The optical radiation of a magnetic dipole coupled to a plasmonic nanoantenna composed of stacked gold nanodisks is investigated. The nanoantenna is fed by a point magnetic dipole properly situated between two nanodisks. It is found that the magnetic dipole efficiently couples to the antenna due to the excitation of an antiphase localized surface plasmon resonance mode. This leads to a remarkable enhancement of the optical radiation at the resonant wavelength associated with the hybridized mode of the nanoantenna. By introducing a slight structural asymmetry in the nanoantenna, the optical radiation is substantially altered. Interestingly, it is shown that the omnidirectional radiation of the magnetic dipole becomes almost unidirectional, further enhanced, and accessible in three-dimensions by the asymmetric nanoantenna. Based on the classical model, quasistatic theory, and electrodynamics calculations, the electromagnetic interaction of the magnetic dipole and the nanoantennas is studied. A remarkable forward-to-backward directionality is achieved by the proposed nanoantenna at visible wavelengths.
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