  Polaritons are compositional light-matter quasiparticles that have recently
enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in
material science. Despite the enormous progress, however, a direct
nanometer-scale visualization of polaritons has remained an open challenge.
Here, we demonstrate that plasmon-exciton polaritons, or plexcitons, generated
by a hybrid system composed of an individual silver nanoparticle and a
few-layer transition metal dichalcogenide can be spectroscopically mapped with
nanometer spatial resolution using electron energy loss spectroscopy in a
scanning transmission electron microscope. Our experiments reveal important
insights about the coupling process, which have not been reported so far. These
include nanoscale variation of Rabi splitting and plasmon-exciton detuning, as
well as absorption-dominated extinction signals, which in turn provide the
ultimate evidence for the plasmon-exciton hybridization in the strong coupling
regime. These findings pioneer new possibilities for in-depth studies of
polariton-related phenomena with nanometer spatial resolution.
