Plasmonic Nanocavity Induced Coupling and Boost of Dark Excitons in Monolayer WSe₂ at Room Temperature

Abstract

Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.

Manipulating the light-matter interactions in plasmonic nanocavities at 1 nm spatial resolution

“…In Fig. S10 , we estimated the PL lifetimes of the plasmonic NCoM cavity at different positions 21 , 42 . PL lifetime can be described as , where Γ is radiative decay rate and Γ nr is nonradiative decay rate.…”

Microcavity exciton polaritons at room temperature

“…Other widely used plasmonic cavities usually consist of paired plasmonic structures, such as bowties [73,74] or nanoparticle-onmirror geometry [75] . When the gap between the paired metal nanoparticles is in sub-nanometer range, an ultra-low mode volume and ultra-strong field enhancement can be obtained, which is more likely to overcome the limit of high emitter scattering at room temperature to realize strong coupling [75][76][77] . These kinds of ultra-compact plasmonic cavities are widely used to study strong coupling with different materials [62,78] .…”

“…Right panel: PL spectra for a single etched WSe 2 -NPoM nanocavity. Peaks at 1.6 eV and 1.66 eV correspond to the emission of dark exciton and bright exciton, respectively [76] .…”

“…boosted at room temperature, finding emission comparable to that of spin-allowed bright excitons [Fig. 10(d)] [76] . Moreover, it is important to note that apart from the above-discussed works, many other plasmonic structures have been experimentally and theoretically investigated [81,[341][342][343] .…”

Nanoscale Valley Modulation by Surface Plasmon Interference

“…The evanescent field converges and forms SPI modes in the center of the SPW. The interference zone was situated in a nanoparticle-overmirror (NPOM) geometry, − which coupled the near-field SPI to the WS 2 layer. Within the monolayer of WS 2 , the excitonic population in the two different valleys (K and K′) can be directly accessed by the mode of localized SPI (Figure d).…”