This dissertation provides what the author believes to be perhaps the first experimental evidence of single electrons passing both ends of a suspended split-ring resonator, with many applications for future nanophotonic devices, and novel implications for research into surface plasmons. / Swift electrons passing near metal-dielectric interfaces can excite travelling electromagnetic waves confined to the charge density at the interface, known as surface plasmon polaritons (SPPs). The excitation process retards the swift electrons to an extent which can be resolved using electron energy loss spectroscopy (EELS). Scanning transmission electron microscopy (STEM) paired with EELS can characterize these losses with high energy and spatial resolution but is a time-averaged technique. Hence, spectral data provides statistical information about the number of electrons which underwent a particular energy exchange, but their existences are otherwise spatiotemporally ambiguous. We have developed and tested a scheme which partially lifts this veil. This thesis details the steps taken to fabricate a split-ring resonator so that single electrons sequentially pass by both ends of the resonator, enabling what we refer to here as double-pass EELS. We provide evidence that single electrons have excited SPPs in both events, and that this aloof analogue to common plural scattering may also lead to an amplification in the second event. This suggests that our technique could be considered a single electron pump-probe spectroscopy, with wide-ranging applications, particularly in quantum research. / Dissertation / Doctor of Philosophy (PhD) / The future of computing may reside in a foundation of quantum technologies, where circuits which use light instead of electrical currents hold much promise. However, minimization of photonic (i.e. light based) components is limited by diffraction; we cannot guide light through channels that have arbitrarily small widths. We can exceed this limit by coupling the light into a ripple in the sea of electrons at the surface of a metal. These are known as surface plasmon polaritons, which can be confined to scales much smaller than what is accessible to light. Surface plasmon polaritons can alternatively be initiated when fast electrons pass by the metal surface and generate tides in the sea of electrons. The electrons lose a measurable amount of energy in this interaction which we can track to study the surface plasmon polaritons. I have worked with staff at the Canadian Centre for Electron Microscopy to design and fabricate U-shaped structures which have allowed a new class of experiments in which a single fast electron can pass by both ends, one after the other. This thesis explains the observed signatures in the energies of the collected electrons, including evidence that single electrons can excite two separate surface plasmon polaritons with a designed delay of around ten millionths of a billionth (10^-14) of a second. We also discuss the implications of these experiments for a wide range of potential applications. Additional works herein include simulations of the entanglement between fast electrons and surface plasmons, and analysis of spectral properties for ensembles of gold and silicon nanoparticles, given that the split-ring resonators are primarily made from those materials. Ultimately, this work introduces a new domain of possible experiments for electron energy loss spectroscopy in which we can characterize some of the ultrafast dynamics of surface plasmon polaritons within a time-averaged technique.