Multi-step cation substitution facilitating the exploration of potential infrared nonlinear optical materials

Abstract

Crystal structure regulation and optical performance enhancement are huge challenges, especially for the laborious and inefficient trial-and-error method, in the research on infrared nonlinear optical (IR NLO) materials. In this work, multi-step cation substitutions were adopted to modulate the crystal structure for the effective design of well-performing IR NLO materials. Specifically, starting from the famous AgGaS₂ (AGS, I2d) crystal, Ag₂CdSiS₄ (Pmn2₁) with diamond-like crystal structure, BaAg₂SiS₄ (I2m) with three-dimensional tunnel structure and LaAgSiS₄ (Ama2) with two-dimensional layer structure were designed and experimentally synthesized through multi-step cation substitutions. Structural analysis reveals the fundamental reason of their tetrahedral framework transformation: the size effect of cations and the reduction of component tetrahedra caused by cationic substitution change the assembly mode of the tetrahedral units. Additionally, three non-centrosymmetric Ag-based thiosilicates exhibit wide transmittance range (0.5–17 μm), higher laser-induced damage thresholds (2 times that of AGS) and significant phase-matchable second harmonic generation (SHG) enhancement from 0.6 to 3.2 times that of AGS. This work demonstrates that multi-step cation substitution is an effective way to extend non-centrosymmetric structures, which facilitate the exploration of potential infrared nonlinear optical materials.