From Monofluorophosphates A<sub>2</sub>PO<sub>3</sub>F to Difluorophosphates APO<sub>2</sub>F<sub>2</sub> (A = alkali metal): Design of a Potential Deep-Ultraviolet Nonlinear Optical Materials System with a Shortened Phase-Matching Wavelength
Wenqi Jin, Congwei Xie, Xueling Hou, Meng Cheng, Evgenii Tikhonov, Mengfan Wu, Shilie Pan, Zhihua Yang
Abstract
Exploring new nonlinear optical (NLO) crystals for the output of deep-ultraviolet (DUV) lasers via the frequency conversion technique is of great interest. Monofluorophosphate is a new chemical system for exploring DUV NLO crystals while difluorophosphate is still lacking in research but attracting attention. Herein, difluorophosphate is proved as a new potential member of the DUV NLO materials system based on high-throughtput crystal structure prediction combined with first-principles calculations. Our high-throughtput screening identified 34 structures for difluorophosphate APO 2 F 2 and monofluorophosphate A 2 PO 3 F (A = Li, Na, K, Rb, Cs) with good thermodynamical (meta)stability and promising NLO properties. Among them, six dynamically stable APO 2 F 2 structures show DUV performance as the shortest second harmonic generation (SHG) phase-matching (PM) wavelength around 185–199 nm and SHG coefficient larger than or comparable to that of KH 2 PO 4 (KDP). Notably, the APO 2 F 2 system possesses enhanced birefringence and shortened SHG PM wavelength compared to the A 2 PO 3 F system. It reveals that dual-fluorine drives stronger P–O bonding electron density along the direction of n max in PO 2 F 2 than that in PO 3 F, which is responsible for the enhanced birefringence and shortened SHG PM wavelength. These results provide a new direction and insight for exploring DUV NLO crystals.