Probing the electromagnetic response of dielectric antennas by vortex electron beams

TL;DR

This paper explores the use of vortex electron beams in electron energy-loss spectroscopy to distinguish electric and magnetic optical modes in dielectric nanoantennas, revealing new capabilities for probing complex photonic responses.

Contribution

It provides a theoretical framework connecting quantum scattering of vortex electron beams to classical models, enabling enhanced characterization of optical excitations in nanostructures.

Findings

VEBs can selectively probe electric or magnetic modes.

Adjusting beam vorticity or voltage changes excitation sensitivity.

Chiral nanostructures exhibit dichroism in v-EELS spectra.

Abstract

Focused beams of electrons, which act as both sources, and sensors of electric fields, can be used to characterise the electric response of complex photonic systems by locally probing the induced optical near fields. This functionality can be complemented by embracing the recently developed vortex electron beams (VEBs), made up of electrons with orbital angular momentum, which could, in addition, probe induced magnetic near fields. In this work, we revisit the theoretical description of this technique, dubbed vortex Electron Energy-Loss Spectroscopy (v-EELS). We map the fundamental, quantum-mechanical picture of the scattering of the VEB electrons to the intuitive classical models, which treat the electron beams as a superposition of linear electric and magnetic currents. We then apply this formalism to characterise the optical response of dielectric nanoantennas with v-EELS. Our calculations reveal that VEB electrons probe electric or magnetic modes with different efficiency, which can be adjusted by changing either beam vorticity or acceleration voltage to determine the nature of the probed excitations. We also study a chirally-arranged nanostructure, which in the interaction with electron vortices produces dichroism in electron energy loss spectra. Our theoretical work establishes VEBs as versatile probes that could provide information on optical excitations otherwise inaccessible with conventional electron beams.