Interaction of light waves of arbitrary polarisation on a photorefractive grating in a cubic gyrotropic crystal in an external alternating electric field

The symmetric two-wave interaction is considered, for small modulation indices of an interference light pattern, on a transmission photorefractive grating with stationary amplitude produced in a crystal of symmetry group 23 in an external alternating electric field. For an arbitrary off-duty ratio of the external field or arbitrary elliptic polarisation of incident light waves, the space-charge field is shown to contain both shifted and unshifted components, while the polarisation state and intensity of a weak light wave change upon the external field switching. The space-charge-field amplitude and the weak-wave intensity gain are analysed as functions of the interaction wavelength, the off-duty ratio of the external field, and the photorefractive-grating spacing for the longitudinal and transverse interaction geometry at a wavelength of 633 nm in a Bi₁₂TiO₂₀ crystal.