Temporal And Spatial Valley Dynamics In Two-Dimensional Semiconductors Probed Via Kerr Rotation

Monolayer transition metal dichalcogenides (TMDCs) offer a tantalizing platform for control of both spin and valley degrees of freedom, which may enable future optoelectronic devices with enhanced and novel functionalities. Here, we investigate the valley dynamics of two prototypical members of TMDCs, namely MoS2 and WSe2, using time-resolved Kerr rotation (TRKR) at temperatures from 10 K to 300 K. This pump-probe technique enables sub-picosecond temporal resolution, providing insight into ultrafast valley dynamics, which is inaccessible by polarized and time-resolved photoluminescence spectroscopy. Bi-exponential decay dynamics were observed for both materials at low temperatures, and the fast decay component indicated a rapid exciton valley depolarization time (< 10 ps) due to strong Coulomb exchange interactions between theK valleys. However, the slow decay components (several tens of picoseconds) were attributed to different origins in the two materials, which were further elucidated by temperature-dependent TRKR measurements. Moreover, the spatial dependence of the TRKR intensity across MoS2 monolayer flakes indicated a weaker valley polarization near the edges, which is likely associated with quenched excitons near the grain boundaries or a disordered edge region in chemical vapor deposition-grown monolayers. These temporal and spatial TRKR measurements reveal insight into the complex dynamics of valley excitonic states, which will be critical for valleytronic applications of monolayer TMDCs.