抄録
Effects of the impurities and defects on the promising optical properties of LiNbO_3 are reviewed from our studies. The crystals grown at nitrogen gas ambient which contains 0.001 vol% oxygen, just after the growth, had a strong and broad optical absorption around 500 nm, which was perfectly vanished after a few minutes of the heat treatment in air. The optical absorption in the range of 400-500 nm increased with the partial pressure of oxygen during the crystal growth. However, the absorption in this range could not be reduced by heat treatment in air even for more than 100 h. The origin of this optical absorption is considered not to be Fe^<2+>, but to be intrinsic defects, which are remarkably reduced by the crystal growth in the poor oxygen atmosphere. The photorefractive effect of LiNbO_3, at least in the bulk form, had no relation to the atmosphere during the growth. Large suppression of the optical damage is characteristic for the Ti-diffused waveguides of the crystal which was grown in low oxygen concentration. In Zn doped LiNbO_3, the optical absorption around 400-500 nm was also reduced. High Zn doping could strongly suppress the optical damage as Mg doping. But the refractive indices of Zn doped crystals are different from those of Mg doped ones. The ordinary refractive index is increased with Zn content, and the extraordinary one shows nonmonotonic dependence.
