LiGeBO4: a nonlinear optical material with a balance between deep-ultraviolet cut-off edge and large SHG response induced by hand-in-hand tetrahedra

Abstract

The chemical-electronic structures of functional modules and their spatial arrangement mainly determine the functional performance of materials. Tetrahedral structural units in borates can be used to widen the band gap for designing deep-ultraviolet/ultraviolet (UV) nonlinear optical (NLO) materials, but they are rarely favored due to their weak optical nonlinearity. In this study, we investigate the second-harmonic generation (SHG) response of LiGeBO₄ with a combined experimental and computational approach. LiGeBO₄ achieves a balance between a large band gap (6.2 eV) and a strong SHG coefficient (double that of KH₂PO₄ (KDP)), which rarely exists in tetrahedral-based crystals. The origin of the SHG effect in LiGeBO₄ is analyzed in detail using density functional theory (DFT) calculations. The calculated frequency dependent SHG efficiency of LiGeBO₄ is 1.1 pm V⁻¹, which is in accordance with the experimental value. Based on the modern polarization theory, the nonlinear electronic polarization induced by an external electric field along the [0 0 1] direction is about 4 pm V⁻¹, which indicates the SHG susceptibility is 2 pm V⁻¹ along the z-direction. Moreover, the SHG-density result shows that the SHG effect of LiGeBO₄ mostly comes from the Ge–B–O tetrahedral units.