Dilute Soft-Cation Anchoring Stabilizes γ-NaAsSe2 against Moisture While Preserving Giant Mid-Infrared Nonlinearity

Abstract

Designing nonlinear optical materials that combine large second-harmonic generation (SHG) efficiency with long-term stability remains a challenge for infrared photonic applications. Chalcogenide crystals such as γ-NaAsSe2 (γ-NAS) have recently emerged as outstanding candidates, exhibiting an exceptionally high SHG susceptibility of ∼590 pm/V at 2 μm. Despite this exceptional SHG response, poor ambient stability has hindered practical use, largely because the labile Na/Se interchain environment is vulnerable to moisture-assisted degradation. Here, we address this limitation by combining Sb substitution at the As site, which stabilizes the polar γ framework, with dilute Ag substitution at the Na site to introduce stronger chalcogenophilic Ag–Se contacts without disrupting the electronic structure responsible for the large nonlinear response. Because phase analysis shows Ag has limited solubility in the γ-NAS lattice (≤2%), we selected γ-Na0.99Ag0.01As0.95Sb0.05Se2 for Bridgman crystal growth. Large single crystals remain stable under ambient conditions for 365 days while preserving a giant SHG response of ∼600 ± 15 pm/V, with stable SHG intensity confirmed during continuous ambient measurements. Dilute Ag substitution introduces more covalent Ag–Se contacts that act as local anchors between chalcogenide chains, suppressing moisture-assisted lattice opening while preserving the polar framework. Sb substitution stabilizes the polar γ polymorph, whereas dilute Ag substitution introduces more covalent Ag–Se contacts at the alkali-metal sublattice to improve ambient durability. Phase-matching calculations further suggest Type I phase-matchability. This work introduces dilute soft-cation anchoring as a chemical strategy for stabilizing moisture-sensitive polar chalcogenides while preserving the electronic structure responsible for large nonlinear optical responses.