Spectral modulation of stimulated Raman scattering signal: Beyond weak Raman pump limit

Stimulated Raman scattering has recently been used in label-free vibrational imaging studies of live cells. Important attempts to develop super-resolution Raman imaging with intense Raman pump or decoherence beam have been made. However, the intense beams beyond weak stimulated Raman scattering limit could complicate spectral characteristics of Raman gain or loss signals. Here, the dependences of stimulated Raman loss signal on Raman pump intensity and pump-probe delay time (t) are specifically investigated through both experimental and simulation studies. In the strong pump regime, a pronounced spectral modulation in stimulated Raman loss is observed at the Raman peaks of aromatic compounds with relatively large optical Kerr non-linearity. We perform a numerical simulation for the coupled wave propagation to investigate the effect of cross-phase modulation (XPM) on the stimulated Raman scattering (SRS). Our simulation involving both vibrationally resonant SRS and non-resonant XPM effects in part accounts for the t-dependent peak shift and spectral broadening as well as the appearance of a gain signal at negative time delays (t<0). At the high pump intensity, it is found that the anti-symmetric XPM signal with respect to the time zero (t=0) cannot be selectively eliminated to produce pure SRS signal by time-integration over t because of the increased interference term of the SRS and XPM at the Raman-active frequency region. We believe that the experimental and simulation results discussed here would be of use in developing a super-resolution coherent Raman imaging microscope in the near future.