Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals (Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu (Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.