Aliovalent Al³⁺ substituted ultra-wide bandgap CuAlX₄ (X = Cl, Br) for infrared nonlinear optics

The development of high-performance infrared (IR) nonlinear optical (NLO) crystals with ultra-wide bandgaps (>4.0 eV) is crucial for advancing laser technologies, yet achieving a balance between wide bandgaps and strong NLO responses remains challenging. A trivalent aliovalent substitution strategy is reported in this work to engineer diamond-like (DL) halides, CuAlX₄ (X = Cl, Br), which simultaneously achieve record-wide bandgaps (4.25 eV for CuAlCl₄ and 4.09 eV for CuAlBr₄), high laser-induced damage threshold (LIDT) values (46.2 MW cm⁻² and 33.9 MW cm⁻², respectively), and competitive second-harmonic generation (SHG) coefficients (3.52 pm V⁻¹ and 7.12 pm V⁻¹ at 2.09 μm, respectively). Structural characterization and first-principles calculations reveal that the introduction of Al³⁺ induces asymmetric [CuX₄] tetrahedra and cationic vacancies, breaking inversion symmetry and enhancing hyperpolarizability. The synergistic effects of increased valence electron concentration and optimized tetrahedral alignment result in exceptional optical properties, including broad transparency and type-I phase-matching behavior. This work demonstrates the potential of high-valence substitution for designing DL halides with ultra-wide bandgaps and robust NLO performance, offering new insights for IR optoelectronic applications.