Strong plasmon coupling to high-energy band-nested excitons in 2D TMDCs*

We show Rabi splitting of 244 meV at the C-exciton in the two-dimensional (2D) transition metal dichalcogenide (TMDC) MoS₂. The splitting is 9.5 % of the transition energy (2.57 eV) in k-space (607 meV or 23.6 % in θ-space), which is ~2.5× the ratio observed in similar experiments at the A- and B-excitons and approaches the ultrastrong coupling limit. The large enhancement in the coupling strength is likely due to the nested conduction and valence bands found in the 2D TMDCs which leads to strong absorption. We measured the k-space dispersion of few-layer MoS₂ in a tunable plasmonic cavity at room temperature to observe the Rabi splitting. The band-nesting region of 2D TMDCs is under study for spontaneous charge-separation and slowed hot carrier cooling. Our observation suggests that strong coupling of the C-exciton in the 2D TMDCs may improve carrier lifetimes as well as enhance other quantum coherent phenomena such as polariton condensation or molecular hybridization for chemical reactions.