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 LiAlS 2 through systematic experimental characterizations and the first principle study. The results confirmed the congruently melting feature of LiAlS 2 , type-I phase matching (PM) performance, and its second-harmonic generation (SHG) response (0.2 × AgGaS 2 at 2.09 µm). Notably, LiAlS 2 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 AgGaS 2 . The structure-property relationship analysis indicates that the [AlS 4 ] tetrahedra determine the optical band gap and primarily contribute to the SHG response. Moreover, the calculated effective SHG coefficient d eff , the refractive index dispersion relation, and the shortest PM wavelength are reported. The results of this study indicate that LiAlS 2 can be a promising IR NLO material in high-energy laser systems.