Active performance of tetrahedral groups to SHG response: theoretical interpretations of Ge/Si-containing borate crystals

Abstract

As potential candidates for deep-UV nonlinear optical (NLO) crystals, borosilicates and borogermanates, which contain NLO-active groups such as B–O, Si–O and Ge–O, have fascinated many scientists. The crystal structures, electronic structures and optical properties of seven borates in different B/R (R = Si, Ge) ratios have been studied using DFT methods. Through the SHG-density, we find that besides the recognized contribution of the π-conjugation configuration of BO₃ to second harmonic generation (SHG), the tetrahedra have a non-negligible influence. This is because the non-bonding p orbitals of the bridging oxygen in the tetrahedra are observably closer to the Fermi level than those in BO₃, which is observed in the PDOS of Rb₄Ge₃B₆O₁₇ and RbGeB₃O₇. This conclusion would be very meaningful in the understanding of the relationship between the crystal structure and nonlinear optical properties.