Functional Unit Design of Deep-UV NLO Crystals With Short Phase-Matching and Large SHG Response.

Abstract

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are crucial for generating deep-UV lasers, and their performance is determined by the type, ratio, and arrangement of microscopic NLO functional units. Currently, there are no suitable materials capable of achieving deep-UV phase-matching (PM) laser output via direct second harmonic generation (SHG) at around 148.3 nm - a key requirement for the ² ² ⁹Th nuclear clock. Here, we proposed a functional-units-ratio design principle to address this bottleneck. Applying this strategy to the Li-B-O-F system, we designed two novel compositions, LiB₃O₄F₂ and Li₂B₄O₅F₄. Subsequent crystal structure prediction identified C2-LiB₃O₄F₂ as an exceptional candidate, exhibiting a record-short PM wavelength of 145.2 nm and a strong SHG response of 3.4 × KH₂PO₄. The prediction also revealed several other metastable phases with outstanding performance, including Cc-LiB₃O₄F₂ (149.7 nm), P2₁-Li₂B₄O₅F₄ (151.6 nm), P2₁-LiB₃O₄F₂-5 (156.1 nm), P2₁-LiB₃O₄F₂-9 (156.8 nm), and Cm-LiB₃O₄F₂-7 (158.2 nm), all of which surpass the previous record and have a high synthesis probability. Crucially, the combination of [BO₃]  and [BO₂F₂] functional units enables deep-UV PM with a moderate birefringence (∼0.05 @1064 nm), effectively circumventing the traditional performance trade-off. This work provides a generalizable design strategy for next-generation deep-UV NLO materials and paves the way for the practical development of the ²²⁹Th nuclear clock.