Unidirectional ultracompact optical nanoantennas.

We report on a dramatic directionality effect in a simple and ultracompact optical nanoantenna consisting of a pair of interacting plasmonic nanoparticles. We found that the emission from a dipole source positioned close to one of the particles in the pair exhibits an essentially unidirectional radiation pattern for emission wavelengths close to the antiphase hybridized plasmon. We analyze this unique effect in terms of radiation, reception, and reciprocity concepts using electrodynamics simulations and dipole analysis. A forward-backward directionality of approximately 18 dB at 665 nm is obtained for a nanoantenna that consists of two 90 nm wide and 20 nm thick gold nanodisks separated by a 10 nm gap.