Real-time observation of ultrafast Rabi oscillations between excitons and plasmons in metal nanostructures with J-aggregates

Surface plasmon polaritons (SPPs), optical excitations at the interface between a metal and a dielectric, carry significant potential for guiding and manipulating light on the nanoscale(1-3). However, their weak optical nonlinearities hinder active device fabrication, for example, for all-optical switching(4-7) or information processing(8,9). Recently, strong optical dipole coupling has been demonstrated between SPPs and nonlinear quantum emitters with normal mode splittings of up to 700 meV (refs 10-15). The predicted ultrafast energy transfer between quantum emitters and SPP fields could be a crucial microscopic mechanism for switching light by light on the nanoscale. Here, we present the first real-time observation of ultrafast Rabi oscillations in a J-aggregate/metal nanostructure, indicating coherent energy transfer between excitonic quantum emitters and SPP fields. We demonstrate coherent manipulation of the coupling energy by controlling the exciton density on a 10 fs timescale, which represents a step towards coherent, all-optical ultrafast plasmonic circuits and devices.