Two novel deep-ultraviolet nonlinear optical crystals with shorter phase-matching second harmonic generation than KBe₂BO₃F₂. A first-principles prediction

In this work, we propose a strategy to design new deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials through the combination of borate and phosphate NLO material genome. Accordingly, a series of borophosphates are predicted with very large energy bandgaps, strong second harmonic generation (SHG) effects, and moderate birefringence in the deep-UV region. In particular, two novel fluoride borophosphates, PBO₃F₂ and PB₃O₆F₂, are able to achieve the deep-UV SHG laser output with shorter phase-matching wavelengths (∼155 and 137 nm) than that of KBe₂BO₃F₂ (KBBF, ∼161 nm). Further calculations show that these two structures are dynamically and thermodynamically stable, thus triggering possible synthesis, growth, and applications of them. The results attest that the proposed strategy is instructive and pave a comprehensive road map for us to discover new NLO crystals, especially in the borophosphate catalog with excellent deep-UV phase-matching capabilities beyond KBBF. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)