CaBOCl and SrBOCl: Nonlinear Optical Crystals with Deep-Ultraviolet Transparency Windows.

MBOX is a prominent family with excellent nonlinear optical (NLO) responses, just as the PbBOI crystal with a large second harmonic generation (SHG) of 13.5 times that of KHPO. 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 CaBOCl and SrBOCl 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 MBOX family. Furthermore, CaBOCl 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.