On the nature of the second-order optical nonlinearity of nanoinhomogeneous glasses in the Li2O-Nb2O5-SiO2 system

The submicroscopic structure of lithium niobium silicate glasses of the compns. 2xLiNbO3.(1-x)(Li2O.2SiO2) (x = 0.40, 0.45, 0.50) and 30Li2O.25Nb2O5.45SiO2 in the initial state and after heat treatment for different times at temps. in the vicinity of the glass transition point Tg are investigated using X-ray powder diffraction, small-angle neutron scattering (SANS), synchrotron small-angle X-ray scattering (SAXS), and electron microscopy. A nanostructure with inhomogeneities .apprx.40 .ANG. in size is formed in glasses at the initial stages of phase sepn. at 600-670ﾰ. This structure is responsible for the appearance of the second-order optical nonlinearity. The SANS, SAXS, and electron microscopic data on the inhomogeneity size are in good agreement with each other. According to the X-ray diffraction, SANS, and SAXS data, the ordering of the glass structure and the difference between the d. of inhomogeneities and the d. of the matrix increase in the course of heat treatment. At the initial stage of amorphous phase sepn., the glass decomps. into regions enriched in SiO2 and regions with an increased content of lithium and niobium. An increase in the temp. or time of heat treatment results in the pptn. of LiNbO3 ferroelec. crystals. The results obtained allow us, for the first time, to make the inference that nanoscale changes in the glass structure lead to considerable changes (by one order of magnitude and more) in the quadratic optical nonlinearity, which can be controlled by heat treatment. The origin of the second-order optical nonlinearity is assocd. with both the nanosized modulations of the polarizability due to the inhomogeneous glass structure and the polarity of structural nanoinhomogeneities from which the LiNbO3 phase ppts. at the later stages of phase sepn.