Colossal Optical Second Harmonic Generation from Stable γ-Na₀.₉₈Ag₀.₀₂As₀.₉₅Sb₀.₀₅Se₂

Designing nonlinear optical (NLO) materials that unite high second harmonic generation (SHG) with long-term stability remains a key challenge for infrared (IR) photonics. Chalcogenides like γ-NaAsSe₂ exhibit exceptional SHG (d₁₁~590 pm/V at 2 µm), nearly 20× higher than AgGaSe₂, but suffer from poor ambient stability. Here, we employ a dual-doping strategy with 2% Ag at Na and 5% Sb at As sites to overcome this limitation. Large γ-Na₀.₉₈Ag₀.₀₂As₀.₉₅Sb₀.₀₅Se₂ crystals grown via the Bridgman method retain a giant SHG response (d₁₁~600 ± 15 pm/V) and remarkable stability for 246 days in air. Structural and spectroscopic analyses reveal that Ag enhances covalency and stabilizes the γ-phase, while Sb suppresses the γ→δ transition, improving crystal quality without degrading optical performance. Calculations confirm Type I phase matching, establishing γ-Na₀.₉₈Ag₀.₀₂As₀.₉₅Sb₀.₀₅Se₂ as a durable, high-performance IR NLO crystal for spectroscopy, telecommunications, and laser technologies.