Nonlinear optical susceptibilities of epitaxially grown polydiacetylene measured by femtosecond time-resolved spectroscopy

Ultrafast nonlinear optical responses in polydiacetylene-3-butoxycarbonyl methylurethane prepared by epitaxial vacuum deposition on a KCl substrate are investigated by femtosecond absorption spectroscopy. Absorbance changes due to ``instantaneous'' processes which decay within the pulse duration of 100 fs, self-trapped exciton, and triplet exciton are resolved in time. The spectra of the corresponding nonlinear optical susceptibilities are obtained considering the pulse duration and response times. The ``instantaneous'' nonlinear optical response consists of several nonlinear effects, i.e., hole burning, coherent coupling between pump polarization and probe field, Raman gain, inverse Raman scattering, and induced-phase modulation.