Mapping plasmons in nanoantennas via cathodoluminescence

We analyze cathodoluminescence (CL) emission from silver nanorod antennas induced by energetic electron beams. The dependence of the emission spectra on particle morphology and electron energy is explored by means of full electromagnetic calculations based on the boundary element method (BEM). We present light-emission image maps obtained by using a light detection system that is incorporated into a transmission electron microscope (TEM), operated in scanning mode. The intensity of each pixel in these maps corresponds to the photon counting rate when the electron beam is focused at that position of the sample. The maps exhibit strong dependence on the polarization of the emitted light and reveal standing-wave patterns of surface plasmons sustained by the nanorods, leading to characteristic spatial variations that correspond to the actual plasmon-mode symmetries. We thus demonstrate direct mapping of plasmon-mode symmetries by observing the variation of the CL intensity as the electron beam scans the sample. Good agreement between experimental and theoretical results is obtained, including the spatial modulation of the intensity along the direction perpendicular to the rods. In particular, plasmon modes of different azimuthal nature are resolved via their characteristic spatial dependence in our polarization-sensitive light detection system.