Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

TL;DR

This study combines theory and experiments to understand how dark excitonic states and phonon interactions influence the coherence lifetime and linewidth of bright excitons in monolayer TMDs, revealing material-specific relaxation mechanisms.

Contribution

It provides a microscopic explanation for excitonic coherence lifetime in TMDs, highlighting the role of dark states and phonon-induced scattering, supported by combined theoretical and experimental analysis.

Findings

Dark excitonic states significantly affect optical properties.

Efficient phonon-mediated exciton relaxation observed in WS$_2$.

Coherence lifetime is governed by intra- and intervalley scattering.

Abstract

Atomically thin transition metal dichalcogenides (TMDs) are direct-gap semiconductors with strong light-matter and Coulomb interaction. The latter accounts for tightly bound excitons, which dominate the optical properties of these technologically promising materials. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. In a recent study, an experimental evidence for the existence of such dark states has been demonstrated, as well as their strong impact on the quantum efficiency of light emission in TMDs. Here, we reveal the microscopic origin of the excitonic coherence lifetime in two representative TMD materials (WS$_2$ and MoSe$_2$) within a joint study combining microscopic theory with optical experiments. We show that the excitonic coherence lifetime is determined by phonon-induced intra- and intervalley scattering into dark excitonic states. Remarkably, and in accordance with the theoretical prediction, we find an efficient exciton relaxation in WS$_2$ through phonon emission at all temperatures.