Three-Dimensional Multipole Rotation in Spherical Silver Nanoparticles Observed by Cathodoluminescence

Abstract

A spherical metallic nanoparticle is the simplest and most frequently used example of plasmonic nanostructures. In such a highly symmetric structure the plasmon modes consist of degenerate multipoles, which cannot be separately observed by only utilizing energy resolved means. We here demonstrate nanoscale optical field mappings of degenerate multipole modes in spherical silver nanoparticles using an angle- and polarization-resolved cathodoluminescence technique combined with scanning transmission electron microscopy. By properly selecting the detection angle and polarization, the observed optical field maps of spherical silver nanoparticles exhibit dipole or quadrupole features, depending on the energy. The angle-dependent multipole patterns visualize their three-dimensional rotation, which can be explained as superposition of the degenerate modes due to the spherical symmetry.