Elimination of the background from polariton CARS spectra by spatial separation of the exciting and probe beams

A method for elimination of the background (resulting from direct four-photon processes) from polariton CARS spectra is proposed and demonstrated experimentally. The method is based on the ability of polaritons to propagate in a medium over macroscopic distances and it requires a geometry of the interacting waves in which the regions of biharmonic excitation of polaritons and probing of the excited polaritons by a probe field are spatially separated in a crystal.