  Optical nanoantennas efficiently convert confined optical energy into
free-space radiation. The polarization of the emitted radiation depends mainly
on nanoantenna shape, so it becomes extremely difficult to manipulate it unless
the nanostructure is physically altered. Here we demonstrate a simple way to
synthetize the polarization of the radiation emitted by a single nanoantenna so
that every point on the Poincar\'e sphere becomes attainable. The nanoantenna
consists of a single scatterer created on a dielectric waveguide and fed from
its both sides so that the polarization of the emitted optical radiation is
controlled by the amplitude and phase of the feeding signals. Our nanoantenna
is created on a silicon chip using standard top-down nanofabrication tools, but
the method is universal and can be applied to other materials, wavelengths and
technologies. This work will open the way towards the synthesis and control of
arbitrary polarization states in nano-optics.
