LAUSR

Monolayer transition metal dichalcogenides (TMDs) provide 2D semiconductors with a rich interplay of electron spin and valley degrees of freedom. Along with strong optical coupling strength, it provides an ideal platform to study exciton-related physics in the 2D limit. Intensive experimental approaches on exciton dynamics in monolayer TMDs have been focusing on the lifetime of the exciton, exciton valley/spin polarization, and coherence, whereas the 'hot' exciton relaxation is rarely addressed. Here we probe the dynamics of bright excitons in monolayer tungsten diselenides, by pumping at exciton ground state and probing at higher energy, where excitonic excited states can be identified from the pump-probe spectroscopy. Remarkably, the rise time of the differential reflection signal increases significantly with the index of the excited states, which we attribute to the phase-space filling effect that probes the momentum distribution of ground state exciton. Our results reveal fast exciton momentum relaxation as a signature of the strong Coulomb interaction in 2D TMDs.