Single-Crystalline Gold Nanodisks on WS2 Mono- and Multilayers for Strong Coupling at Room Temperature

Engineering light-matter interactions up to the strong-coupling regime at room temperature is one of the cornerstones of modern nanophotonics. Achieving this goal could enable new platforms for potential applications such as quantum information processing, quantum light sources, and even quantum metrology. Layered materials like transition metal dichalcogenides (TMDCs) and, in particular, tungsten disulfide (WS2), possess strong dipole moments which are comparable to semiconductor-based quantum dots, but the former also exhibit 1.6 large exciton binding energies, thereby making TMDCs suitable candidates for exploring light-matter interactions at ambient conditions. Furthermore, the combination of TMDCs with plasmonic nanocavities, which tightly confine light down to nanometer scale, has recently emerged as a suitable platform for achieving strong coupling between plasmons and excitons at room temperature. Here, we use ultrathin single-crystalline gold nanodisks featuring large in-plane electric dipole moments aligned with the exciton's dipole moments in monolayer WS2. By performing both scattering and reflection spectroscopy, we demonstrate strong coupling at room temperature with a Rabi splitting of similar to 408 meV. In addition, when the plasmonic resonance of these nanodisks is coupled with few-layer WS2, a Rabi splitting of similar to 175 meV is observed, with a major increase of 62% relative to the monolayer configuration. Our results therefore suggest that ultrathin single-crystalline gold nanodisks coupled to WS2 constitute an attractive platform to explore light-matter interactions in the strong-coupling regime.