Investigation of Structural and Optical Homogeneity of LiNbO_3:ZnO Crystals of Different Genesis

摘要

Nominally pure congruent LiNbO_3 crystal and LiNbO_3:ZnO ([ZnO] ～ 5.4-6.4 mol % in the melt) crystals of different genesis are studied using photoinduced light scattering, laser conoscopy, and X-ray diffraction (method of moments). The photorefractive properties and optical and structural uniformity are analyzed. It is found that LiNbO_3:ZnO crystals obtained by direct solid phase doping have the best optical and structural uniformity, the lowest photorefractive sensitivity, and a high compositional uniformity along the polar axis. Crystals obtained by homogeneous doping are compositionally uniform, although their optical and structural uniformity is worse than those of LiNbO_3:ZnO crystals obtained by direct solid phase doping. Anomalies of conoscopic patterns are caused by the presence of charged structural defects in LiNbO_3:ZnO crystals and the distortion of the optical indicatrix induced by mechanical stresses and compositional inho-mogeneity of the crystals. High temperature annealing in short-circuited state of LiNbO_3:ZnO crystals with high dopant concentration leads to healing of charged defects and improvement of optical characteristics in general. The distribution coefficient for LiNbO_3:ZnO crystals obtained by the homogeneous doping method is significantly higher than that for the crystals obtained by the direct solid phase doping method.