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, [B2Ox(OH)5–x], 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.

硼磷酸盐（Borophosphates）因具备极短的紫外（UV）截止边而广受关注，已成为极具应用前景的紫外及深紫外功能晶体候选材料；然而，四面体构型的[PO4]与[BO4]基团极化各向异性较弱，不利于实现大双折射率，这限制了其在短波区域的应用。本研究的核心目标为在不降低材料带隙的前提下提升硼磷酸盐的双折射率。
通过将[B2O5]、[BO2]∞链状基团、[B2Ox(OH)5–x]等优异双折射功能基团引入硼磷酸盐体系，研究团队成功合成了7种双折射率得到优化的硼磷酸盐材料（Δn>0.05@532 nm）。
尤为值得关注的是，其中Rb3B8PO16可制备得到厘米级晶体，其具备极短的深紫外（deep-UV）截止边（175 nm）与大双折射率（实验双折射率Δn(exp.)≈0.072@589.3 nm），最短相位匹配波长可达222 nm，使其成为极具应用前景的紫外非线性光学（Nonlinear Optics, NLO）晶体；而KB6PO10(OH)4同样具备深紫外截止边，其双折射率在已报道的硼磷酸盐体系中达到最高（Δn(exp.)≈0.103@546 nm），是一类极具潜力的深紫外双折射晶体。
本研究不仅为提升硼磷酸盐的双折射率提供了可行策略，更将相位匹配窗口拓展至短波区域。