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

TL;DR

This work demonstrates optical brightening of spin-forbidden dark trions in monolayer WS$_2$ at elevated temperatures by coupling to Anderson-like localized surface plasmons in a disordered Au substrate. A temperature-dependent PL study reveals a spectral doublet separated by about 45 meV, interpreted as semi-dark and bright trions arising from intervalley electron–electron scattering mixing of dark and bright states. A two-state model, supported by measured spin-orbit splittings and coupling strengths, quantitatively accounts for the observed splitting and the PL intensities, while polarization analysis shows a negative circular polarization at the semi-dark energy. The findings establish a scalable plasmonic approach to access dark excitonic complexes in 2D semiconductors, with potential applications in valleytronics and nanophotonics.

Abstract

Optically inactive dark trions in two-dimensional semiconductors 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 optical activation remains challenging, specially at elevated temperatures. Here, we demonstrate the brightening of dark trions from ML WS$_2$ in the temperature range, 83 K-115 K, via localized surface plasmon modes in a disordered gold substrate. The resulting photoluminescence (PL) spectrum reveals a distinct spectral doublet with the twin peaks separated by ~ 45 meV. We propose that the peaks represent semi-dark and bright trion states, the origin of which lies in intervalley electron-electron scatterings. We also report on the experimental evidence of a negative degree of circular polarization in ML WS$_2$ at the energy of the semi-dark trion state.

Figures (11)

• Figure 1: (a) singlet bright trion (X$_{\mathrm{S}}^{-}$), (b) triplet bright trion (X$_{\mathrm{T}}^{-}$), (c) dark trion (X$_{\mathrm{D}}^{-}$).
• Figure 2: (a) A schematic diagram of the experimental arrangement of the ML WS$_2$ sample on a disordered Au film with an underlying Si/SiO$_2$ substrate. (b) A 3D AFM profile of the rough gold surface with the sample covering the dotted rectangular area. (c) PL spectra at T= 83 K of ML WS$_2$ placed on the Si/SiO$_2$ substrate (colored blue), and on the Si/SiO$_2$/Au substrate (colored red).
• Figure 3: (a) PL spectra at 83 K from different regions of the sample, along the line shown in the right panel. (b) PL integrated intensity of $E_+$ and $E_-$peaks at 83 K as a function of laser power. (c) False color plot for normalized PL intensity as a function of temperature from ML WS$_2$ on rough Si/SiO$_2$/Au substrate. (d) Integrated PL intensity of $E_+$ and $E_-$ states as a function of temperature.
• Figure 4: Schematic diagram of the intervalley scattering processes that couple the dark and bright states of trions. $E_{g}$ is the band gap and $\Delta_{SO}$ stands for the CB spin-splitting. (b) Circular polarization-resolved PL spectrum of ML WS$_2$ on disordered Si/SiO$_2$/Au substrate, as detected in the $\sigma+$ and $\sigma-$ channels on excitation by $\sigma+$ 488 nm (2.54 eV) laser irradiation. (c) Spin conserving scattering process from X$_{\mathrm{S}}^{-}$ to X$_{\mathrm{D}}^{-}$. (d) Scattering from X$_{\mathrm{T}}^{-}$ to X$_{\mathrm{S}}^{-}$ via e-h exchange interaction.
• Figure S1: (a) Room-temperature PL spectrum of ML WS$_2$ on a Si/SiO$_2$ substrate. (b) Room-temperature PL spectra from ML WS$_2$ on the disordered Au film and from the sole disordered Au film, as indicated in the figure.