Role of large Rashba spin-orbit coupling in second-order nonlinear optical effects of polar BiB3O6

The alpha-BiB3O6 (BIBO) is a widely used nonlinear optical (NLO) crystal in ultraviolet (UV) energy region. Although the spin-orbit coupling (SOC) effect on the band structure of BIBO has been investigated, the role of SOC effect on the second-order NLO properties is still unclear. In this article, using the first-principles calculations, we discover the unusual polar lone-pair electron can induce large Rashba SOC effect in the electronic structure of alpha-BiB3O6 (BIBO), which can fundamentally change its NLO responses around the band edge. For the second harmonic generation (SHG), it is found that the inclusion of SOC effect can significantly improve the accuracy of SHG coefficients calculations in BIBO, closer to the experimentally measured values. Importantly, we also find the large Rashba SOC effect can strongly modulate the photodriven high-order charge/spin currents in BIBO, giving rise to large bulk photovoltaic effect, large circular photogalvanic effect, and an unusual spin photocurrent effect. Interestingly, the roles of SOC in tuning the charge and spin currents are quite different. Accordingly, we propose the spin current switching around the band edge induced by the large Rashba SOC in a BIBO crystal. Our study not only provides a deep and complete understanding on the unique roles of Rashba SOC effect in the second-order NLO properties of BIBO, but also suggests the BIBO crystal may be used for UV-range spintronics and optoelectronics.

Automated summary

This study reveals the large Rashba SOC effect induced by the polar lone-pair electron in BIBO, fundamentally changing its NLO responses. The SOC effect plays an important role in tuning the second-order nonlinear optical properties of BIBO, suggesting potential applications in UV-range spintronics and optoelectronics.