Fluorooxoborates: Ushering in a New Era of Deep Ultraviolet Nonlinear Optical Materials

Borates are one of the most important classes of functional materials, and several hundreds of artificial borates have been synthesized. The substitution of oxygen by fluorine leads to manifold classes of borates. Fluorooxoborates (also known as fluoroborates), in which the F atoms covalently connect with the B atoms, show additional compositional and structural diversity compared to classic borates. Recently, owing to the large polarizability anisotropy, large HOMO-LUMO gaps, and high hyperpolarizability of the oxyfluoride BOxF4-x building blocks, fluorooxoborates have received unprecedented attention in the search for new ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials. Specifically, some compounds have excellent NLO properties that are comparable or superior to KBe2BO3F2, which is the only usable crystal that generates coherent light below 200 nm through a direct second harmonic generation (SHG) process. This Minireview illustrates recent progress on the synthesis, crystal structures, structure-properties relationships and applications of fluorooxoborates. This paper concludes by highlighting the outstanding opportunities offered by NLO fluorooxoborate crystals as an innovative avenue for DUV all solid-state coherent light generation.