Rb3B11P2O23: Materials Design of a New Chemically Benign Deep-Ultraviolet Nonlinear Optical Material

Deep-ultraviolet (DUV) coherent light is urgently needed for many important technological applications such as lithography and advanced scientific instruments. Its generation, however, strongly depends on the frequency conversion of nonlinear optical (NLO) materials. Currently, KBe2BO3F2 (KBBF) is sole material that can produce the DUV coherent light by direct second-harmonic generation (SHG), i.e., sixth harmonic generation of Nd:YAG (1064 nm/6 = 173 nm). The strong layering growth habit, and the highly toxicity of BeO make the safe growth of KBBF a great challenge. In this communication, we report the first chemically benign DUV three-dimensional mixed-coordinated borophosphate Rb3B11P2O23 (RBPO). This material inherits all the excellent structural genes of KBBF and achieves comparable optical properties, such as a large SHG response (2.5 × KDP; 1.2 × KDP for KBBF), short absorption edge (168 nm; 147 nm for KBBF) and suitable birefringence (0.071@1064 nm; 0.077@1064 nm for KBBF). More importantly, RBPO melts congruently and can overcome all the processing limitations of KBBF. Theoretical calculations and structure analysis reveal that its superior optical properties can be attributed to the KBBF-like B4P2O11∞ layers, where NLO-active B3O7 groups adopt a coplanar configuration with well-ordered arrangement. These indicate RBPO will be the new generation of DUV NLO crystal.