Observation of Phonon Resonances in the Optical Nonlinearity in Sexithienyl Thin Film

Phonon resonances in the optical nonlinearity induced by the mixing of pump and probe pulses are investigated in an ?-sexithienyl vacuum evaporated thin film. We have performed femtosecond differential transmission spectroscopy at room temperature and at 20 K using excitation energies from 2.14 to 2.4 eV. We observe simultaneously an induced transmission at the Stokes and at the anti-Stokes energetic position of the pump pulse which is only present during the temporal overlap of the pump and the probe pulse. The phonon associated with the C=C stretching mode of ?-sexithienyl is involved in the coherent light-matter interaction for both the stimulated inverse Raman scattering at the anti-Stokes energetic position and for the Raman gain at the Stokes energetic position.