  We report on the investigation of an advanced circular plasmonic nanoantenna
under ultrafast excitation using nonlinear photoemission electron microscopy
(PEEM) under near-normal incidence. The circular nanoantenna is enhanced in its
performance by a supporting grating and milled out from a gold film. The
considered antenna shows a sophisticated physical resonance behavior that is
ideal to demonstrate the possibilities of PEEM for the experimental
investigations of plasmonic effects on the nanoscale. Field profiles of the
antenna resonance for both possible linear polarizations of the incident field
are measured with high spatial resolution. In addition, outward propagating
Hankel plasmons, which are also excited by the structure, are measured and
analyzed. We compare our findings to measurements of an isolated plasmonic
nanodisc resonator and scanning near-field optical microscopy (SNOM)
measurements of both structures. All results are in very good agreement with
numerical simulations as well as analytial models that are also discussed in
our paper.
