Strong coupling between WS$_2$ monolayer excitons and a hybrid plasmon polariton at room temperature

Light-matter interactions in solid-state systems have attracted considerable interest in recent years. Here, we report on a room-temperature study on the interaction of tungsten disulfide (WS$_2$) monolayer excitons with a hybrid plasmon polariton (HPP) mode supported by nanogroove grating structures milled into single-crystalline silver flakes. By engineering the depth of the nanogroove grating, we can modify the HPP mode at the A-exciton energy from propagating surface plasmon polariton-like (SPP-like) to localized surface plasmon resonance-like (LSPR-like). Using reflection spectroscopy, we demonstrate strong coupling between the A-exciton mode and the lower branch of the HPP for a SPP-like configuration with a Rabi splitting of 68 meV. In contrast, only weak coupling between the constituents is observed for LSPR-like configurations. These findings demonstrate the importance to consider both the plasmonic near-field enhancement and the plasmonic damping during the design of the composite structure since a possible benefit from increasing the coupling strength can be easily foiled by larger damping.