SrZnSiS₄: Shattering the 4.0 eV Threshold toward Dual-Optimized Bandgap and Second-Harmonic Generation

Infrared (IR) nonlinear optical (NLO) crystals are crucial for mid-IR lasers, yet their intrinsic narrow band gaps cause low laser-induced damage thresholds (LIDTs) and detrimental two-photon absorption, thus limiting their widespread application. Herein, a synergistic strategy involving d-orbital exclusion and polarization motif construction is employed to synthesize novel chalcogenide SrZnSiS₄ by introducing [SiS₄] and [ZnS₄] tetrahedral polarization subunits. Performance evaluation demonstrates that SrZnSiS₄ breaks through the "4.0 eV" band gap threshold and holds the broadest band gap of 4.35 eV among the second-harmonic generation (SHG) > 0.5 × AgGaS₂ sulfur compound series, accompanied by an SHG response of 0.7 × AgGaS₂ and high LIDTs (>10 × AgGaS₂). Theoretical calculations reveal that SrZnSiS₄ can achieve phase matching with Δn = 0.05 at 1064 nm. Significantly, this material overcomes limitations of existing samples that offer an SHG coefficient above 0.5 × AgGaS₂, and it realizes synergistic optimization of a wide band gap and high nonlinearity.