Thermal Redistribution of Exciton Population in Monolayer Transition Metal Dichalcogenides Probed with Plasmon–Exciton Coupling Spectroscopy

Abstract

Inversion symmetry breaking and spin–orbit coupling result in spin-splitting of both valence and conduction bands in transition metal dichalcogenide (TMDC) monolayers. The optical transitions between band edges with opposite spins are termed dark excitons that are decoupled with in-plane polarized photons. Here, we find that the presence of dark excitons modifies the temperature-dependent plasmon–bright-exciton coupling strength of a TMDC monolayer interacting with a single plasmonic nanocavity. Quite interestingly, we observe that the modifications are in an opposite manner for WS2 and MoS2 monolayers. Coupled-oscillator analysis reveals that the WS2–nanocavity coupling strength increases with rising temperature, yet that for the MoS2–nanocavity diminishes, which both follow the temperature evolution of the respective exciton oscillator strength obtained by fitting the reflectance spectra of pristine TMDC monolayers with a multi-Lorentz oscillator model. Full-wave electromagnetic simulations with experim...