Researchers progress in the enhancement of birefringence in borophosphate pushing phase-matching into the short-wavelength region--The Xinjiang Technical Institute of Physics & Chemistry.CAS

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Researchers progress in the enhancement of birefringence in borophosphate pushing phase-matching into the short-wavelength region

Editor: | Jul 22,2022

Birefringence is important for ultraviolet (UV) or deep-ultraviolet (DUV) optical crystals that are served as the critical material in all-solid-state laser devices and are the indispensable impetus for the promotion of science and advanced technologies. An appropriate birefringence allows the phase-matching of NLO crystals to achieve in a particular transparency window. BPO₄ possesses an exceptionally optimistic second harmonic generation (SHG) response (2 × KH₂PO₄), and its UV transmittance range extends to around 130 nm. However, its very small birefringence of 0.005@1064 nm makes conventional phase-matching impossible, and the crystal cannot be used as a UV NLO crystal. Therefore, it is still a constant challenge in synthesizing new UV/DUV functional crystals with large birefringence, especially for borophosphates.

Recently, a research group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, by introducing the excellent birefringent functional groups, such as [B₂O₅], [BO₂]_∞ chain, [B₂O_x(OH)₅_？_x], and so forth into borophosphates, seven borophosphates with improved birefringence were successfully synthesized (Δn > 0.05@532 nm). Remarkably, among them, KB₆PO₁₀(OH)₄ with deep-UV cutoff edge features the largest birefringence (Δn(exp.) ～ 0.103@546 nm) in the reported borophosphate system, making KB₆PO₁₀(OH)₄ a promising deep-UV birefringent crystal. The centimeter-sized crystal of Rb₃B₈PO₁₆ with a short deep-UV cutoff edge (175 nm) and large birefringence (Δn(exp.) ～ 0.072@589.3nm) exhibits the shortest phase-matching wavelength (222 nm), which makes Rb₃B₈PO₁₆ a promising UV NLO crystal.

This study not only provides feasible strategies for increasing the birefringence of borophosphates but also pushes phase-matching into the short-wavelength region. The result was published in J. Am. Chem. Soc. with the title of “Enhancement of Birefringence in Borophosphate Pushing Phase-Matching into the Short-Wavelength Region”. This work was financially supported by National Natural Science Foundation of China, West Light Foundation of CAS, Xinjiang Major Science and Technology Project, et. al.

Article link: https://pubs.acs.org/doi/full/10.1021/jacs.2c02310

The strategy of enhancement birefringence (left); shortest phase matching wavelength of borophosphates in this work compared with classical borophosphates (right).

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