Mechanism of linear and nonlinear optical effects of chalcopyrites LiGaX2 (X=S, Se, and Te) crystals

The electronic band structures are calculated for the recently discovered infrared nonlinear optical crystals LiGaX2 (X=S, Se, Te) by using the ab initio plane-wave pseudopotential method. Based on the band structures, the linear and nonlinear optical coefficients for these crystals are obtained, both in reasonable agreement with the available experimental results. Moreover, the origin of the difference for the band gaps and optical properties between the Li-containing crystals and our previous studied infrared optical crystals AgGaX2 (X=S, Se, Te) family with the similar structure [L. Bai , J. Chem. Phys. 120, 8772 (2004)] is elucidated. The d-orbitals of silver ion entering into the very top of the valence band results in a narrower band gap for the Ag-containing crystals narrower than that of the Li-containing crystals where no Li orbital lies at the band gap edge. Meanwhile, due to the different sizes of Li and Ag cations, the structural distortion of the GaX4 groups in the Li-containing crystal is larger than that in the Ag-containing crystal, which explains the fact that the birefringences of the Li-containing crystals are larger than that of the Ag-containing crystals. (C) 2008 American Institute of Physics.