A bifunctional primitive strategy induces enhancements of large second harmonic generation and wide UV transmittance in rare-earth borates containing [B5O10] groups

Abstract

Strong second-harmonic generation (SHG) and a short ultraviolet (UV) cutoff edge are two crucial yet often conflicting parameters that must be finely tuned in the exploration of nonlinear optical (NLO) materials. In this study, two new rare earth borate NLO crystals, K₇BaSc₂B₁₅O₃₀ (KBSBO) and Rb₂₁Sr_3.8Sc_5.2B₄₅O₉₀ (RSSBO), were rationally designed through a bifunctional primitive strategy to achieve an optimized balance between favorable SHG efficiency and UV transparency. As anticipated, both KBSBO and RSSBO exhibit a wide UV transparency window below 190 nm. Notably, these tailored crystals display strong SHG responses, with RSSBO achieving a remarkable enhancement in SHG efficiency (2 × KDP), surpassing that of most deep-UV rare earth borates containing [B₅O₁₀] groups known to date. Theoretical calculations and structural analyses reveal that the impressive SHG activities primarily stem from the [B₅O₁₀] groups and [ScO₆] polyhedra. These findings suggest promising potential for KBSBO and RSSBO crystals as beryllium-free deep UV NLO materials.