  Spin-valley coupling in monolayer transition metal dichalcogenides gives rise
to valley polarization and coherence effect, limited by intervalley scattering
caused by exciton-phonon, exciton-impurity, and electron-hole exchange
interaction (EHEI). We explore an approach to tune the EHEI by controlling
excitons center of mass momentum (COM) utilizing the photon distribution of
higher-order optical vortex beam. By virtue of this, we have observed excitons
COM-dependent valley depolarization and decoherence which gives us the ability
to measure the timescale associated with valley dynamics in the steady-state
measurement. Our steady-state technique to probe the valley dynamics can open
up a new paradigm to explore the physics of excitons in two-dimensional
systems.
