Near-resonant third-order optical nonlinearities in p-toluene sulfonate polydiacetylene

We report single-wavelength subpicosecond and nanosecond excite-probe and four-wave mixing experiments near the exciton resonance in p-toluene sulfonate polydiacetylene. Contributions to the third-order susceptibility χ(3) resulting from phase-space filling, photoinduced absorption, and two-photon absorption are identified. A three-level rate-equation model that accounts for the change in sign of the nonlinear absorption observed at near-resonant frequencies is discussed.