Uniform Alignment of Non-π-Conjugated Species Enhances Deep Ultraviolet Optical Nonlinearity

The precision of array of laser applications and manipulation on smaller scales are limited by the so-called 200 nm wall, and breakthroughs rely on the discovery of new materials with transparency and phase matchability in the ultraviolet and deep ultraviolet region. Herein, we discover an unprecedented alignment of the asymmetric non-π-conjugated species [PO₃F] in NaNH₄PO₃F·H₂O, which allows the best uniform P-F bond orientation that generates a remarkable enhancement of the (010) in-plane anisotropy that yields the largest birefringence (obv.: 0.053) to date in the phosphate and fluorophosphate families. The substance produces second harmonic generation lasers through direct frequency doubling with incident Yb:KGW femtosecond lasers on an as-synthesized (010) wafer with a size of 14 × 10 × 2.1 mm³. According to the calculated refractive index dispersion curve, the shortest second-harmonic generation (SHG) wavelength is estimated to be 194 nm, the shortest among phosphates and monofluorophosphate. These insights may help to design other high-performance non-π-conjugated deep-UV nonlinear optical materials.