Optical properties of metallic nanoparticles are well-known to be dominated by localized surface plasmons – collective electron oscillations. While the radiative plasmonic modes have been widely studied, their non-radiative counterpart, dark plasmons, have not been intensively investigated yet. Due to their lack of net dipole moment, they are inaccessible with light at normal incidence. In this work, the dark radial breathing modes (RBMs) of 20 nm-thin monocrystalline Au nanodisks (NDs) with varying diameters have been systematically studied with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS).
High spatial and spectral resolution EELS maps of a single Au ND reveal several different spatially distributed plasmonic modes, which can be attributed to the bright dipole with maximum intensity at the edge of the ND, the quadrupole with a more confined spatial distribution close to the edge of the ND, and the non-radiative RBM concentrated at the centre of the ND. While the observation of dark modes in EELS, which probes the full electromagnetic local density of optical states (EMLDOS)[1], is not surprising, the same modes are also remarkably evident in CL, which only detects the radiative part of the EMLDOS. However, our experimental results, in conjunction with numerical calculations of EEL and CL spectra, using the discontinuous Galerkin time-domain method[2], indicate that the substrate plays an essential role to enhance the visibility of the plasmonic dark modes in CL rather than previously reported retardation effects[3]. Furthermore, the diameter of the Au NDs mainly affects the spectral position of the plasmonic modes, whereas the substrate thickness seems to be responsible for the visibility of the dark modes in the CL spectra. In principle, this allows exploiting the lossless nature of dark plasmons for practical applications.