Nonunidirectional energy exchange upon the steady-state two-wave interaction in a cubic gyrotropic photorefractive crystal in a dc electric field

The steady-state two-wave interaction on a transmission photorefractive grating produced in a cubic gyrotropic crystal of the symmetry group 23 in an external dc electric field is studied. The analytic solution of the equation for coupled waves is obtained for an arbitrary orientation of the crystal and arbitrary polarisation of incident light beams in the fixed pump approximation. It is shown that a nonunidirectional contribution to the energy exchange between the waves can exist, which is caused by the local component of the photorefractive grating. In a particular case of light waves with a wavelength of 633 nm propagating in the (001) plane of a Bi$_{12}$TiO$_{20}$ crystal and of the orientation of the grating vector along the [$\bar110$] axis, this contribution greatly exceeds a usual unidirectional contribution. The dependence of the two-wave gain on the modulation coefficient of the interference pattern on the input face of the crystal is studied by numerically integrating a total system of coupled-wave equations.