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.