M2B5O9X is a prominent family with excellent nonlinear optical (NLO) responses, just as the Pb2B5O9I crystal with a large second harmonic generation (SHG) of 13.5 times that of KH2PO4. However, most of these compounds are limited to ultraviolet and visible regions because of their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. Using a high-temperature solution method, we obtain Ca2B5O9Cl and Sr2B5O9Cl crystals, which exhibit a deep-ultraviolet (DUV) absorption edge of 170 nm (band gap ≈ 7.29 eV). It is an important breakthrough in the DUV transparency of the M2B5O9X family. Furthermore, Ca2B5O9Cl crystals display a phase-matching SHG response under a 1064 nm laser, which is further confirmed by the balance between the suitable birefringence and the small dispersion of refractive indexes in the wavelength range of 1064-532 nm. Therefore, they are promising DUV transparency windows and NLO candidates.

中文翻译：

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Ca2B5O9Cl和Sr2B5O9Cl：具有深紫外线透明窗口的非线性光学晶体。

M2B5O9X是一个杰出的家族，具有出色的非线性光学（NLO）响应，就像Pb2B5O9I晶体具有23.5倍于KH2PO4的大二次谐波（SHG）一样。然而，由于它们的长吸收边缘（小带隙），这些化合物中的大多数限于紫外线和可见光区域。在这里，我们报告了这个家庭的两名成员，这改变了情况。使用高温溶液法，我们获得了Ca2B5O9Cl和Sr2B5O9Cl晶体，它们的深紫外（DUV）吸收边缘为170 nm（带隙≈7.29 eV）。这是M2B5O9X系列DUV透明性的重要突破。此外，Ca2B5O9Cl晶体在1064 nm激光下显示出相匹配的SHG响应，这可以通过在1064-532 nm波长范围内合适的双折射与折射率的小分散之间的平衡得到进一步证实。因此，它们有望成为DUV透明窗口和NLO候选对象。