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

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 WS ₂ and MoS ₂ monolayers. Coupled-oscillator analysis reveals that the WS ₂ -nanocavity coupling strength increases with rising temperature, yet that for the MoS ₂ -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 experimentally determined exciton resonance energy and line width at elevated temperatures further reveal a quantitative proportionality between the plasmon-exciton coupling strength and exciton oscillator strength as predicted by a thermal dynamic model. On the basis of these experimental, theoretical, and numerical results, we propose that such a dramatic difference in the temperature-dependent plasmon-bright-exciton coupling strengths is due to the reversed sign of energy difference between the bright and dark excitons in WS ₂ and MoS ₂ monolayers, which consequently leads to opposite redistribution of their exciton population (proportional to their oscillator strength) under thermal tuning. Our comparative study provides a unified physics scenario of recent experimental results on the exciton oscillator strengths of these two typical TMDC monolayers, which is of critical importance for fundamental studies such as high-temperature stable polaritons and also for thermally robust photonic applications and nanoscale thermal switching in optical devices.