The Power of Pnictogen Bond: As─As Linkages Drive Wide Bandgap, Strong NLO Response and Large Birefringence in SrGa ₃ Si ₄ As ₁₁

Arsenides represent an advantageous platform for designing long‐wavelength infrared (IR) nonlinear optical (NLO) materials; however, the intrinsic narrow bandgaps hinder their applications. In this work, by introducing As–As homoatomic bonds to eliminate the nonbonding electron pairs on As atoms, the first experimentally verified quaternary arsenide NLO material SrGa 3 Si 4 As 11 was rationally designed within the As‐rich A II –B III –C IV –As system, and synthesized experimentally. The compound crystallizes in the uniaxial hexagonal system ( P 6 3 space group, No. 173) and is composed of [SrAs 9 ] polyhedra, [GaAs 4 ] and [SiAs 4 ] tetrahedra with unique As–As homoatomic bonding. The introduction of As─As bonds simultaneously enhances the NLO response, bandgap, and birefringence, demonstrating a strong phase‐matching second‐order NLO response (∼8.0 × AgGaS 2 ), a wide arsenide bandgap (∼1.59 eV), a high laser‐induced damage threshold (∼1.5 × ZnGeP 2 and ∼4.0 × AgGaS 2 ) and a large birefringence (Δ n = 0.10@2090 nm) in the compound. Both experimental and theoretical results confirm the positive contributions of As─As bonds to these superior properties. The results enrich the structural diversity of arsenides, and establish As‐rich arsenides incorporating homoatomic bonds as an emerging system for high‐performance IR NLO materials, with SrGa 3 Si 4 As 11 serving as a promising candidate.