THG in conjugated polymers and their application to optical waveguide

3201

Synthetic Metals, 41-43 (1991) 3201

THG IN CONJUGATED POLYMERS AND THEIR APPLICATION TO OPTICAL WAVEGUIDE

Tatsuo Wada, Haruki Okawa, A k i r a Yamada, Anthony F. Garito and H i r o y u k i Sasabe F r o n t i e r Research Program, RIKEN, Hirosawa 2-1, Wako, Saitama 351-01, Japan

Conjugated compounds which show large optical n o n l i n e a r i t i e s and u l t r a fast responses are key materials as a c t i v e elements in optical signal processing systems. For these applications, it is i m p o r t a n t to develop processable c o n j u g a t e d materials which show large t h i r d o r d e r nonlinear optical responses. Several processable c o n j u g a t e d polymers: soluble p o l y t h i o p h e n e d e r i v a t i v e s , polyaniline, p o l y ( p - p h e n y l e n e v i n y l e n e ) p o l y merized via p r e c u r s o r polymer r o u t e and p o l y d i y n e s (PDY), were i n v e s t i g a t e d in t h e r m s of t h i r d o r d e r optical n o n l i n e a r i t i e s and t h e p o s s i b i l i ties of optical w a v e g u i d e a p p l i c a t i o n . The t h i r d o r d e r nonlinear optical s u s c e p t i b i l i t i e s X(3)ijkl(-Co4;oJ1,692,~o 3) of polymer t h i n films were e s t i mated from t h e optical t h i r d harmonic generation (THG) measurements at a w a v e l e n g t h of 1.907 /zm. Prism film c o u p l i n g t e c h n i q u e s were a p p l i e d to c h a r a c t e r i z e d t h e p r o p a g a t i o n c h a r a c t e r i s t i c s of polymer slab w a v e guide. The X(3)(-3~;69,69,o9) values f o r an a l k a n e - b r i d g e d PDY is estimated as 10-13 esu, which is not a E - c o n j u g a t i o n system. A f t e r thermal t r e a t ment, however t h i s value increased by one o r d e r of m a g n i t u d e (10 -12 esu) because of t h e f o r m a t i o n of one-dimensional E - c o n j u g a t i o n like a p o l y d i a c e t y l e n e due to i n t e r c h a i n c r o s s l i n k i n g . In case of an a r e n e b r i d g e d PDY, which has f u l l y c o n j u g a t e d E - e l e c t r o n Systems in t h e main chain, t h e X(3) value is 10-11 esu. The similar r e s u l t s were obtained in undoped p o l y t h i o p h e n e p r e p a r e d electrochemically (3x10 -10 esu) and p o l y ( 3 - a l k y l t h i o p h e n e ) s p r e p a r e d chemically (10 -12 esu) due to t h e d i f f e r e n c e in c o n j u g a t i o n l e n g t h of E - e l e c t r o n systems. These r e s u l t s confirm t h a t t h e f o r m a t i o n of E - c o n j u g a t i o n enhances t h e t h i r d o r d e r optical suscepti bilities. The r e f r a c t i v e indices f o r these c o n j u g a t e d p o l y m e r s were d e t e r m i n e d by m o d e - l i n e measurements. We demonstrated t h e p r o p a g a t i o n of l i g h t in these p o l y m e r i c t h i n films. In case of PDY and p o l y t h i o p h e n e d e r i v a t i v e s we can clearly o b s e r v e mode-lines by prism film c o u p l i n g . The p o s s i b i l i t i e s of optical w a v e g u i d e a p p l i c a t i o n were also discussed by a d i r e c t o b s e r v a t i o n of s t r e a k image of a p r o p a g a t i o n l i g h t w i t h a CCD camera. Elsevier Sequoia/Printed in The Netherlands