Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2

TL;DR

This study investigates valley dynamics in monolayer WSe2 by analyzing the emission and polarization of neutral and charged excitons, revealing rapid decay of neutral excitons and stable valley polarization in trions over nanoseconds.

Contribution

It provides new insights into the distinct polarization and decay dynamics of neutral and charged excitons in monolayer WSe2, highlighting stable valley polarization in trions.

Findings

Neutral exciton decay time is about 4 ps.

Trion emission lasts tens of ps.

Valley polarization in trions persists over >1 ns.

Abstract

Optical interband transitions in monolayer transition metal dichalcogenides such as WSe2 and MoS2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K-valley in momentum space. Here we probe the valley dynamics in monolayer WSe2 by monitoring the emission and polarization dynamics of the well separated neutral excitons (bound electron hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of about 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.