Expanding Frontiers of Ultraviolet Nonlinear Optical Materials with Fluorophosphates

If a bucket is to hold more water, its shortest plank must be made longer. This guideline also applies to the exploration of ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials that are limited by multiple criteria. Phosphates are one kind of promising candidate for new NLO materials. Unfortunately, the small birefringence, as the shortest plank, severely restricts the phase-matching of second harmonic generation (SHG) in the UV/DUV region. In this work, fluorophosphates are rationally proposed as substitutes for phosphates to break down the limitation of birefringence and simultaneously enhance SHG response and retain wide UV transmittance. The (PO3F)2– and (PO2F2)− groups are confirmed as superior material genomes to achieve the discussed combination properties. Accordingly, (NH4)2PO3F was screened out by density functional theory calculation, and single crystals with centimeter size have been grown. It possesses a powder SHG efficiency of 1 × KH2PO4 (KDP) and is phase-matchable with output of SHG wavelength at 266 nm. To the best of our knowledge, it is the first time that fluorophosphates are identified and developed as new and ideal candidates to new UV/DUV NLO materials by combining theories and experiments.