Anisotropy of acousto-optic figure of merit for the case of a Bragg-diffracted wave propagating along an optic axis in optically biaxial crystals: Tl3AsS4 crystals

In the present work, we analyze acousto-optic (AO) interactions under the condition when a diffracted optical wave propagates along one of the optic axes in optically biaxial crystals. We find that the number of AO interaction types is reduced from nine, which is typical for the most general case, to six, since some of the isotropic and anisotropic diffraction types manifest themselves simultaneously. We reveal that, in the case of the interactions of quasi-ordinary and quasi-extraordinary optical waves, the dependences of effective elasto-optic coefficients and AO figure of merit on the tracing angle of an incident optical beam around the optic axis are symmetric with respect to the plane that is perpendicular to the plane of optic axes and contains the principal axis Z. We determine the geometry of AO interactions at which the highest AO figure of merit can be expected. Finally, we show that, in the case of propagation of diffracted light along one of the optic axes in the biaxial Tl3AsS4 crystals, the highest AO figures of merit, M2=1290×10-15s3/kg and M2=1305×10-15s3/kg, can be reached when the quasi-longitudinal acoustic wave interacts with the quasi-ordinary or quasi-extraordinary optical waves in the plane of optic axes.