  The rapidly developing field of plasmonics can be roughly categorized into
two branches: surface plasmon polaritons (SPPs) propagating in plasmonic
waveguides and localized surface plasmons (LSPs) supported by scattering
plasmonic particles. Investigations along these two directions usually employ
quite different approaches and techniques, resulting in more or less a dogma
that the two branches progress almost independently of each other, with few
interactions. Here in this work we interpret LSPs from a Bohr model based
geometric perspective relying on SPPs, thus establishing a connection between
these two sub-fields. Besides the clear explanations of conventional scattering
features of plasmonic nanoparticles, based on this geometric model we further
demonstrate other anomalous scattering features (higher order modes supported
at lower frequencies, and blueshift of the resonance with increasing particle
sizes) and multiple electric resonances of the same order supported at
different frequencies, which have been revealed to originate from backward SPP
modes and multiple dispersion bands supported in the corresponding plasmonic
waveguides, respectively. Inspired by this geometric model, it is also shown
that, through solely geometric tuning, the absorption of each LSP resonance can
be maximized to reach the single channel absorption limit, provided that the
scattering and absorption rates are tuned to be equal. The Bohr model based
geometric picture offers new insights into the understanding of the localized
resonances, and may shed new light to many related applications based on
particle scattering, such as biosensing, nanoantennas, photovoltaic devices and
related medical treatment.
