Abstract: We demonstrate experimentally and theoretically that the sign of two-wave coupling gain in sillenite crystal at some polarization of light changes with the increase of the amplitude of square-wave field applying along the axis. It is shown that the nonunidirectional energy exchange is the reason of this effect in Bi$-12$/TiO$-20$/ single crystal. Also strong influence of this effect on the polarization properties of the two-wave coupling gain was it revealed at the experimental investigations of the efficiency of the two-wave interaction in Bi$-12$/SiO$-20$/ single crystal. At the theoretical analysis we used the model obtained on the basic of the mode approach. Made the numerical calculation of dependencies of the two-wave coupling gain has good agreement with the experimental data. This calculation has allowed estimating the material parameters of the investigated samples of the crystal. !14
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