(CN ₄ H ₇ )(NH ₂ SO ₃ ): A Solar‐Blind Ultraviolet Nonlinear Optical Crystal With Short UV Cutoff Edge and Full‐Wavelength Phase‐Matching

It is a challenge to develop solar‐blind ultraviolet (UV) nonlinear optical (NLO) crystals for the conflicting among wide bandgap, strong second‐harmonic generation (SHG) response and sufficient birefringence. The double‐amine substitution strategy by decreasing the local symmetry of both [SO 4 ] tetrahedron and π‐conjugated [C(NH 2 ) 3 ] group has been employed to obtain a solar‐blind UV NLO crystal, (CN 4 H 7 )(NH 2 SO 3 ). It achieves good balance among three key mutually exclusive properties, including a wide bandgap up to 5.88 eV, a strong phase‐matching SHG response reaching 0.5 × β ‐BaB 2 O 4 and large birefringence of 0.15@532 nm. Concurrently, (CN 4 H 7 )(NH 2 SO 3 ) displays high transparency down to 260 nm, a short UV cutoff edge extending to 211 nm, a high laser‐induced damage threshold of 1.23 GW cm −2 and favorable crystal growth habit. Notably, (CN 4 H 7 )(NH 2 SO 3 ) can realize full‐wavelength phase‐matching, in which its phase‐matching wavelength completely covers the transparency window down to 211 nm. (CN 4 H 7 )(NH 2 SO 3 ) is an intriguing candidate as a solar‐blind UV NLO crystal for the generation of 266 nm laser output‐the fourth harmonic of the widely used 1064 nm YAG laser.