Electron beam interrogation and control of ultrafast plexcitonic dynamics

n the strong coupling regime, photons and excitons exchange energy very rapidly such that they blend together, forming hybrid plexcitonic states. Recently, strong coupling of plasmons with a single fluorescent molecule was realized for the first time at room temperature using plasmonic nanocavities. Here, we show that the electron beam excitation of a plasmonic nanocavity allows us to dynamically control the light-matter interaction forming the plexciton without changing the plasmonic nanocavity. Unlike dark-field microscopy, an electron beam has the ability to excite dark plasmons which, due to their non-radiative nature, allow for more energy cycles between the plasmon and the emitter to survive in a very lossy system. Directly comparing effective 2D (nanowire) and 3D (nanosphere) plasmonic nanocavities we demonstrate that the dark modes inherently present at lower frequencies in the nanowire-nanocavity can be used as a means to extend the quantum coherence between the plasmon and emitter, opening the way towards dynamic tuning of the strong-coupling dynamics via the electron beam velocity. This opens new routes for dynamically addressing and controlling quantum emitters in the strong coupling regime via (the velocity of) the electron beam.