We report on the experimental and the theoretical investigation of multipole surface plasmon resonances in metal nanowires conductively connected by small junctions. The influence of a conductive junction on the resonance energies of nanowire dimers was simulated using the finite element method based software CST Microwave Studio and experimentally measured by electron energy-loss spectroscopy in a transmission electron microscope. We extend the analysis of conductively connected structures to higher order multipole modes up to third order, including dark modes. Our results reveal that an increase in junction size does not shift significantly the antibonding modes, but causes a strong blue shift of the bonding modes, leading to an energetic rearrangement of the modes compared to those of a capacitively coupled dimer with similar dimensions.

Recently, there has been significant interest in using dielectric nanocavities for the controlled scattering of light, owing to the diverse electromagnetic modes that they support. For plasmonic systems, electron energy-loss spectroscopy (EELS) is now an established method enabling structure–optical property analysis at the scale of the nanostructure. Here, we instead test its potential for the near-field mapping of photonic eigenmodes supported in planar dielectric nanocavities, which are lithographically patterned from amorphous silicon according to standard photonic principles. By correlating results with finite element simulations, we demonstrate how many of the EELS excitations can be directly corresponded to various optical eigenmodes of interest for photonic engineering. The EELS maps present a high spatial definition, displaying intensity features that correlate precisely to the impact parameters giving the highest probability of modal excitation. Further, eigenmode characteristics translate into their EELS signatures, such as the spatially and energetically extended signal of the low Q-factor electric dipole and nodal intensity patterns emerging from excitation of toroidal and second-order magnetic modes within the nanocavity volumes. Overall, the spatial–spectral nature of the data, combined with our experimental–simulation toolbox, enables interpretation of subtle changes in the EELS response across a range of nanocavity dimensions and forms, with certain simulated resonances matching the excitation energies within ±0.01 eV. By connecting results to far-field simulations, perspectives are offered for tailoring the nanophotonic resonances via manipulating nanocavity size and shape.

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