Nonlinear optical inorganic sulfates: The improvement of the phase matching ability driven by the structural modulation

Abstract

Nonlinear optical (NLO) crystals are an essential support for laser applications, as they could extend the tunable range of all-solid-state lasers. NLO inorganic sulfates always exhibit a large band gap and a moderate second harmonic generation response, which should be a rich source of NLO materials. However, due to the high symmetry of the [SO4] tetrahedron, most of the inorganic sulfates present insufficient phase matching ability, the improvement of which will bring the rapid advance of NLO materials. Therefore, in this review, the development of NLO inorganic sulfates is briefly discussed and the structures with a good phase matching ability are carefully investigated. On the basis of the structure–property relationships, four design strategies to improve their phase matching ability are summarized, including (1) the combination of the planar group; (2) the cooperation of highly polarized cations; (3) the synergy of multiple functional motifs such as [IO3] triangular pyramids and highly polarized cations; (4) the development of polar structures with ferroelectricity. All these strategies effectively improve the phase matching ability of sulfates, while most of them also bring the reduction of the band gap. In order to give full play of the large band gap of sulfates and develop deep-ultraviolet NLO inorganic sulfates, we propose that much attention should be paid to the combination of the planar group based on the birefringence phase matching and the polar structures with ferroelectricity based on the quasi-phase matching. Finally, we hope that this review could provide some inspiration for the design of NLO crystals.