  High spatial and energy resolution EELS can be employed for detailed
characterization of both localized and propagating surface plasmon excitations
supported by metal nanostructures, giving insight into fundamental physical
phenomena involved in various plasmonic effects. Here, applying EELS to
ultra-sharp convex grooves in gold, we directly probe extremely confined gap
surface plasmon (GSP) modes excited by swift electrons in nanometer-wide gaps.
Both experimental and theoretical EELS data reveal the resonance behavior
associated with the excitation of the antisymmetric (with respect to the
transverse electric-field component) GSP mode for extremely small gap widths,
down to ~5 nm. It is argued that the excitation of this mode, featuring very
strong absorption, plays a crucial role in the experimental realization of
non-resonant light absorption by ultra-sharp convex grooves with
fabrication-induced asymmetry. Occurrence of the antisymmetric GSP mode along
with the fundamental GSP mode exploited in plasmonic waveguides with extreme
light confinement is a very important factor that should be taken into account
in the design of plasmonic nanophotonic circuits and devices.
