Spatio-spectral metrics in electron energy loss spectroscopy as a tool to resolve nearly degenerate plasmon modes in dimer plasmonic antennas

TL;DR

This paper introduces spatio-spectral metrics in electron energy loss spectroscopy (EELS) to improve the resolution of nearly degenerate plasmon modes in dimer antennas, combining simulations and experiments to enhance mode identification.

Contribution

It proposes novel spatio-spectral metrics that enable better resolution and identification of closely spaced plasmon modes in EELS, surpassing traditional spectral analysis methods.

Findings

Metrics successfully resolve three modes in experimental data

Simulations confirm the metrics accurately identify mode energies

Transverse bonding and antibonding modes remain unresolved

Abstract

Electron energy loss spectroscopy (EELS) is often utilized to characterize localized surface plasmon modes supported by plasmonic antennas. However, the spectral resolution of this technique is only mediocre, and it can be rather difficult to resolve modes close in the energy, such as coupled modes of dimer antennas. Here we address this issue for a case study of the dimer plasmonic antenna composed of two gold discs. We analyze four nearly degenerate coupled plasmon modes of the dimer: longitudinal and transverse bonding and antibonding dipole modes. With a traditional approach, which takes into account the spectral response of the antennas recorded at specific points, the modes cannot be experimentally identified with EELS. Therefore, we employ the spectral and spatial sensitivity of EELS simultaneously. We propose several metrics that can be utilized to resolve the modes. First, we utilize electrodynamic simulations to verify that the metrics indeed represent the spectral positions of the plasmon modes. Next, we apply the metrics to experimental data, demonstrating their ability to resolve three of the above-mentioned modes (with transverse bonding and antibonding modes still unresolved), identify them unequivocally, and determine their energies. In this respect, the spatio-spectral metrics increase the information extracted from electron energy loss spectroscopy applied to plasmonic antennas.