Controlling Multipolar Surface Plasmon Excitation through the Azimuthal Phase Structure of Electron Vortex Beams

We have theoretically studied how the azimuthal phase structure of an electron vortex beam excites surface plasmons on metal particles of different geometries as observed in electron energy loss spectroscopy. To do so, we have developed a semi-classical approximation combining an azimuthal phase factor and the dielectric formalism. Our results indicate that the vortex beam order may be used to modify and control surface plasmon multipole excitation in nanoparticles. In favorable cases, specific plasmon modes can even attain enhancement factor of several orders of magnitude. Since, electron vortex beams interact with particles mostly through interference effects due to azimuthal symmetries, i.e. in the plane perpendicular to the electron beam, anisotropy information (longitudinal and transversal) of the sample can be derived in EELS studies by comparing non-vortex and vortex beam measurements.