First polar quaternary sulphide CsLiGa6S10 with mixed ordered alkali cations displaying excellent infrared nonlinear optical properties

Abstract

Infrared (IR) nonlinear optical (NLO) materials with simultaneous strong NLO coefficient and high laser-induced damaged threshold (LIDT) are the urgently desired optical performance NLO materials in laser application. Here, two different electropositive alkali elements are concurrently incorporated into a tetrahedral GaS₄ anionic framework, affording the first polar quaternary sulphide CsLiGa₆S₁₀ (CLGS) with mixed ordered alkali cations through the solid-state method. CLGS displays a wide band gap (3.25 eV) originating from the GaS₄ anionic framework and assistant of the ‘dimensional reduction’ effect by involving two alkali cations. This merit, accompanied by extremely small anisotropy of the thermal expansion of crystal CLGS (0.05) enhances the LIDT (51.0 MW cm²) of 4.5 times that of benchmark AgGaS₂ (11.3 MW cm²) at 1064 nm with the pulse width of 10 ns. Phase CLGS adopts a diamond-like anionic framework in which all tetrahedra are arranged in a highly oriented manner, providing a critical contribution to modest second-harmonic generation 0.7 times that of AgGaS₂ with a phase-matchable behaviour at the 1910 nm laser. These attributes, along with broad transmittance, warrant the further exploration of potential IR NLO chalcogenides containing multiple alkali cations.