Photorefractive optical properties of volume phase gratings induced in sillenite crystals, when the grating vector lies on the (111) plane

We study the diffraction efficiency of volume phase gratings in Bi12GeO20, when the grating vector, lying on a (111) plane, is at an angle with respec t to the crystallographic direction. An external bias field parallel to the direction is applied during recording, thus, the recording conditions depe nd on the grating vector orientation. The basic parameters determining the diffraction efficiency are the grating vector orientation, the rotatory pow er and the field-induced linear birefringence (primary and secondary). Anal ytic expressions for the diffraction efficiency have been obtained by takin g into account all the above-mentioned parameters, provided that linearly p olarized light is incident on the crystal. In this configuration, the influ ence of the secondary electro-optic effect (inverse piezoelectric and photo elastic effects) to the diffraction efficiency is actually stronger than th e influence of the primary effect. Experimental results are given.