No abstract

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices.

We present a femtosecond optical heterodyne detection of the optical Kerr effect study on Chemical Vapor Deposition-grown monolayer MoS2 films at 800 nm. The third-order nonlinear optical susceptibility (i.e., χxxyy(3)+χxyyx(3)) of monolayer MoS2 is determined to be 1.4 × 10−9 esu, and the ultrafast temporal response process indicates that the susceptibility origins from nonresonant electronic polarization. Based on Kleinman symmetry, susceptibility tensor elements are determined, and further, the nonlinear refractive indexes of any elliptically polarized light could be calculated for MoS2. These results will benefit the application of MoS2 in nonlinear photonic devices.