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

Borophosphates are very known for the short ultraviolet (UV) cutoffedge and have become the promising UVand deep-UV functional crystals candidates; however, tetrahedral[PO4] and [BO4] groups own weak anisotropy of polarizability andare not conducive to large birefringence, which hinders theirapplication in the short-wavelength region. Improving theirbirefringence without compromising the band gap is the mainresearch objective. By introducing the excellent birefringentfunctional groups, such as [B2O5], [BO2]infinity chain,[B2Ox(OH)5-x], and so forth into borophosphates, sevenborophosphates with improved birefringence were successfully synthesized (Delta n> 0.05@532 nm). Remarkably, among them, thecentimeter-sized crystal of Rb3B8PO16with a short deep-UV cutoffedge (175 nm) and large birefringence (Delta n(exp.)similar to 0.072@589.3nm) exhibits the shortest phase-matching wavelength (222 nm), which makes Rb3B8PO16a promising UV NLO crystal, whileKB6PO10(OH)4with deep-UV cutoffedge features the largest birefringence (Delta n(exp.)similar to 0.103@546 nm) in the reportedborophosphate system, making KB6PO10(OH)4a promising deep-UV birefringent crystal. This study not only provides feasiblestrategies for increasing the birefringence of borophosphates but also pushes phase-matching into the short-wavelength region