Imaging Plasmon Hybridization in Metal Nanoparticle Aggregates with Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) provides detailed nanoscopic spatial and spectral information on plasmonic nanoparticles that cannot be discerned with far-field optical techniques. Here we demonstrate that EELS is capable of mapping the relative phases of individual localized surface plasmons that are hybridized within nanoparticle assemblies. Within the context of an effective plasmon oscillator model, we demonstrate the relationship between the self-induced back-force on the electron due to the plasmon and the EEL probability and use this to present a rubric for determining the relative phases of hybridized localized surface plasmons in EELS. Comparison between the analytical oscillator model, experiment, and numerical electrodynamics simulation is made across a variety of nanoparticle monomer, dimer, and trimer systems.