a) The central symmetry produced by the [GdO3F4] polyhedra; b) the [B3O6] groups connected by the same O atoms on the [GdO3F4] polyhedra are arranged in reverse directions; c) the translation operation between the [LuO3F3] polyhedra; d) the [B3O6] groups connected by the same O atoms on the [LuO3F3] polyhedra exhibit the same translation operation and orientation. Credit: Advanced Functional Materials (2025). DOI: 10.1002/adfm.202520516
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., O2-, F-, BO33-, BO3F4-) 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.
More information:
Qianzhen Zhang et al, Unlocking Advanced UV NLO Crystals in Rare‐Earth Metal Borate Fluorides via [B3O6]‐Mediated Structural Modulation, Advanced Functional Materials (2025). DOI: 10.1002/adfm.202520516
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Three nonlinear optical materials achieve sub-200-nm cutoff edges for advanced photonics (2025, November 6)
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