Fluorooxoborates are considered a promising system for exploring deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals. However, the regulatory rules governing the ratio of different π- and non-π-conjugated functional units and their impact on optical properties, as well as the underlying microscopic mechanisms, remain poorly understood. Furthermore, there is currently no effective method for the directional design of chemical compositions with specific functional unit ratios.

The Crystal Materials Research Center at the Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (XTIPC), has proposed the "functional unit ratio design" principle. Based on this principle, they have screened and designed advantageous fluorinated building unit configurations. Focusing on the system comprising π-conjugated [BO₃] and non-π-conjugated [BO₂F₂], they successfully designed two novel fluorooxoborate materials: LiB₃O₄F₂ and Li₂B₄O₅F₄. For the LiB₃O₄F₂ composition, 26 candidate structures were obtained, and for the Li₂B₄O₅F₄ composition, 9 candidate structures were obtained.

The study found that 20 of the LiB₃O₄F₂ crystals and 5 of the Li₂B₄O₅F₄ crystals exhibited birefringence greater than 0.05, with maximum values of 0.111 and 0.07 at 1064 nm, respectively. The powder second-harmonic generation (SHG) responses ranged from 0.3 to 3.4 × KDP for LiB₃O₄F₂ and 0.7 to 1.6 × KDP for Li₂B₄O₅F₄. The sufficient birefringence enables 25 of these crystal structures to achieve deep-UV phase-matching. More importantly, the C2-LiB₃O₄F₂ crystal achieved a phase-matching wavelength as low as 145.2 nm, surpassing the 148.3 nm threshold required for ²²⁹Th nuclear clock applications, while exhibiting a strong SHG response 3.4 times that of KDP, positioning it as an excellent candidate material for deep-UV NLO crystals.

This research not only validates the general applicability of the "functional unit ratio design" strategy for designing deep-UV NLO materials but also achieves the synergistic optimization of phase-matching wavelength, band gap, birefringence, and SHG response. It provides a novel approach for the development and application of next-generation deep-UV laser sources.

Figure. Correspondence between functional units and chemical compositions in fluorooxoborates.

The related research findings were published in an international renowned journal Advanced Materials under the title "Functional Unit Design of Deep-UV NLO Crystals With Short Phase-Matching and Large SHG Response". The XTIPC, is the sole corresponding institution. The first author is Abudukadi Tudi, and the corresponding authors are Congwei Xie, Shilie Pan, and Zhihua Yang. This research was supported by the National Natura Science Foundation of Chin, National Key Research and Development Program of China, CAS Project for Young Scientists in Basic Researc, th Strategic Priority Research Program of the Chinese Academy of Science, International Partnership Program of CAS.