Direct Measurement of Exciton Valley Coherence in Monolayer WSe$_2$

TL;DR

This study directly measures the valley coherence time of excitons in monolayer WSe$_2$, revealing a coherence duration of about 100 femtoseconds, which is crucial for advancing valleytronics technologies.

Contribution

It is the first to experimentally determine the valley coherence time in monolayer WSe$_2$, highlighting the role of electron-hole exchange interaction in decoherence.

Findings

Valley coherence persists for approximately 100 femtoseconds.

Electron-hole exchange interaction significantly contributes to decoherence.

Provides insights for optical control of valley pseudospin.

Abstract

In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time, a key quantity for manipulating the valley pseudospin, has never been measured in any material. In this work, we use a sequence of laser pulses to resonantly generate a coherent superposition of excitons (Coulomb-bound electron-hole pairs) in opposite valleys of monolayer WSe$_2$. The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population decay. Our work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.