RAMAN SPECTROSCOPY IN GUIDED OPTICAL WAVES

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RAMAN SPECTROSCOPY IN GUIDED OPTICAL WAVES

J. Swalen, J. Rabolt

To cite this version:

J. Swalen, J. Rabolt. RAMAN SPECTROSCOPY IN GUIDED OPTICAL WAVES. Journal de

Physique Colloques, 1983, 44 (C10), pp.C10-501-C10-503. �10.1051/jphyscol:198310102�. �jpa-

00223462�

JOURNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome 44, décembre 1983 page C10-501

RAMAN SPECTROSCOPY IN GUIDED OPTICAL WAVES

J . D . Swalen and J . F . R a b o l t

IBM Research Laboratory, 5600 Cottle Road, KS2/281, San Jose, California 95193, U.S.A.

Résumé - Nous avons obtenu et analysé les spectres Raman de polymères en films minces et de la région interfaciale entre des films de polymères lamellaires, en couplant le faisceau laser avec un guide d'onde optique par 1 Mntermêdaire d'un prisme. Des informations sur les orientations et la répartition des molécules sont obtenues en modifiant les modes et la polarisation du champ électromagnétique incident. Nous présentons les résultats concernant différents films et leurs inte- ractions interfaciales.

Abstract - By coupling l a s e r l i g h t w i t h a prism i n t o an o p t i c a l waveguide, we have observed and analyzed Raman spectra o f t h i n polymer f i l m s , a n d of the i n t e r f a c i a l region between laminate p o l y m e r f i l m s . Both o r i e n t a t i o n a l and s p a t i a l i n f o r m a t i o n s are obtained by o p t i c a l f i e l d s f o r d i f f e r e n t modes and p o l a r i z a t i o n s . Results on various f i l m s and t h e i r i n t e r f a c i a l i n t e r a c t i o n s w i l l be presented.

INTRODUCTION

Considerable i n t e r e s t has developed i n the p r o p e r t i e s and uses o f surfaces and t h i n f i l m s f o r both s c i e n t i f i c and t e c h n i c a l proposes. For example, s c i e n t i f i c a l l y the s t r u c t u r e at the s u r f a c e s , i n c l u d i n g the species p r e s e n t , t h e i r e l e c t r o n i c and v i b r a t i o n a l p r o p e r t i e s , and t h e i r o r i e n t a t i o n , requires d e t a i l e d c h a r a c t e r i z a t i o n . And energy t r a n s f e r and charge t r a n s f e r are dependent on the c h a r a c t e r i s t i c s o f i n t e r a c t i o n between the surface and the species on the s u r f a c e . Now f o r the t e c h n i c a l s i d e , there are a whole host o f new requirements f o r devices which demand very exact and d e t a i l e d understanding and manipulation o f the surface and a t h i n f i l m on a s u r f a c e . Some o f the p r o j e c t e d needs are f o r e l e c t r i c a l or o p t i c a l conduction or m o d i f i c a t i o n o f s i g n a l s , f o r i n s u l a t i o n , p r o t e c t i o n o r e n c a p s u l a t i o n , f o r p a s s i v a t i o n or c a t a l y s i s , or f o r adhesion or l u b r i c a t i o n .

For mainly s c i e n t i f i c purposes, but also w i t h t e c h n i c a l a p p l i c a t i o n i n mind, we have been applying Raman spectroscopy t o the d e t a i l e d study of the species present and t h e i r o r i e n t a t i o n a t e i t h e r the s u r f a c e , i . e . s u b s t r a t e - a i r i n t e r f a c e , or the i n t e r f a c i a l region between two f i l m s , e . g . two polymeric f i l m s . I n the l a t e r case the i n t e r f a c i a l i n t e r a c t i o n can give us i n f o r m a t i o n about energy t r a n s f e r and adhesion. Because Raman cross sections are low, i t i s f r e q u e n t l y very d i f f i c u l t or impossible t o o b t a i n s u f f i c i e n t s i g n a l t o noise r a t i o s to measure s p e c t r a .

Consequently, we have been applying guided o p t i c a l waves, i . e . i n t e g r a t e d o p t i c s , t o increase the i n t e n s i t y ( 1 - 5 ) , t h i s technique being f i r s t explored by Y. Levy e t a l . ( 6 , 7 ) . Polymer f i l m s down t o thicknesses o f 800 A (2) have been observed by t a k i n g advantage o f the high o p t i c a l f i e l d , long o p t i c a l path l e n g t h , complete f i l l i n g o f the s l i t s o f the monochromator, and m u l t i p l e scan signal averaging w i t h our o p t i c a l waveguide Raman spectrometer system.

In a d d i t i o n , resonance Raman on t h i n monolayer f i l m s (4,5) has made possible the observation o f Raman s i g n a l s on f i l m s less than 30 A t h i c k . O r i e n t a t i o n o f each v i b r a t i o n a l mode was determined by the d i f f e r e n c e i n absorption between the transverse e l e c t r i c (TE) mode and the transverse magnetic mode (TM).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:198310102

CI 0-502 JOURNAL DE PHYSIQUE

Here we r e p o r t some r e s u l t s on Raman spectroscopy o f l a m i n a t e polymer f i l m s where we probe t h e i n t e r f a c i a l r e g i o n between two f i l m s . As we w i l l see, t h e l i g h t i s p a r t i t i o n e d between t h e two d i f f e r e n t l a y e r s i n d i f f e r i n g amounts and p r o f i l e s f o r each o p t i c a l mode o f p r o p a g a t i o n . From measurements o f t h e Raman s p e c t r a a t s e v e r a l modes and b y m a t h e m a t i c a l l y d e c o n v o l u t i n g w i t h t h e known o p t i c a l f i e l d d i s t r i b u t i o n s , t h e s p e c t r a o f t h o s e specie% a t t h e i n t e r f a c e can be seen, s e p a r a t e d f r o m t h o s e of each o f t h e b u l k t h i n f i l m s .

EXPERIMENTAL

The p o l y m e r i c f i l m s were p r e p a r e d by e i t h e r s p i n n i n g f r o m s o l u t i o n o r d o c t o r b l a d i n g ( s p r e a d i n g w i t h a k n i f e edge a c c u r a t e l y p o s i t i o n e d above t h e s u b s t r a t e ) f r o m

s o l u t i o n . The f i l m w i t h t h e h i g h e r r e f r a c t i v e i n d e x i s c a s t on t o p so t h a t c o u p l i n g of l i g h t ( 8 ) can be accomplished w i t h t h e evanescent o p t i c a l f i e l d f r o m a h i g h i n d e x prism. ( I f t h e l o w e r i n d e x f i l m were on top, t h e evanescent f i e l d t o couple t o t h e low modes i n t h e h i g h e r i n d e x f i l m would be t o o weak). Care must be taken as t o s o l v e n t s , such t h a t t h e s o l v e n t f o r t h e t o p f i l m does n o t d i s s o l v e t h e l o w e r f i l m . T h i s i s , i n p a r t , based on m o l e c u l a r s t r u c t u r e s and, i n p a r t , based on e x p e r i m e n t a t i o n . The c o u p l i n g i s shown s c h e m a t i c a l l y i n t h e f i g u r e .

By v a r y i n g t h e a n g l e o f i n c i d e n c e , t h e p r o p e r p r o p a g a t i o n v e c t o r , k, can be s e l e c t e d f o r each p r o p a g a t i n g mode and energy t r a n s f e r r e d by t h e evanescent f i e l d across t h e v e r y narrow a i r gap.

When t h e two f i l m s f o r m i n g t h e l a m i n a t e have s i m i l a r i n d i c e s o f r e f r a c t i o n , t h e p a r t i t o n i n g o f r a d i a n t energy i s d i v i d e d a p p r o x i m a t e l y e q u a l l y between t h e two l a y e r s . On t h e o t h e r hand, when t h e i n d i c e s a r e s i g n i f i c a n t l y d i f f e r e n t , p r o p a g a t i o n b e g i n s i n t h e l a y e r w i t h t h e h i g h e r index. Only when t h e e f f e c t i v e i n d e x i n t h e f i l m becomes s m a l l e r t h a n t h a t f o r t h e l a y e r w i t h t h e l o w e r i n d e x does p r o p a g a t i o n commence i n t h i s l a y e r . E v e n t u a l l y w i t h e f f e c t i v e i n d i c e s f o r h i g h o r d e r modes i n b o t h l a y e r s does t h e l i g h t d i s t r i b u t e s i t s e l f i n a somewhat u n i f o r m f a s h i o n . I n o u r Raman experiments, a c o l l e c t i o n l e n s ( f 1.8) is p o s i t i o n e d such t h a t a p o r t i o n o f t h e o p t i c a l s t r e a k i n t h e waveguide i s imaged o n t o t h e e n t r a n c e s l i t s o f a Jobin-Yvon HG-2S double monochromator. Data c o l l e c t i o n i s w i t h a Hammamatsu R955 p h o t o m u l t i p l i e r and s t a n d a r d p h o t o n c o u n t i n g e l e c t r o n i c s i n t e r f a c e d t o a N i c o l e t 1180 d a t a system t o p r o v i d e m u l t i p l e scan c a p a b i l i t i e s . Data i s t h e n t r a n s f e r r e d t o a l a r g e computer f o r d e t a i l e d d e c o n v o l u t i o n and a n a l y s i s .

RESULTS

We have measured the s p e c t r a f o r laminate polymer f i l m s o f p o l y s t y r e n e on p o l y v i n y l alcohol and f i l m s o f p o l y v i n y l alcohol on p o l y methyl methacrylate (3). I n both cases the s p e c t r a l i n t e n s i t i e s f o r t h e bands o f each l a y e r f o l l o w e d q u a l i t a t i v e l y the o p t i c a l f i e l d i n t e n s i t i e s w i t h i n each l a y e r . Deconvolution c a l c u l a t i o n s r e c e n t l y performed showed t h a t the Raman i n t e n s i t i e s o n l y approximately agreed w i t h t h e c a l c u l a t e d i n t e n s i t i e s based on the o p t i c a l f i e l d s w i t h i n the o p t i c a l waveguide, the problem being i n a c c u r a t e l y determining t h e o p t i c a l f i e l d s throughout the laminate s t r u c t u r e .

The method o f c a l c u l a t i o n i n v o l v e s measuring the angles f o r c o u p l i n g i n t o t h e v a r i o u s modes and then determining t h e r e f r a c t i v e i n d i c e s and thicknesses o f each l a y e r by a s o l u t i o n o f t h e eigenvalue equation. U n f o r t u n a t e l y even w i t h more than f o u r modes, the parameters are n o t s u f f i c i e n t l y independent t o reach a unique s o l u t i o n . A number o f approaches are being t r i e d t o overcome t h i s problem. One i s t o measure t h e f i r s t f i l m , determine i t ' s constants, and use these r e s u l t s t o c a l c u l a t e the o t h e r l a y e r o f the laminate. This i s s a t i s f a c t o r y i f i n t h e process o f d e p o s i t i n g t h e second l a y e r the f i r s t l a y e r ' s thickness o r r e f r a c t i v e index i s n o t modified. Another method i s t o simultaneously c a s t s i n g l e f i l m s o f each l a y e r a t the same time t h e laminate i s cast. From each o f these s i n g l e f i l m s we can determine the r e f r a c t i v e i n d i c e s and assume t h a t they do n o t change on lamination.

C u r r e n t l y we are attempting t o v e r i f y these approaches and have done some s p e c t r a l s u b t r a c t i o n o f the i n d i v i d u a l components. Evidence f o r i n t e r f a c i a l v i b r a t i o n s and r e s i d u a l s o l v e n t were observed i n t h e PVA-PMMA system. Questions o f f i l m a n i s o t r o p y need a l s o t o be addressed.

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