Probing Bright and Dark Surface Plasmon Modes in Au Nanoparticles Using a Fast Electron Beam

We have studied the surface plasmons (SPs) of individual and coupled Au nanoparticles (NPs) with various sizes and shapes by electron energy-loss spectroscopy (EELS) using a 2-nm monochromatized fast electron beam in a scanning transmission electron microscope (STEM). EELS spectra were investigated with the beam in grazing incidence, the optical near-field setup, and \textit{bright} SP modes (visible by light) were identified in both individual, coupled NPs ranging from near-IR to visible regimes. In an individual long rod with an aspect ratio of $\sim $6, a \textit{dark} SP mode invisible by light was surprisingly characterized, supported by calculations of the macroscopic dielectric responses. With the NPs coupling, rich bright, dark SP modes emerge and the suppression of one mode over the other is dictated by the beam position when approaching a geometrically centered site. The electron scattering geometry thus plays the role the SP-mode selection, never documented before.