Factors controlling the nonlinear optical (NLO) responses of infrared materials remain unclear due to the complex systems spanning the whole main group elements and also transition elements. The microscopic mechanism of cation-affected optical properties is consequently necessary to the mid-infrared region. In this research work, the covalence, electronic structure and mechanism of nonlinear optical (NLO) responded for a typical IR system, In-containing chalcogenides with the formula MInS2 (M = Ag, Li), are systemically investigated. It reveals that the orbitals hybridization, size effect and the cation electronegativity induce the changes of band gaps. Via tailoring the A-site cations, the microscopic factors affecting the optical band gap are analyzed. In addition to the effect of anionic groups on optical properties, we have found that the contributions of the A-site cations are not negligible. Furthermore, the electronic structures of sulfur in the compounds are susceptible to the type of cations, and consequently affect the NLO properties.
