PHOTOREFRACTIVE EFFECT IN LITHIUM NIOBATE CRYSTALS OF VARIABLE COMPOSITIONS AND ITS MANIFESTATION IN RAMAN SPECTRA

Results of Raman studies of ordering of the cation sublattice and photorefractive properties of LiNbO3 single crystals are reported. Photorefraction of crystals containing small concentrations of Gd3+ or Y3+ and Mg2+ is found to be essentially determined by ordering of structural units of the cation sublattice. Intensity of the stretch vibration band of the oxygen bridge between NbO6 NbO6 octahedrons is found to be sensitive to dipole ordering in the cation sublattice of the crystal. Recent findings have led to the development of new approaches to growing highly perfect single crystals of compositions close to the stoichiometric (R=Li/Nb=1), revealing a noticeable influence of intrinsic structural defects on photorefractive properties of the crystal [1]. Raman studies of photorefractive effect in lithium niobate crystals of various compositions (including those moderately doped with nonphotorefractive cations) are reported in the paper presented. Photorefraction is revealed in Raman spectra by appearance of forbidden bands [2-7]. It is interesting that intensity of the forbidden bands increases with time, as the optical damage grows. We have studied the spectra of nominally pure single crystals of congruent (R=0.946) and stoichiometric (R=1) compositions grown from melt (either of stoichiometric composition dope