AlGaSe₃: Tuning Stability via Partial Cation Substitution and Achieving a Nonlinear Optical Property Leap

Simultaneously achieving a large second-harmonic generation (SHG) response and high laser-induced damage threshold (LIDT) remains a critical scientific challenge in mid-infrared nonlinear optical (NLO) materials. Computational studies demonstrate that Al2Se3 exhibits a large NLO coefficient and an ultrawide bandgap among selenides. However, its instability excludes the application potential, which attracts us to design its derivatives. Novel AlGaSe3 can be obtained via a chemical substitution strategy when taking Al2Se3 as the template. AlGaSe3 crystallizes in the noncentrosymmetric space group Cc, isostructural with Al2Se3, and its three-dimensional diamond-like structure consists of two distinct MSe4 (M = Al/Ga) tetrahedral units. AlGaSe3 exhibits an NLO effect of 1.7 × Al2Se3 and 0.7 × AgGaS2 (AGS) at 2.1 μm and an LIDT of 4.3 × AGS while maintaining a relatively wide bandgap of 2.46 eV among selenides. Theoretical calculations and structural analyses reveal that the NLO performances of Al2Se3 and AlGaSe3 primarily originate from the synergistic interaction between two types of tetrahedral units. The enhanced SHG effect in AlGaSe3 is attributed to the more distorted tetrahedra, distinct orientation differences among them, and closer packing of them. This work provides an effective strategy for designing high-performance NLO materials.