Observation of Exciton–Exciton Interaction Mediated Valley Depolarization in Monolayer MoSe₂

The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long time scales (hundreds of nanoseconds). However, time-resolved measurements report valley lifetimes of only a few picoseconds. This has been attributed to mechanisms such as phonon-mediated intervalley scattering and a precession of the valley pseudospin through electron–hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe₂. We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton–exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs.