Excitation of surface polaritons by nondegenerate four-wave mixing of evanescent waves

We propose a method for exciting a surface polariton wave by nonlinear mixing of three evanescent waves. The nonlinear polarization and phase matching conditions are analyzed for diatomic cubic crystals and a numerical example is given for GaP showing that this scheme is possible. Furthermore, by consideration of the energy balance in the crystal, the gain coefficient for the surface wave and the intensity of the wave coupled to the vacuum are also calculated.