Role of self-phase modulation in stimulated Raman scattering from more than one mode

Stimulated Stokes spectra obtained with picosecond pulses in organic liquids often exhibit transient gain in more than one mode. A theory is developed here to describe the important effect of self-phase modulation on such multiple-component spectra. The nonlinear refractive index generated by a two-component Stokes field produces additional spectral components, which are separated by the frequency spacing between the two initial components. Calculations indicate that the new components are observable at considerably lower intensities than that necessary to produce noticeable phase modulation of the pulse envelope and may be an intrinsic part of the excitation process in molecules exhibiting Stokes gain in more than one mode. A comparison between the calculated spectra and experimentally obtained stimulated Raman spectra in methanol confirms the theoretical predictions, and the consequences of the theory with regard to the stimulated Raman spectra of large molecules are discussed.