Anomalous second-harmonic generation response in SrBPO5 and BaBPO5

Abstract

In order to study the mechanism of the second harmonic generation (SHG) effect in borophosphates, two large ultraviolet nonlinear optical crystals, SrBPO₅ and BaBPO₅, have been successfully obtained using a top-seeded solution growth method. Optical and thermal properties have been measured, and SrBPO₅ and BaBPO₅ are shown to display large experimental gaps (6.44 eV and 6.14 eV, respectively) and moderate SHG coefficients (0.57 and 0.43 times that of KH₂PO₄). More importantly, based on a further understanding of the relationship between nonlinear optical properties and crystal structure within density functional theory (DFT), we have demonstrated that BO₄ groups and metal cations play an essential role in the SHG effect of SrBPO₅ and BaBPO₅, but not PO₄ groups as anticipated. Indeed, PO₄ groups play a counteractive role in BaBPO₅ due to the indirect influence of metal cations.