Anti-Stokes stimulated Raman scattering of light by polaritons

A theoretical analysis is made of anti-Stokes stimulated Raman scattering (STRS) of light by polaritons in noncentrosymmetric crystals. This is due to the superposition of parametric four-photon and two-stage three-photon processes, in addition to three-photon processes associated with the possible existence of thermal polariton excitations. These processes are mutually coherent and their contributions interfere. The fluctuation-dissipation theory of polariton STRS is applied to derive and analyze general formulas for the frequency-angular distribution of the Stokes and anti-Stokes radiation intensity at the exit face of a plane-parallel crystal layer pumped by a given plane monochromatic wave. Unlike the phonon process, frequency-tunable polariton anti-Stokes STRS is found in scattering at any angles within the polariton range of scattering angles. The anti-Stokes processes also affect the Stokes scattering. In particular, a doublet "fine structure" is observed near the center of the gain line with a "dip" on one side and a "peak" on the other. If the "peak" is sufficiently large, it may determine the line observed in strong STRS. The anti-Stokes lines are generally considerably narrower than the Stokes lines but their peak intensities are of the same order of magnitude. They also have a doublet structure with a "dip" and "peak" whose width, in this case, is of the order of the total line width. Spontaneous anti-Stokes scatterng is attributable only to thermal polariton excitations in the medium.