OPTIMUM ORIENTATION OF VOLUME PHASE GRATINGS IN SILLENITE CRYSTALS - IS IT ALWAYS [111]

We study the simultaneous influence of optical activity, piezoelectric effect, and elasto-optic effect on two-wave mixing (TWM) under diffus ion recording in photorefractive Bi12TiO20 and Bi12SiO20 crystals and find numerically the maximum of the TWM gain as a function of the orie ntation of the grating vector. Contrary to widespread belief, the grat ing orientation KI[111] is the optimum orientation only if optical act ivity is negligibly small. Nonzero optical activity results in a stron g dependence of the optimum grating orientation on the crystal thickne ss. The strongest deviation of the optimum from the [111] direction is achieved for rho d = 180 degrees, where rho is the rotatory power and d is the crystal thickness. Our theory explains spell prior results f or crystals of moderate thickness and predicts new effects for thick ( e.g., fiberlike) crystals. (C) 1998 Optical Society of America [S0740- 3224(98)01210-7].