Ultrafast Carrier Dynamics of Exfoliated Transition Metal Dichalcogenides with Optical-Pump Terahertz-Probe Microscopy

Atomically thin two-dimensional transition metal dichalcogenides (TMD) exhibit extraordinary properties similar to graphene but features an intrinsic bandgap, opening vast potential applications in photonics and optoelectronics as semiconductors. The characterization of these materials is essential for developing such technologies and the emerging technique of terahertz (THz) microscopy enables contactless probing that directly reveals the carrier dynamics of TMDs. We employ a unique, optical-pump THz-probe microscope to measure the electric field response of mechanically exfoliated bulk TMD MoS₂. We measure the decay lifetime of bulk MoS₂ electron carriers with a temporal resolution of approximately 500 femtoseconds in the 1 THz range. We observe the relaxation of free carriers to be uneven on a single sample and the decay constant was found to vary with position from 36.9 ps to 82.5 ps. Using atomic force microscopy, we demonstrate a correlation between the thickness of a given position on the MoS₂ sample with the decay rate, where a thinner region or sample edge position corresponded to a faster decay. We attribute this nonuniform carrier relaxation rate to the edge state and surface defect effects.