Femtosecond Nonlinear Optical Properties of 2D Metallic NbS₂ in the Near Infrared

The understanding of the nonlinear optical (NLO) properties of photonic materials is relevant and necessary for basic studies and technological developments. Among the materials with high optical nonlinearities, layered transition-metal dichalcogenides (LTMDs) have attracted considerable attention from the viewpoint of both synthesis as well as characterization and applications. We report here the third-order NLO properties of one LTMDthe metallic NbS₂that was prepared by a modified redox exfoliated method, suspended in acetonitrile. The LTMD was morphologically and compositionally characterized using transmission electron microscopy, atomic force microscopy, X-ray diffraction, Raman scattering, and linear absorption. The Z-scan technique was employed to characterize the nonlinear refraction (NLR) and nonlinear absorption (NLA) behavior in the femtosecond regime. Metallic NbS₂ presented a sign inversion of the nonlinear refractive index from negative (self-defocusing) to positive (self-focusing) with the increase of intensity up to 100 GW/cm². Two distinct features characteristic of NLA, two-photon absorption and saturated absorption, were also observed for different intensity ranges, with the former evolving into the latter as the intensity increased in the same range as for the NLR measurements. The material’s band structure of NbS₂ was calculated using the density functional theory, and the origin of the nonlinearities is discussed by comparison with the experimental data.