LiAlS2: A promising infrared frequency-conversion material with ultrawide band gap and high laser-induced damage threshold

The development of infrared (IR) nonlinear optical (NLO) crystals could foster their wide applications in civil and military fields. Thus, IR NLO materials with a wide band gap, high laser-induced damage threshold, and lack of two-photon absorption are urgently needed. This work reports the comprehensive optical properties of LiAlS2 through systematic experimental characterizations and the first principle study. The results confirmed the congruently melting feature of LiAlS2, type-I phase matching (PM) performance, and its second-harmonic generation (SHG) response (0.2 × AgGaS2 at 2.09 µm). Notably, LiAlS2 shows the widest band gap of 5.13 eV among the selected IR NLO materials and an ultrahigh laser-induced damage threshold about 28 times that of AgGaS2. The structure-property relationship analysis indicates that the [AlS4] tetrahedra determine the optical band gap and primarily contribute to the SHG response. Moreover, the calculated effective SHG coefficient deff, the refractive index dispersion relation, and the shortest PM wavelength are reported. The results of this study indicate that LiAlS2 can be a promising IR NLO material in high-energy laser systems.