Analysis and design of an optical trapped nanodipole using plasmonic core-shell particles

In this paper we adapt to optical wavelengths the principles of operation of trapped dipole antennas, which typically operate in the low MHz frequency range. The proposed nanoantenna consists of a plasmonic nanorod as its baseline element. Along its length the nanorod is loaded with plasmonic core-shell particles also referred to as 'traps'. These function as nanocircuits that create the equivalent response of a parallel LC circuit at resonance. When the traps resonate, open-circuit conditions are established at the two ends of the nanorod section defined in between them. This naturally results in the excitation of the shorter section's λ/2 resonance. In this way trapped dipoles, apart from their original λ/2 resonance (due to their total length), exhibit an additional radiating mode which is excited when the traps resonate. This property enables the dual-mode operation of the dipole antenna. Our analysis clearly demonstrates the tuning capabilities that plasmonic core-shell particles can offer while it further introduces a simple and practical approach to engineer dual-mode optical sensors.