Third-order optical nonlinearity in regio-controlled polythiophene films

We have investigated the third-order optical nonlinearity of poly(3-hexylthiophenes) with various head-to-tail coupling ratios (r), using the third-harmonic generation method. An increase in r leads to a reduction in optical gap energy (E-g) and an increase in the third-order nonlinear susceptibility (chi((3))). For r=0 to 0.80, chi((3)) is scaled by E-g as chi((3))proportional to E-g(-6.7), while for r similar to 1, chi((3)) is considerably enhanced beyond this scaling law. We discuss how the behavior of chi((3)) is based upon the conjugation-length dependence of the transition dipole moments. (c) 2005 American Institute of Physics.