The Achievement of a Strong Second Harmonic Generation Response and a Large Birefringence in Sulfates

In the solar-blind region, the development of short-wave ultraviolet (UV) lasers relies critically on high-performance nonlinear optical (NLO) crystals, which require a wide bandgap, a strong second harmonic generation (SHG) response, and a sufficient birefringence to fulfil phase-matching requirements. Although sulfates possess a wide bandgap necessary for short-wave UV applications, the inherent symmetry of [SO 4 ] 2− tetrahedron typically results in an insufficient birefringence. To optimize their birefringence, the introduction of π -conjugated ligands was implemented with a good structural inducer of 4-hydroxypyridine group. It is an effective strategy to markedly augment the birefringence of sulfates as demonstrated in (C 5 H 5 NO) 2 ZnSO 4 ·H 2 O crystal with a birefringence of 0.48 @ 546 nm. Then, based on it, a new UV NLO crystal, (C 5 H 5 NO)ZnSO 4 ·3H 2 O, was obtained by modulating the coordination mode of zinc atoms to mitigate the steric hindrance and increase hydrogen bond sites. It achieves the collaborative optimization of SHG response (1.5 × KH 2 PO 4 (± 0.1)) and optical anisotropy (birefringence ∼ 0.27 @ 546 nm), originating from the synergetic contribution of 4-hydroxypyridine ligands and [SO 4 ] 2− groups. Therefore, this study establishes an innovative strategy for the structural design and performance optimization in NLO materials.