Brightening of dark trions in monolayer WS$_2$ via localization of surface plasmons

TL;DR

This paper demonstrates a method to brighten dark trions in monolayer WS$_2$ using localized surface plasmons, enabling their observation at higher temperatures and advancing quantum and valleytronic technologies.

Contribution

The study introduces a scalable plasmonic approach to access and observe semi-dark trions in monolayer WS$_2$ at elevated temperatures, which was previously challenging.

Findings

Brightening of dark trions via localized surface plasmons in WS$_2$.

Observation of a spectral doublet indicating semi-dark and bright trions.

Enhanced out-of-plane electromagnetic field enables visibility of semi-dark trions.

Abstract

Among all excitonic complexes in transition-metal dichalcogenides (TMDs), dark and semi-dark trions are poised to play a stellar role in future quantum technologies due to their long lifetimes, about two orders of magnitude greater than those of their bright counterparts. In monolayer (ML) tungsten disulphide (WS$_2$), accessing these states via a suitable brightening mechanism remains challenging, specially, at elevated temperatures. Here, we demonstrate the brightening of dark trions from ML WS$_2$ over the temperature range, 83 K-115 K, enabled by localized surface plasmon modes in a disordered gold substrate. The resulting photoluminescence (PL) spectrum reveals a distinct spectral doublet with the twin peaks of semi-dark and bright trion states, separated by $\sim$ 45 meV. The origin of the semi-dark trion state lies in intervalley electron-electron scatterings, while its visibility in the PL spectrum is made possible by the enhanced out-of-plane electromagnetic field associated with plasmon localization. We also report on the negative degree of circular polarization in ML WS$_2$ at the energy of the semi-dark trion state. Our results establish a scalable plasmonic route to access valley-polarized semi-dark trions, opening new opportunities for quantum and valleytronic applications.