Cs3[(BOP)2(B3O7)3]: A Deep-Ultraviolet Nonlinear Optical Crystal Designed by Optimizing Matching of Cation and Anion Groups

Deep-ultraviolet nonlinear optical (DUV NLO) crystals are indispensable for solid-state lasers to produce coherent light with wavelengths shorter than 200 nm, yet their structure design still faces great challenges because two groups of conflicting properties must be satisfied simultaneously, i.e., "large second harmonic generation (SHG) response-large band gap" and "large birefringence-weak growth anisotropy". Clearly, hitherto, no crystal can perfectly satisfy these properties, including KBe₂BO₃F₂. Herein, we design a new mixed-coordinated borophosphate Cs₃[(BOP)₂(B₃O₇)₃] (CBPO) by optimizing the matching of cation and anion groups, which unprecedentedly achieves a balance for two groups of contradictions concurrently for the first time. In the structure of CBPO, it has the coplanar and π-conjugated B₃O₇ groups, which can make it possess a large SHG response (3 × KDP) and large birefringence (0.075@532 nm). Then, terminal O atoms of these B₃O₇ groups are connected by BO₄ and PO₄ tetrahedra, which eliminates all dangling bonds and blue shifts the UV absorption edge to the DUV region (165 nm). More importantly, owing to the judicious selection of cations, the size of cations and void of anion groups is a perfect match, which makes CBPO possess a very stable three-dimensional anion framework and thus reduces the crystal growth anisotropy. A CBPO single crystal with a size of up to 20 × 17 × 8 mm³ has been successfully grown, through which a DUV coherent light has also been achieved in Be-free DUV NLO crystals for the first time. These indicate CBPO will be the next generation of DUV NLO crystals.