Acousto-optic interaction in the directions forbidden by crystal symmetry

Abstract

It is shown that two types of acousto-optic diffraction (isotropic and anisotropic) are not independent. Speaking generally, the existence of one type testifies that the other type is possible also, though with less efficiency. The values of Raman-Nath parameter both for isotropic and anisotropic diffraction were calculated. The dependence of each Raman-Nath parameter value on every photoelastic constant is demonstrated. It is indicated that the divergent beam can interact in the forbidden direction in crystal, where acousto-optic figure of merit is equal to zero due to the crystal symmetry. The coefficient determining Raman-Nath parameter was calculated in the vicinity of Euler angles defining the forbidden [110] direction in paratellurite crystal. The results of experimental investigation of a collinear diffraction in paratellurite crystal are presented which proved the existence of this type of acousto-optic interaction with the efficiency up to 15%.