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 (Eg) and an increase in the third-order nonlinear susceptibility (χ(3)). For r=0 to 0.80, χ(3) is scaled by Eg as χ(3)∝Eg−6.7, while for r∼1, χ(3) is considerably enhanced beyond this scaling law. We discuss how the behavior of χ(3) is based upon the conjugation-length dependence of the transition dipole moments.