Recently, the Crystal Materials Research Center at the XTIPC has made progress in the study of solar-blind ultraviolet nonlinear optical crystals. The research team innovatively proposed and implemented a "π-conjugated cation engineering" strategy, successfully developing two high-performance phosphite nonlinear optical crystals—GUPO and GPO. This breakthrough provides a key material solution for advancing all-solid-state, compact solar-blind ultraviolet lasers.

Nonlinear optical crystals are core components for laser wavelength extension. Lasers operating in the solar-blind ultraviolet region (wavelength < 280 nm) hold substantial application value in fields such as precision machining and quantum communication. However, conventional phosphate crystals are limited by their inherently low birefringence (Δn < 0.05), which results in a phase-matching wavelength significantly shorter than their UV absorption edge. This impedes effective frequency conversion across their entire transmission range, severely restricting their application in the solar-blind ultraviolet spectrum. The Crystal Materials Research Center has long been committed to the design and development of novel nonlinear optical materials. In this study, researchers adopted a novel approach by moving beyond traditional reliance on anionic groups. They introduced a new strategy utilizing π-conjugated organic cations to substantially enhance material optical anisotropy. By incorporating guanidinium and guanylurea cations with strong π-conjugation into the phosphite system, they successfully synthesized non-centrosymmetric GPO and GUPO crystals.

Among these, the GUPO crystal demonstrates exceptional comprehensive performance: 1. Unprecedented optical anisotropy with a birefringence of 0.19 @589.3 nm, significantly surpassing all known inorganic phosphates. 2. Achieves full-wavelength phase matching, covering its entire transmission range from 215 nm to 1600 nm, extending into the solar-blind UV region. 3. Its phase-matching capability shows a 43 nm blue shift compared to commercial KDP crystals. 4. Exhibits strong second-order nonlinear optical effects, with a powder second-harmonic generation efficiency 2.2 times that of commercial KDP (@1064 nm) and equivalent to β-BBO (@532 nm), showing promise for direct 266 nm laser generation. 5. Possesses excellent physicochemical properties, including facile crystal growth via aqueous solution methods, good stability, and ease of processing.

Theoretical studies elucidate the origin of these properties: the large birefringence and second-harmonic generation effects primarily arise from the directional alignment and synergistic polarization of the π-conjugated cations. Meanwhile, the [H₂PO₃]⁻ anionic framework, assembled via hydrogen bonding guidance, ensures the formation of the non-centrosymmetric structure and low UV optical absorption. This research successfully resolves the long-standing challenge in phosphate systems of balancing "wide bandgap and high birefringence," opening a new pathway for designing next-generation ultraviolet nonlinear optical materials.

The related research findings have been published in Angewandte Chemie International Edition (64, e202510363) and selected as a VIP paper. The Xinjiang Institute of Physics and Chemistry is the sole corresponding institution. Researcher Shilie Pan and Researcher Jian Han from the Crystal Materials Research Center are the corresponding authors. Doctoral student Guangsheng Xu is the first author. This work was supported by the Tianshan Talent Training Program, Key Research and Development Program of Xinjiang, Strategic Priority Research Program of the Chinese Academy of Sciences, National Natural Science Foundation of China, and the Xinjiang Major Science and Technology Project.

Figure 1: π-conjugated cation engineering achieves a significant blue shift in the phase-matching wavelength of phosphate nonlinear optical crystals