Role of Photoinduced Exciton in the Transient Terahertz Conductivity of Few-Layer WS2 Laminate

The exciton effect in two-dimensional (2D) transition metal dichalcogenides (TMDs) plays a dominating role in describing the optical and optoelectronic properties. However, the interplay between the excitons and free carriers has not yet been understood upon photoexcitation in 2D TMDs. Here, we first present a study of the dynamical interplay of excitons and unbound electron–hole pairs using time-resolved terahertz (THz) spectroscopy (TRTS) in a few-layer WS2 laminate. Our experimental results demonstrate that the Auger recombination is observed in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation. The transient complex THz photoconductivity spectroscopy of WS2 is well described by the Drude–Lorentz model of free carriers modulated by the exciton polarization field. Our results provide a comprehensive understanding of nonequilibrium carrier kinetics (both excitons and free carriers) in WS2 laminate, and should be applicable to other 2D systems.