Direct Visualization of Counter-Propagating Surface Plasmons in Real Space-Time

We deploy two-dimensional nanohole arrays as resonant surface plasmon polariton (SPP) couplers that enable counter-propagation and excitation field interference-free imaging of SPP wave packets. We monitor the spatiotemporal evolution of the resulting SPPs using two-color photoemission electron microscopy. The measurements track the electric field envelope of the SPP in real space and time and enable direct characterization of their spatiotemporal properties in a regime where the SPP wave packet is the principal observable. We provide an analysis of the observables for both the co- and counter-propagating directions via SPP trajectories that are recorded in tandem. Our results highlight the advantages of isolating SPPs through counter-propagation, where excitation field-SPP interactions are suppressed.