Adjusting birefringence to push the phase-matching (PM) of nonlinear optical (NLO) crystals blue-shifting to the deep-ultraviolet (DUV) region is the inorganic chemists and material scientists' pursuit, while refractive dispersion as an important but easily overlooked optical properties generally brings the unexpected result. Herein, two new Y-based borates, KYBOF and RbYBOF have been synthesized by the modulation of A-site cation. In both structures, owing to the structure-directing properties of Y-based polyhedra, the natural advantage of π-conjugated [BO] groups was pushed to the greatest extent, theoretical calculations and structure analysis revealed that the coplanar arrangement of [BO] groups results in the large birefringence of KYBOF and RbYBOF. Importantly, the aligned orientation of [BO] rings makes RbYBOF possess a strong SHG response of 5.0 × KDP, meanwhile the small dispersion of the refractive indices makes RbYBOF the first borate crystal containing the strong π-conjugation [BO] rings with the PM limit breaking through the ‘‘200 nm DUV wall’’. This work indicates that refractive dispersion can be effectively reduced through the cation effect, thereby broadening the PM range.

中文翻译：

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阳离子结构调制突破相位匹配区“200 nm深紫外墙”


调节双折射以推动非线性光学（NLO）晶体的相位匹配（PM）蓝移至深紫外（DUV）区域是无机化学家和材料科学家的追求，而折射色散作为重要但容易被忽视的光学属性通常会带来意想不到的结果。在此，通过A位阳离子的调节合成了两种新的Y基硼酸盐KYBOF和RbYBOF。在这两种结构中，由于Y基多面体的结构导向特性，π共轭[BO]基团的天然优势被最大程度地发挥，理论计算和结构分析表明[BO]基团的共面排列导致KYBOF和RbYBOF具有较大的双折射。重要的是，[BO]环的排列取向使得RbYBOF具有5.0×KDP的强SHG响应，同时折射率色散小使得RbYBOF成为第一个包含具有PM极限的强π共轭[BO]环的硼酸盐晶体突破“200 nm DUV 墙”。这项工作表明，通过阳离子效应可以有效降低折射色散，从而拓宽PM范围。