ACu4AsS4 (A = K, Rb): Two infrared nonlinear optical crystals with strong second-harmonic generation responses originating from multiple chromophores

Infrared (IR) nonlinear optical (NLO) crystals are of great significance in modern laser technology, and metal chalcogenides are receiving increasing interest as promising IR NLO candidates. To explore new IR NLO materials with enhanced properties, two quaternary chalcogenides with multiple chromophores, KCu4AsS4 and RbCu4AsS4, were synthesized using the solvothermal method. KCu4AsS4 and RbCu4AsS4 feature 2D [Cu6As2S6]∞ anionic layers with multiple chromophores, and exhibit strong phase-matched second-harmonic generation (SHG) responses (1.8–2.0 × AgGaS2), wide band gaps, suitable birefringence (0.083 and 0.074 @ 2050 nm) and wide infrared transparency. The theoretical calculation results suggest that the strong SHG responses are ascribed to the 2D [Cu6As2S6]∞ anionic layers.