Influence of Faraday elliptical birefringence on the acousto-optic diffraction efficiency: a case of isotropic interaction with quasi-longitudinal acoustic waves in KH₂PO₄ crystals

It is shown for the first time that acousto-optic efficiency can be enhanced by the magnetic field-induced ellipticity of optical eigenwaves due to a Faraday effect. We find that the acousto-optic figure of merit for KH2PO4 crystals can be increased about two times when a magnetic field is applied along the optic axis and the incident and diffracted optical waves propagate along the directions close to the optic axis with the polarizations being the same as the magnetically induced polarizations of elliptical eigenwaves. We also analyze the angular behaviour of the peaks of effective elasto-optic coefficients and their full-width half maximums with changing magnetic field. Our results demonstrate that it is possible to control the efficiency of the acousto-optic Bragg diffraction using the magnetic field. The magnetic field needed for enhancing the acousto-optic figure of merit depends on the Faraday-tensor component of a material.