Temporal Evolution of Low-Temperature Phonon Sidebands in Transition Metal Dichalcogenides

Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungstendiselenide (WSe_2) shows a multitude of sharp emission peaks below the brightexciton. Some of them have been recently identified as phonon sidebands ofmomentum-dark states. However, the exciton dynamics behind the emergence ofthese sidebands has not been revealed yet. In this joint theory-experimentstudy, we theoretically predict and experimentally observe time-resolved PLproviding microscopic insights into thermalization of hot excitons formed afteroptical excitation. In good agreement between theory and experiment, wedemonstrate a spectral red-shift of phonon sidebands on a timescale of tens ofpicoseconds reflecting the phonon-driven thermalization of hot excitons inmomentum-dark states. Furthermore, we predict the emergence of a transientphonon sideband that vanishes in the stationary PL. The obtained microscopicinsights are applicable to a broad class of 2D materials with multiple excitonvalleys.