Polariton CARS with spatially separated exciting and probe beams

The spectra of coherent anti-Stokes Raman scattering (CARS) on polaritons belonging to the lower dispersion branch of a BeO crystal were investigated experimentally and theoretically as a function of the distance d between a probe beam and the region of biharmonic excitation of polaritons. The dependence of the CARS amplitude on the distance d was used to determine the mean free path of coherent polaritons and the ratio of the contributions of direct four-photon and cascade three-photon processes to the resultant CARS signal. 'Escape' of polaritons from the excitation region broadened the CARS spectrum, which depended on the ratio of the dimensions of the excitation region to the mean free path of coherent polaritons.