  Recent years have seen a growing interest in strong coupling between plasmons
and excitons, as a way to generate new quantum optical testbeds and influence
chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has
been achieved even with single quantum emitters. Dark plasmonic modes fare
better in some applications due to longer lifetimes, but are difficult to probe
as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy
to demonstrate that a dark mode of an individual plasmonic bowtie can interact
with a small number of quantum emitters, as evidenced by Rabi-split spectra.
Coupling strengths of up to 85 meV place the bowtie-emitter devices at the
onset of the strong coupling regime. Remarkably, the coupling occurs at the
bowtie gap periphery, even while the electron beam probes their center. Our
findings pave the way for using EEL spectroscopy to study exciton-plasmon
interactions involving non-emissive photonic modes.
