Hideyuki KAWAHARA,¹ Yoshihiro UENO,¹ Nanae ABE,¹ Shiho KISHINO,¹ Kazuhiro EMA,¹ Masahiro RIKUKAWA,² Yuko TABUCHI² and Naoya OGATA²

¹Department of Physics, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo, 102 Japan, ²Department of Chemistry, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo, 102 Japan

(Received December 11, 1996; Accepted December 16, 1996)

We measured the third-order optical nonlinearity of regioregular head-to-tail coupled poly (3-hexylthiophene) (HT-PHT) and regiorandom poly (3-hexylthiophene) (R-PHT) around exciton resonance using degenerate four wave mixing. The observed value of |χ⁽³⁾| at the exciton resonance peak of HT-PHT is in the order of 10^-9 esu which is approximately three-fold larger than that of R-PHT. We observed for the first time the enhancement in optical nonlinearity that occurs through the control of regularity of the polymeric structure. Response time of the nonlinearity was also measured using the transient grating method. The decay time of the transient grating in R-PHT and HT-PHT was less than 2 ps.

Key words : poly(3-hexylthiophene), head-to-tail coupling, conjugation length, Frenkel exciton, third-order nonlinearity