  Plasmonic nanostructures attract tremendous attention as they confine
electromagnetic fields well below the diffraction limit while simultaneously
sustaining extreme local field enhancements. To fully exploit these properties,
the identification and classification of resonances in such nanostructures is
crucial. Recently, a novel figure of merit for resonance classification has
been proposed 1 and its applicability was demonstrated mostly to toy model
systems. This novel measure, the energy-based plasmonicity index (EPI),
characterizes the nature of resonances in molecular nanostructures. The EPI
distinguishes between either a single-particle-like or a plasmonic nature of
resonances based on the energy space coherence dynamics of the excitation. To
advance the further development of this newly established measure, we present
here its exemplary application to characterize the resonances of graphene
nanoantennas. In particular, we focus on resonances in a doped nanoantenna. The
structure is of interest, as a consideration of the electron dynamics in real
space might suggest a plasmonic nature of selected resonances in the low doping
limit but our analysis reveals the opposite. We find that in the undoped and
moderately doped nanoantenna, the EPI classifies all emerging resonances as
predominantly single-particle-like and only after doping the structure heavily,
the EPI observes plasmonic response.
