3RD-ORDER OPTICAL NONLINEARITY IN REGIOREGULAR HEAD-TO-TAIL COUPLED POLY(3-HEXYLTHIOPHENE)

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 f our wave mixing. The observed value of \chi((3))\ at the exciton reson ance peak of HT-PHT is in the order of 10(-9) esu which is approximate ly three-fold larger than that of R-PHT. We observed for the first tim e the enhancement in optical nonlinearity that occurs through the cont rol of regularity of the polymeric structure. Response time of the non linearity was also measured using the transient grating method. The de cay time of the transient grating in R-PHT and HT-PHT was less than 2 ps.