RESUMO
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.
RESUMO
In this work, a potential semiorganic nonlinear optical candidate NH4B[D-(+)-(C4H4O5)]2·H2O (NBC) has been studied using Density Functional Theory. The origin of the second harmonic generation (SHG) effect of NBC crystals for the NH4B[D-(+)-(C4H4O5)]2·H2O molecular complex is explained by employing a combination of the density of states, SHG density and molecular orbital analysis. It reveals a way in which the organic and ammonium groups affect the SHG processes in a significantly different manner in the crystals and the molecular complex. In particular, the role of hydrogen bonding interaction in influencing the electronic structure and nonlinear optical properties is explicitly identified and explained.