Controlling Electric and Magnetic Resonances for Ultracompact Nanoantennas with Tunable Directionality

We design and numerically demonstrate an ultracompact plasmonic nanoantenna with tunable high directionality. The antenna consists of a metallic trimer that can support a highly spectrally tunable magnetic dipole mode with its amplitude comparable to that of an electric dipole mode. Superior forward radiation is achieved when these modes satisfy the Kerker conditions, leading to a very low side lobe level of -22 dB. It is shown that by moving one of the three particles by less than 10 nm the resonance wavelength of the magnetic mode will shift dramatically, resulting in the change of the interference conditions and hence the radiation characteristics. From the evolution of the resonant modes, we find optimized designs that reverse the radiation direction at the same wavelength. Meanwhile, the enhancement of spontaneous emission and radiated power of a nanoemitter adjacent to the antenna can reach approximately 4 and 3 orders of magnitude, respectively. Analyses based on a simple dipole model are performed, and the reconstructed radiation patterns agree well with the simulation results.