NH4Be2BO3F2 and -Be2BO3F: Overcoming the Layering Habit in KBe2BO3F2 for the Next-Generation Deep-Ultraviolet Nonlinear Optical Materials

KBe2BO3F2 (KBBF) is still the only practically usable crystal that can generate deep-ultraviolet (DUV) coherent light by direct second harmonic generation (SHG). However, applications are hindered by layering, leading to difficulty in the growth of thick crystals and compromised mechanical integrity. Despite efforts, it is still a great challenge to discover new nonlinear optical (NLO) materials that overcome the layering while keeping the DUV SHG available. Now, two new DUV NLO beryllium borates have been successfully designed and synthesized, NH4Be2BO3F2 (ABBF) and -Be2BO3F (-BBF), which not only overcome the layering but also can be used as next-generation DUV NLO materials with the shortest typeI phase-matching second-harmonic wavelength down to 173.9nm and 146nm, respectively. Significantly, -BBF is superior to KBBF in all metrics and would be the most outstanding DUV NLO crystal.