Strong coupling between WS₂ monolayer excitons and a hybrid plasmon polariton at room temperature

Light-matter interactions between plasmonic and excitonic modes have attracted considerable interest in recent years. A major challenge in achieving strong coupling is the identification of suitable metallic nanostructures that combine tight field confinement with sufficiently low losses. Here, we report on a room-temperature study on the interaction of tungsten disulfide (WS₂) 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 change the character of the HPP mode from propagating surface plasmon polariton-like (SPP-like) to localized surface plasmon resonance-like (LSPR-like). Using reflection spectroscopy, we demonstrate strong coupling with a Rabi splitting of 68 meV between the WS₂ monolayer excitons and the lower HPP branch for an optimized nanograting configuration with 60 nm deep nanogrooves. In contrast, only weak coupling between the constituents is observed for shallower and deeper nanogratings since either the field confinement provided by the HPP is not sufficient or the damping is too large. The possibility to balance the field confinement and losses render nanogroove grating structures an attractive platform for future applications.