Influence of two-photon processes on the nonlinear optical response of polydiacetylene single crystals in the near-resonant spectral range Original Research Article

The nonlinear optical response of single crystals of the polydiacetylene p-TS6 in the near-resonant spectral range was investigated by degenerate four-wave mixing and nonlinear absorption. Time resolved and intensity-dependent measurements showed that induced absorption from a dominant singlet exciton state to higher energetic states has a significant influence on the nonlinear optical behaviour. This can be expressed by the susceptibilities χ(3) and χ(5). The results were interpreted within the framework of the phase-space filling model with an extension from a two- to a three-level system.