o f incidence, as demonstrated i n a r e l a t e d experiment 131. Whether t h e i n s t a b i l i t y plays a d e c i s i v e r o l e i n p r a c t i c a l s i t u a t i o n s has y e t t o be demonstrated.

However, t h e s t r o n g p o l a r i z a t i o n dependence observed i n s t e e l c u t t i n g w i t h C02 l a s e r s /4/ may be a d i r e c t consequence o f a n i s o t r o p i c energy t r a n s p o r t 151 through s t r o n g l y p o l a r i z e d s u r f a c e plasmons.

References

1. KEILMANN F., i n Surface Studies w i t h Lasers, Springer Ser. Chem. Phys., eds. F.R. Aussenegg, A. L e i t n e r and M.E. L i p p i t s c h , Heidelberg 1983.

2. HUTLEY Y.C. and MAYSTRE D., Optics Corn. 19 (1976) 431. - 3. KEILMANN F. and BAI Y.H., Appl. Phys. A 29 (1982) 9.

4. OLSEN F.O., DVS B e r i c h t e 63 (1980) 197. - 5. KEILYANN F., Verhandl . DPG ( V I ) 17 (1982) 481. -