Observation of phonon resonances in the optical nonlinearity in an alpha-sexithienyl thin film

Phonon resonances in the optical nonlinearity induced by the mixing of pump and probe pulses are investigated in an alpha-sexithienyl vacuum evaporate d thin film. We have performed femtosecond differential transmission spectr oscopy 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 p resent during the temporal overlap of the pump and the probe pulse. The pho non associated with the C=C stretching mode of alpha-sexithienyl is involve d in the coherent light-matter interaction for both the stimulated inverse Raman scattering at the anti-Stokes energetic position and for the Raman ga in at the Stokes energetic position.