Nonlinear optical (NLO) materials play a vital role in modern photonic technology, driving advancements in applications such as laser frequency conversion, ultrafast optical switching, and quantum information processing. Among NLO crystals, borate-based systems have long remained at the forefront of short-wavelength (<280 nm) NLO materials due to their structural adaptability, broad transparency window, and high laser damage threshold. However, the development of next-generation NLO materials faces a critical challenge: achieving sufficient birefringence for short-wavelength phase matching while preserving a strong second-harmonic generation (SHG) effect.

Prior research has shown that synergistic interactions between various anions (e.g., O^2-, F^-, BO₃^3-, BO₃F_4-) can unlock unprecedented optical functionalities. Meanwhile, the planar π-conjugated [B₃O₆] group exhibits high hyperpolarizability and significant polarizability anisotropy—key properties that enable efficient SHG and the sufficient birefringence required for short-wavelength phase matching.

Building on these insights, a research team from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (CAS) adopted a synergistic optimization strategy to successfully synthesize three novel rare-earth metal borate fluorides: K₂GdB₃O₆F₂, Rb₂LuB₃O₆F₂, and Cs₂LuB₃O₆F₂. Their findings were recently published in the journal Advanced Functional Materials.

All three compounds feature short cutoff edges below 200 nm. Notably, Cs₂LuB₃O₆F₂ demonstrates a large experimental frequency-doubling effect, measuring 1.5 times that of KH₂PO₄. For Rb₂LuB₃O₆F₂ and Cs₂LuB₃O₆F₂, the shortest Type-I phase-matching wavelengths are evaluated at 210 nm and 202 nm, respectively, indicating their potential to enable direct output of 213 nm coherent light via the fifth harmonic generation process of a Nd:YAG laser.

A key observation from the study is the structural evolution from centrosymmetric K₂GdB₃O₆F₂ to non-centrosymmetric Rb₂LuB₃O₆F₂ and Cs₂LuB₃O₆F₂. This transition reveals that the arrangement and orientation of [B₃O₆] groups are strongly influenced by the coordination of rare-earth metal polyhedra, which plays a decisive role in determining the overall structural symmetry of the materials.

This research was supported by the National Natural Science Foundation of China, the CAS Strategic Priority Research Program, and other funding sources.