Integrated research of structural and optical homogeneities of the lithium niobate crystal with low photorefractive effect

Stoichiometric (Li/Nb = 1) and congruent (Li/Nb = 0.946) single crystals of lithium niobate (LiNbO3) doped with cations of Mg2+, Zn2+, Cu2+, B3+, Gd3+, Y3+, Fe3+, Ta5+ were investigated by means of photoinduced light scattering (PILS), Raman spectroscopy (RS), electron spectroscopy and laser conoscopy. It was established that investigated specimens can be divided into three groups depending on the type of PILS painting. The first group includes crystals that manifest small photorefractive effect. PILS indicatrix of the third group crystals opens in first few seconds after laser irradiation and for the second group – after about a minute. First group of crystals is characterized by the steep rise of transmission edge in comparison with other crystals, which indicates high volume homogeneity of these specimens. Anomalous behavior of an edge transmission is typical for the LiNbO3:Cu (0.015 wt.%) crystal which indicates its high optical inhomogeneity. The edge position of the optical transmission of the third group crystals (LiNbO3:Y (0.46 wt.%) and LiNbO3:Y(0.24 wt.%):Mg(0.63 wt.%)) is in a good agreement with the crystals of the first group, but the slope of the curve to the x-axis is substantially less. It is shown that asymmetry of PILS patterns and Raman spectra of photorefractive lithium niobate crystals in scattering geometries is due to the birefringence of the exciting laser radiation propagating perpendicular to the polar axis of the crystal.