  Plasmonic nanoantennas, metallodielectric structures with engineered size and
shape, have attracted much attention lately as they make the control of the
directionality and temporal characteristics of light emitted by fluorophores
possible. Nanoantennas exploit light-matter interactions mediated by Localized
Surface Plasmon Resonances and, so far, have been demonstrated using metallic
nanoparticles or other metallic nanostructures. Plasmonic picocavities, i.e.,
plasmonic cavities with mode volumes below 1 cubic nanometer, could act as
antennas to mediate light-matter interaction even more efficiently than their
nanoscale counterparts due to their extreme field confinement, but the
directionality on their emission is difficult to control. In this work, we show
that the plasmonic picocavity formed between the tip of a Scanning Tunnelling
Microscope and a metal surface with a monoatomic step shows directional
emission profiles and, thus, can be considered as a realization of a
picoantenna. Comparison with electromagnetic calculations demonstrates that the
observed directionality arises from light emission tilting of the picocavity
plasmons. Our results, thus, pave the way to exploiting picoantennas as an
efficient way to control light-matter interaction at the nanoscale.
