Non-equilibrium diffusion of dark excitons in atomically thin semiconductors

TL;DR

This study reveals that dark excitons in atomically thin semiconductors exhibit a unique, time-dependent diffusion behavior shortly after excitation, significantly impacting their transport properties and potential applications.

Contribution

The paper provides the first combined theoretical and experimental analysis of dark exciton spatial dynamics, uncovering a transient increase in diffusion coefficient during initial picoseconds.

Findings

Dark excitons are initially populated via phonon emission from bright states.

A transient, rapid expansion of excitons leads to a temporary increase in diffusion coefficient.

Unconventional, time-dependent diffusion deviates from steady-state behavior.

Abstract

Atomically thin semiconductors provide an excellent platform to study intriguing many-particle physics of tightly-bound excitons. In particular, the properties of tungsten-based transition metal dichalcogenides are determined by a complex manifold of bright and dark exciton states. While dark excitons are known to dominate the relaxation dynamics and low-temperature photoluminescence, their impact on the spatial propagation of excitons has remained elusive. In our joint theory-experiment study, we address this intriguing regime of dark state transport by resolving the spatio-temporal exciton dynamics in hBN-encapsulated WSe$_2$ monolayers after resonant excitation. We find clear evidence of an unconventional, time-dependent diffusion during the first tens of picoseconds, exhibiting strong deviation from the steady-state propagation. Dark exciton states are initially populated by phonon emission from the bright states, resulting in creation of hot excitons whose rapid expansion leads to a transient increase of the diffusion coefficient by more than one order of magnitude. These findings are relevant for both fundamental understanding of the spatio-temporal exciton dynamics in atomically thin materials as well as their technological application by enabling rapid diffusion.