Polarization-Independent Plasmon-Exciton Strong Coupling in a Transition-Metal Dichalcogenide-Plasmonic Heterodimer Sandwich

Optical excitons in transition-metal dichalcogenide monolayers, when combined with plasmonic nanocavities, are excellent platforms for achieving the plasmon-exciton strong coupling regime at room temperature. Previous reports show strategies where either multiple layers of WSe2 are used or the optical response shows strong polarization dependence. Here, we investigate a precisely positioned array of vertically integrated plasmonic nanocavities with monolayer WSe2 sandwiched between them, similar to a nanoparticle-on-mirror geometry but with crucial design differences. We utilize optical retardation across isolated heterodimers to achieve record-high field enhancement within a 10–50 nm3 mode volume, thus showing polarization-insensitive plasmon-exciton strong coupling with an experimentally measured Rabi splitting greater than 170 meV. The large Rabi splitting and the polarization insensitivity at nonzero angles of incidence are further confirmed by numerical simulations. The strongly coupled WSe2-heterodimers are electrically disconnected and thus suitable for optoelectronic device applications, in addition to possible applicability in plexciton-based nanodevices such as low-threshold lasers.