We study the simultaneous influence of optical activity, piezoelectric effect, and elasto-optic effect on two-wave mixing (TWM) under diffusion recording in photorefractive Bi<sub>12</sub>TiO<sub>20</sub> and Bi<sub>12</sub>SiO<sub>20</sub> crystals and find numerically the maximum of the TWM gain as a function of the orientation of the grating vector. Contrary to widespread belief, the grating orientation <b>K</b>∥ is the optimum orientation only if optical activity is negligibly small. Nonzero optical activity results in a strong dependence of the optimum grating orientation on the crystal thickness. The strongest deviation of the optimum from the  direction is achieved for ϱd = 180°, where ϱ is the rotatory power and <i>d</i> is the crystal thickness. Our theory explains well prior results for crystals of moderate thickness and predicts new effects for thick (e.g., fiberlike) crystals.
© 1998 Optical Society of America
(090.7330) Holography : Volume gratings
(190.4400) Nonlinear optics : Nonlinear optics, materials
(190.5330) Nonlinear optics : Photorefractive optics
(190.7070) Nonlinear optics : Two-wave mixing
E. Shamonina, V. P. Kamenov, K. H. Ringhofer, G. Cedilnik, A. Kiessling, and R. Kowarschik, "Optimum orientation of volume phase gratings in sillenite crystals: is it always ?," J. Opt. Soc. Am. B 15, 2552-2559 (1998)