Photorefractive Properties of Congruent Lithium Niobate Crystals Doped with Zinc

摘要

Optical and structural homogeneity and photorefractive properties of LiNbO_3:Zn crystals (0.03 to 4.5 mol % Zn) were investigated by the methods of laser conoscopy, photoinduced light scattering, and Raman spectroscopy. It is established that the photorefractive effect nonmonotonically depends on the concentration of Zn~(2+) and is almost absent in LiNbO_3:Zn crystals containing 0.05 and 4.5 mol % Zn. The suppression of photorefraction is explained by a decrease in the number of defects with the electrons localized on them due to the replacement of the NbLi and VLi defects by Zn~(2+) cations, by the appearance of shallow traps near the bottom of the conduction zone, and by the radiative recombination of laser-induced photo-electrons. The conoscopic pattern of a single-axis crystal of high optical quality was observed in a LiNbO_3:Zn crystal containing 4.5 mol % Zn upon irradiation by the laser with a power of 90 mW. This pattern is of much better quality than the one observed with a laser radiation power of 1 mW, which is explained by curing of the defects by the laser radiation. An area of highly ordered structure with high alternation ordering of main and doped cations and vacancies along the polar axis in the cation sublattice is detected.