First-principles calculation of the second-harmonic-generation coefficients of berate crystals

We report the calculation of the second-harmonic-generation (SHG) coefficie nts of LiB3O5 (LBO), CsB3O5 (CBO), and BaB2O4 (BBO) using the linearized au gmented plane-wave band method in the local-density approximation with a sc issors operator that includes the renormalization of the momentum operator. The analysis that is based on the spectral and spatial decomposition of th e calculated results reveals that, for the large component of SHG coefficie nts, the dominant source of the optical nonlinearities for these berate cry stals is the nonlinear response of the high-lying 2p electrons of oxygen at oms, while the cations play a minor role even in the heavier Cs and Ba case s, though they dominate the conduction-band minimum. But for the small SHG component, the role of the cation became important, particularly when the i solated anionic group has little contribution due to the restriction of the symmetry. In the case of LBO and CBO, due to the linkage of anionic groups , the contributions of off-ring O atoms are almost the same as those of in- ring O atoms. Yet for BBO where there is no such linkage, the off-ring O at om plays a much more important role than the in-ring O atom does. We also f ind that the contribution of the virtual-hole process cannot be ignored as is usually done in the semiconductors case. [S0163-1829(99)05937-8].