Multi-slot Nanoantenna for Unidirectional Visible Radiation to Free-space
Plasmonic nanoantennas are key elements in nanophotonics capable of directing radiation or enhancing the transition rate of a quantum emitter. The magnetic-dipole-type nanoantennas, which are the Babinet complementary structures of typical electric-dipole-type, have received little attention leaving their antenna properties largely unexplored. We present a multi-slot nanoantenna with a magnetic dipole feed exhibiting highly unidirectional free-space radiation. This antenna does not suffer from the directivity limit arising from a natural tendency to radiate into high-refractive-index substrates. Its geometry is engineered to achieve an optimal front-to-back ratio by adjusting the phase of the plasmon field and couplings between slot elements. We achieve a maximum front-to-back ratio of 8.5 (9.2 dB) with a five-slot structure at 664 nm. A multi-slot nanoantenna provides a new possibility of an electrically excitable feed and of energy transfer from one waveguide to another by working as a future optical via.