  Controlling directional emission of nanophotonic radiation sources is
fundamental to tailor radiation-matter interaction and to conceive highly
efficient nanophotonic devices for on-chip wireless communication and
information processing. Nanoantennas coupled to quantum emitters have proven to
be very efficient radiation routers, while electrical control of unidirectional
emission has been achieved through inelastic tunneling of electrons. Here we
prove that the radiation emitted from the interaction of a high-energy electron
beam with a graphene-nanoparticle composite has beaming directions which can be
made to continuously span the full circle even through small variations of the
graphene Fermi energy. Emission directionality stems from the interference
between the double cone shaped electron transition radiation and the
nanoparticle dipolar diffraction radiation. Tunability is enabled since the
interference is ruled by the nanoparticle dipole moment whose amplitude and
phase are driven by the hybrid plasmonic resonances of the composite and the
absolute phase of the graphene plasmonic polariton launched by the electron,
respectively. The flexibility of our method provides a way to exploit graphene
plasmon physics to conceive improved nanosources with ultrafast reconfigurable
radiation patterns.
