Continuous control of third-order optical nonlinearity in charge-transfer-type conjugated polymers

Third-order optical nonlinearity was investigated in two charge-transfer (CT)-type conjugated polymers, poly(aryleneethynylene) and poly(thiophene-alt-thiazole), using the third-harmonic generation (THG) method. Comparison of the results with previously obtained THG data for polythiophene and the strong CT polymer poly(thiophene-alt-quinoxaline) revealed that the ratio of the maximum value of third-order nonlinear susceptibility and the absorption coefficient max parallel to chi((3))parallel to/alpha(max) increased as the degree of CT character increased for the four polymers. This variation in max parallel to chi((3))parallel to/alpha(max) is discussed based on the magnitude of the transition dipole moment between the one-photon-allowed and the one-photon-forbidden excited states. (C) 2008 American Institute of Physics.