Optical Stark shifts of Raman gain spectra in polydiacetylene

We report that the stimulated Raman gain signals of a single-crystal polydiacetylene with the side groups 2,5-bis-trifluoromethyl-phenyl, PDA-DFMP, exhibit an optical Stark shift of ≈50 cm−1. Using an ultrashort, near-resonant high-intensity laser, we find that the intensity-dependent signal shifts to higher probe photon energies for both the C=C- and C≡C-stretching modes. Semiquantitative agreement between the data and a three-level-system density-matrix calculation is achieved with a nonlinear-optical mechanism in which a pump-laser field is optically coupled to both the exciton ← zero-phonon ground-state and the exciton ← one-phonon ground-state transitions.