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 9 Th 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, LiB 3 O 4 F 2 and Li 2 B 4 O 5 F 4 . Subsequent crystal structure prediction identified C 2‐LiB 3 O 4 F 2 as an exceptional candidate, exhibiting a record‐short PM wavelength of 145.2 nm and a strong SHG response of 3.4 × KH 2 PO 4 . The prediction also revealed several other metastable phases with outstanding performance, including Cc ‐LiB 3 O 4 F 2 (149.7 nm), P 2 1 ‐Li 2 B 4 O 5 F 4 (151.6 nm), P 2 1 ‐LiB 3 O 4 F 2 ‐5 (156.1 nm), P 2 1 ‐LiB 3 O 4 F 2 ‐9 (156.8 nm), and Cm ‐LiB 3 O 4 F 2 ‐7 (158.2 nm), all of which surpass the previous record and have a high synthesis probability. Crucially, the combination of [BO 3 ] and [BO 2 F 2 ] 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 229 Th nuclear clock.