Anisotropy of acousto-optic figure of merit for LiNbO₃ crystals: anisotropic diffraction

We have developed a method for the analysis of anisotropy of an acousto-optic figure of merit (AOFM), which is valid for the case of anisotropic diffraction in the trigonal crystals of the point symmetries 3m, 32, and 3¯m. The method is verified via the example of LiNbO₃ crystals. The relations for the effective elasto-optic coefficients and the AOFM are obtained for the three types of acousto-optic (AO) interactions peculiar for the anisotropic AO diffraction: the interaction of a so-called type VII with a quasi-longitudinal acoustic wave and the interactions of types VIII and IX with two quasi-transverse acoustic waves. The AO diffraction geometries providing maximal AOFM values have been determined for each of the mentioned interaction types. We have found that the maximum AOFM proper for LiNbO₃ is equal to 15.9×10^-15  s³/kg. This value is achieved at the type IX of AO interactions in the interaction plane rotated by 60.0 deg around the principal X axis with respect to the principal X-Z plane. The type VIII of AO interactions is characterized by a comparable AOFM (15.1×10^-15  s³/kg), which is realized in the Y-Z interaction plane. A close comparison of our results with the available experimental data demonstrates their fairly good agreement.