  Plasmonic nanoantennas are key elements in nanophotonics capable of directing
radiation or enhancing the transition rate of a quantum emitter. Slot-type
magnetic-dipole nanoantennas, which are complementary structures of typical
electric-dipole-type antennas, have received little attention, leaving their
antenna properties largely unexplored. Here we present a novel
magnetic-dipole-fed multi-slot optical Yagi-Uda antenna. By engineering the
relative phase of the interacting surface plasmon polaritons between the slot
elements, we demonstrate that the optical antenna exhibits highly
unidirectional radiation to free space. The unique features of the slot-based
magnetic nanoantenna provide a new possibility of achieving integrated features
such as energy transfer from one waveguide to another by working as a future
optical via.
