Brightening of dark trions in monolayer WS₂via localization of surface plasmons

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 disulfide (WS), accessing these states via a suitable brightening mechanism remains challenging, specially, at elevated temperatures. Here, we demonstrate the brightening of dark trions from ML WSover the temperature range, 83-115 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 by45 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 WSat 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.