MgPO2F3: An Ultraviolet Nonlinear Optical System with an Extremely Short Phase-Matching Wavelength Achieved by Introducing Dual-Type Fluorine via Structure Prediction

Non-pi-conjugated systems with a short ultraviolet (UV) cutoff edge have been a hot research topic, while achieving second-harmonic generation (SHG) phase-matching (PM) in the UV/deep-ultraviolet (DUV) regions is a challenge for them because of small birefringence. Fluorination of [PO4] to [PO2F2] group can greatly shorten the SHG PM wavelength of the phosphate system. In this work, the alkaline-earth metal difluorophosphate system MgPO2F3 composition was designed by introducing dual-type fluorine into the phosphate system. Four thermodynamically metastable structures are obtained. All of them show UV/DUV SHG PM ability and balanced optical properties compared to phosphates. Among them, MgPO2F3-IV shows excellent comprehensive performance, with SHG coefficients as low as 0.92 x KDP and the shortest type I SHG PM wavelength down to 195 nm. The enhanced optical properties mainly originate from the [PO2F2] groups and F anions through the analysis of the response electron distribution anisotropy method and SHG density. This work elucidates the possibility that alkaline-earth metal difluorophosphates can be used as an alternative system for UV/DUV nonlinear optical (NLO) materials.