  Dark plasmonic modes have interesting properties, such as a longer lifetime
and a narrower linewidth than their radiative counterpart, as well as little to
no radiative losses. However, they have not been extensively studied yet due to
their optical inaccessibility. Using electron-energy loss (EEL) and
cathodoluminescence (CL) spectroscopy, the dark radial breathing modes (RBMs)
in thin, monochrystalline gold nanodisks are systematically investigated in
this work. It is found that the RBMs can be detected in a CL set-up despite
only collecting the far-field. Their visibility in CL is attributed to the
breaking of the mirror symmetry by the high-index substrate, creating an
effective dipole moment. The outcoupling into the far-field is demonstrated to
be enhanced by a factor of 4 by increasing the thickness of the supporting SiN
membrane from 5 to 50 nm due to the increased net electric dipole moment in the
substrate. Furthermore, it is shown that the resonance energy of RBMs can be
easily tuned by varying the diameter of the nanodisk, making them promising
candidates for nanophotonic applications.
