Novel [BO2] superhalogen module-enabled deep-ultraviolet nonlinear optical material: rational design for tailored birefringence and phase matching

Abstract

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals are key materials for creating tunable DUV lasers for frequency conversion technologies. However, limited by the transparency range, very few crystals can satisfy this technical requirement, and the innovation of novel candidates has become a challenge for researchers. Hence, studies to date have chiefly focused on finding potential competitive candidates by changing the coordination environment through the use of highly electronegative anionic functional groups. Herein, we introduce a DUV-NLO concept by exploring a new chemical space with the [BO₂] unit, and present the design of a novel NLO crystal, CsBe₂B₃O₇, by combining molecular engineering and first-principles calculations. CsBe₂B₃O₇, with a new DUV NLO functional [BO₂] module, exhibits excellent DUV optical properties such as a suitable SHG response of ∼0.8 × KH₂PO₄ (KDP), a short DUV cutoff edge of 192 nm and a large birefringence of 0.108 at 1064 nm. The optimized optical properties endow CsBe₂B₃O₇ with a DUV frequency doubling ability close to its cutoff edge (∼192 nm). Additionally, the first-principles calculations demonstrate that our constructed [Be₂B₃O₇] units maintain a densely stacked layered structure without F atoms. This work not only extends structural and functional diversity but also provides a new direction for the invention of novel high-performance NLO crystals with the [BO₂] unit.