Enhancement of Birefringence in Borophosphate Pushing Phase-Matching into the Short-Wavelength Region

Borophosphates are very known for the short ultraviolet (UV) cutoff edge and have become the promising UV and deep-UV functional crystals candidates; however, tetrahedral [PO₄] and [BO₄] groups own weak anisotropy of polarizability and are not conducive to large birefringence, which hinders their application in the short-wavelength region. Improving their birefringence without compromising the band gap is the main research objective. By introducing the excellent birefringent functional groups, such as [B₂O₅], [BO₂]_∞ chain, [B₂O(OH)_5-], and so forth into borophosphates, seven borophosphates with improved birefringence were successfully synthesized (Δn > 0.05@532 nm). Remarkably, among them, the centimeter-sized crystal of Rb₃B₈PO₁₆ with a short deep-UV cutoff edge (175 nm) and large birefringence (Δn_(exp.) ∼ 0.072@589.3 nm) exhibits the shortest phase-matching wavelength (222 nm), which makes Rb₃B₈PO₁₆ a promising UV NLO crystal, while 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. This study not only provides feasible strategies for increasing the birefringence of borophosphates but also pushes phase-matching into the short-wavelength region.