Ultrafast nonlinear absorption and carrier relaxation in ReS2 and ReSe2 films

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.