  Plasmonic nanoantenna is of promising applications in optical sensing,
single-molecular detection, and enhancement of optical nonlinear effect,
surface optical spectroscopy, photochemistry, photoemission, photovoltaics,
etc. Here we show that in a carefully-designed dimer gap-antenna made by two
metallic nanorods, the longitudinal plasmon antenna mode (AM) of bonding
dipoles can compete with the transverse plasmonic cavity modes (CMs), yielding
dramatically enhanced or suppressed scattering efficiency, depending on the CMs
symmetry characteristics (e.g., the radial order n and the azimuthal quantum
number m ). More specifically, it is demonstrated that an appropriately loaded
gap layer enables substantial excitation of toroidal moment and its strong
interaction with the AM dipole moment, resulting in Fano- or
electromagnetically induced transparency (EIT)-like profile in the scattering
spectrum. However, for CMs with nonzero azimuthal number, the spectrum features
a cumulative signature of the respective AM and CM resonances. We supply both
detailed near-field and far-field analysis for these phenomena, showing that
the modal overlap and phase relationship between the fundamental moments of
different order play a crucial role. A classical coupled oscillator model is
proposed which clearly reproduces the coexistence of scattering enhancement and
suppression. Finally, we show that the resonance bands of the AM and CMs can be
tuned by adjusting the geometry parameters and the permittivity of the load.
Our results may be useful in plasmonic cloaking, spin-polarized directional
light emission, ultra-sensitive optical sensing, and plasmon-mediated
photoluminescence.
