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.

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Monolayers of transition metal dichalcogenides (TMD) are promising materials for optoelectronics devices. However, one of the challenges is to fabricate large-scale growth of high quality TMD monolayers with the desired properties in order to expand their use in potential applications. Here, we demonstrate large-scale tungsten disulfide (WS) monolayers grown by van der Waals Epitaxy (VdWE). We show that, in addition to the large structural uniformity and homogeneity of these samples, their optical properties are very sensitive to laser irradiation. We observe a time instability in the photoluminescence (PL) emission at low temperatures in the scale of seconds to minutes. Interestingly, this change of the PL spectra with time, which is due to laser induced carrier doping, is employed to successfully distinguish the emission of two negatively charged bright excitons. Furthermore, we also detect blinking sharp bound exciton emissions which are usually attractive for single photon sources. Our findings contribute to a deeper understanding of this complex carrier dynamics induced by laser irradiation which is very important for future optoelectronic devices based on large scale TMD monolayers.

In this work, we have investigated the structural and optical properties of GaAs (1x) Bi x /GaAs single quantum wells (QW) grown by molecular beam epitaxy (MBE) on GaAs (311)B substrates using x-ray diffraction (XRD), atomic force microscopy (AFM), Fourier-transform Raman (FT-Raman) and photoluminescence (PL) spectroscopy techniques. The FT-Raman results revealed a decrease of the relative intensity ratio of transverse (TO) and longitudinal (LO) optical modes with the increase of Bi concentration which indicates reduction of the structural disorder with increasing Bi incorporation. In addition, the PL results show an enhancement of the optical efficiency of the structures as the Bi concentration is increased due to important effects of exciton localization related to Bi defects, non-radiative centers and alloy disorder. These results provide evidence that Bi is incorporated effectively in the QW region. Finally, the temperature dependence of PL spectra has evidenced two distinct types of defects related to the Bi incorporation, namely Bi clusters and pairs, and alloy disorder and potential fluctuation.

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