Temporal evolution of low-temperature phonon sidebands in WSe$_2$ monolayers

TL;DR

This study combines theory and experiment to reveal how hot excitons in WSe$_2$ monolayers thermalize over tens of picoseconds, causing spectral shifts in phonon sidebands observed in low-temperature photoluminescence.

Contribution

It provides the first combined theoretical and experimental analysis of exciton thermalization dynamics and phonon sideband evolution in WSe$_2$ monolayers.

Findings

Spectral red-shift of phonon sidebands over tens of picoseconds.

Observation of a transient phonon sideband that disappears in stationary PL.

Good agreement between theoretical predictions and experimental results.

Abstract

Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe$_2$) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory-experiment study, we theoretically predict and experimentally observe time-resolved PL providing microscopic insights into thermalization of hot excitons formed after optical excitation. In good agreement between theory and experiment, we demonstrate a spectral red-shift of phonon sidebands on a timescale of tens of picoseconds reflecting the phonon-driven thermalization of hot excitons in momentum-dark states. Furthermore, we predict the emergence of a transient phonon sideband that vanishes in the stationary PL. The obtained microscopic insights are applicable to a broad class of 2D materials with multiple exciton valleys.