Abstract
Strong coupling of two-dimensional semiconductor excitons with plasmonic resonators enables control of light-matter interaction at the subwavelength scale. Here we develop such strong coupling in plasmonic nanogap resonators, which allows modification of exciton strength by altering electromagnetic environments in nearby semiconductor monolayers. Using this system, we not only demonstrate a large vacuum Rabi splitting up to 163 meV and splitting features in photoluminescence spectra but also reveal that the effective exciton number contributing to the coupling can be reduced down to the single-digit level (N < 10), which is 2 orders lower than that of previous systems, close to single-exciton based strong coupling. In addition, we prove that the strong coupling process is not affected by the large exciton coherence size that was previously believed to be detrimental to the formation of plasmon-exciton interaction. We provide a deeper understanding of strong coupling in two-dimensional semiconductors, paving the way for room-temperature quantum optics applications.