Molecular orbital approach to the optical nonlinearities of fresnoite-type crystals

Hyperpolarizability (β) values have been evaluated by means of molecular orbital calculation for the clusters which simulate the fresnoit type crystals, Ba2TiGe2O8(BTG), Ba2TiSi2O8 (BTS), and Sr2TiSi2O8 (STS). The experimental results have suggested that the second order optical nonlinearity of the crystals were strongly affected by the distortion of the crystal structure. In this study, β values were calculated for distorted clusters of BTG, BTS, and STS crystals. The obtained β values increase both with axial (c-axis direction) and equatorial (in c-plane) expansions of the unit cell. Especially, the β value of BTG cluster increased significantly with axial expansion.