Morphology effect of metal-insulator-metal nanopatch antennas in strong coupling with monolayer WSe₂

Serving as cavity quantum electrodynamic testbeds at the nanoscale, the past decade has seen a prosperous rise in strong coupling between metallic nanostructures and semiconductor excitons. Within the iteration of the delicate plasmonic nanostructures, metal-insulator-metal (e.g., various nanoparticles-on-mirror) planar structures with highly confined fields and decent quality factors, come to prominence. Excitons in transition metal dichalcogenides (TMDCs) emerge as a favored choice of quantum emitters due to their room-temperature performance and potential integration into optoelectronic devices. In this paper, strong coupling with a Rabi splitting of 137 meV was achieved by coupling radial breathing mode from a nanoplate-on-mirror planar nanoantenna with neutral excitons in monolayer WSe₂. More importantly, we investigated the morphology dependence of the coupling in planar nanopatch antennas. It showed that the efforts taken to change the shapes and sizes of the nanopatch structures (e.g., hexagonal, triangular, round plates, etc.) can not effectively optimize the coupling. This is because only the excitons at the edges could contribute to the coupling strength. It is double-edged that, on the one hand, it sets a limitation for tuning the coupling strength, on the other hand, it prevents involving more excitons which may degrade the plexcitonic nonlinearity.