Breaking Through the Trade‐Off Between Wide Band Gap and Large SHG Coefficient in Mercury‐Based Chalcogenides for IR Nonlinear Optical Application

It is substantially challenging for non‐centrosymmetric (NCS) Hg‐based chalcogenides for infrared nonlinear optical (IR‐NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase‐matching (PM) second‐harmonic‐generation intensity (deff > 1.0 × benchmark AgGaS2) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg‐based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2S4. The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04–3.07 eV), large PM deff (≈2.2–3.0 × AgGaS2), ultra‐high laser‐induced damage threshold (≈14.8–15 × AgGaS2), and suitable Δn (≈0.19–0.24@2050 nm), making them highly promising candidates for IR‐NLO applications. Importantly, such excellent second‐order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4] and [GeS4] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg‐based materials with well‐balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2) and puts forward a new design avenue for the construction of more efficient IR‐NLO candidates. Two novel non‐centrosymmetric chalcogenides, AEHgGeS4 (AE = Sr and Ba) are developed through the diagonal synergetic substitution strategy, which represents the first two examples breaking through incompatibility between large energy gap (Eg > 3.0 eV) and strong SHG response (deff > 1.0 × benchmark AgGaS2) in Hg‐based chalcogenides, owing to the synergistic effect of highly polarized [HgS4] and [GeS4] tetrahedra.