Enhanced third-order optical nonlinear response about polaronic bands in polythiophene films

Abstract

In this work we will measure the third-order nonlinear susceptibility χ(3) directly inside the polaronic bands in an homologous series of polycondensed thiophene-based polymers. Direct information is provided on the possible effect of (bi)polarons on the size and picosecond dynamics of the third-order optical nonlinearity. A novel technique is used for measuring photoinduced midgap state; these gap states are photogenerated in polythiophene samples by an above-gap excitation (2.33 eV) . The nonlinearities are subsequently probed usingby below-gap (1.2eV) degenerate four-wave mixing. Enhancement of the degenerate four-wave mixing signal at 1.2 eV of more than an order of magnitude is observed when the excitation beam (2.33 eV) overlaps with three infrared-probe beams (1.2 eV). The dependence of the enhanced signal on the polarization of the pump and probe beams is studied. Enhancement of χ(3) from the photoinduced polaronic bands is compared with the enhancement of χ(3) brought about by the doping-induced polaronic bands.