  We use spatially and angle-resolved cathodoluminescence imaging spectroscopy
to study, with deep subwavelength resolution, the radiation mechanism of single
plasmonic ridge antennas with lengths ranging from 100 to 2000 nm. We measure
the antenna's standing wave resonances up to the fifth order and measure the
dispersion of the strongly confined guided plasmon mode. By directly detecting
the emitted antenna radiation with a 2D CCD camera we are able to measure the
angular emission patterns associated with each individual antenna resonance. We
demonstrate that the shortest ridges can be modeled as a single point dipole
emitter oriented either upward (m=0) or in-plane (m=1). The far-field emission
pattern for longer antennas (m>2) is well described by two interfering in-plane
point dipoles at the end facets giving rise to an angular fringe pattern, where
the number of fringes increases as the antenna becomes longer. Taking advantage
of the deep subwavelength excitation resolution of the cathodoluminescence
technique, we are able to determine the antenna radiation pattern as function
of excitation position. By including the phase of the radiating dipoles into
our simple dipole model we completely reproduce this effect. This work
demonstrates how angle-resolved cathodoluminescence spectroscopy can be used to
fully determine the emission properties of subwavelength ridge antennas, which
ultimately can be used for the design of more complex and efficient antenna
structures.
