Deep-ultraviolet nonlinear-optical crystal BaNa2[PO3(OH)]2 with large birefringence and strong second-harmonic-generation response

Deep-ultraviolet (DUV, λ ​< ​200 ​nm) nonlinear-optical (NLO) crystals play a pivotal role in modern laser science and technology. Metal phosphates have been considered as fascinating candidates for DUV NLO crystal material. However, how to generate both strong second-harmonic-generation (SHG) response and large birefringence for promising DUV NLO metal phosphates is a long-standing issue. Herein, a new DUV NLO metal phosphate, BaNa2[PO3(OH)]2, was successfully synthesized, which features a two-dimensional [NaPO3(OH)]∞ alveolate layer alternately connected by highly asymmetric [PO3(OH)] groups and NaO5 triangular bipyramids. BaNa2[PO3(OH)]2 can realize a good balance of DUV nonlinear properties covering short UV absorption edge less than 190 ​nm, strong SHG response with the magnitude of ∼1.2 ​× ​benchmark KH2PO4 (KDP), and the largest birefringence with the experimental value of [email protected] ​± ​3 ​nm in reported alkaline/alkaline-earth metal phosphates, producing the shortest phase-matching output down to the wavelength of 164 ​nm. These research results show that BaNa2[PO3(OH)]2 crystal is a potential DUV NLO crystal material.