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.

© 1990 Optical Society of America