Spectrally-Tunable Directionality of Compact Optical Nanoantennas

We propose a novel design of plasmonic compact nanoantenna with an efficiently engineered spectral response for the directive emission or harvesting of light. The nanoantenna comprised of four gold nanodisks, arranged longitudinally, and appropriately spaced. Interestingly, by tuning of the inter-particle distances, it is found that the proposed nanoantenna shows either dual-band or broad-band unidirectional performances. These remarkable spectral effects are due to the tailored energies of the two hybridized out-of-phase LSPR modes and the intrinsic electromagnetic interactions. The theoretical predictions are obtained based on the modified coupled-dipole approximation method. In order to obtain more accurate theoretical results, the primary incident optical field seen by the smaller nanodisks are modified by taking into account the field-enhancement caused by the excited plasmons in the largest nanodisk when it is illuminated first. The theoretical results are confirmed by the electromagnetic simulations.