Nonlinear diffraction under three-wave spontaneous and four-wave coherent scattering of light by polaritons

Summary form only given.In a medium with periodical spatial distribution of the quadratic nonlinear susceptibility /spl chi/((2)) we could observe the phenomenon of nonlinear diffraction. This means that the effectiveness of parametric wave-mixing processes increases greatly in the directions determined by quasi-phase-matching conditions. We studied the nonlinear diffraction under the processes of light scattering by phonon polaritons in multiple domained LiNbO/sub 3/:Nd,Mg crystals with the head-to-head-tail-to-tail laminar domain structure. Periodical reversals of the spontaneous polarization P/sub sp/ lead to the reversals of the /spl chi//sup (2)/ sign with the same period.