Abstract
Plasmonic nanoantenna is of promising applications in optical sensing and detection, enhancement of optical nonlinear effect, surface optical spectroscopy, photoemission, etc. Here we show that in a carefully-designed dimer gap-antenna made by two metallic nanorods, the longitudinal plasmon antenna mode (AM) of bonding dipoles can compete with the transverse plasmonic cavity modes (CMs), yielding dramatically enhanced or suppressed scattering efficiency, depending on the CMs symmetry characteristics. More specifically, it is demonstrated that an appropriately loaded gap layer enables substantial excitation of toroidal moment and its strong interaction with the AM dipole moment, resulting in Fano- or electromagnetically induced transparency (EIT)-like profile in the scattering spectrum. However, for CMs with nonzero azimuthal number, the spectrum features a cumulative signature of the respective AM and CM resonances. We supply both detailed near-field and far-field analysis, showing that the modal overlap and phase relationship between the fundamental moments of different order play a crucial role. Finally, we show that the resonance bands of the AM and CMs can be tuned by adjusting the geometry parameters and the permittivity of the load. Our results may be useful in plasmonic cloaking, spin-polarized directional light emission, ultra-sensitive optical sensing, and plasmon-mediated photoluminescence.
Highlights
Plasmonic nanoparticles and their assemblies are well-known optical nanoantennas, and have been intensively studied in nanophotonics due to the fascinating optical properties originated from localized surface plasmon resonance (LSPR)[1,2]
We have systematically explored the anomalous scattering properties of nanoantennas made by silver nanorod homodimer that are deliberately designed to support spectrally overlapping longitudinal electric dipolar antenna mode (AM) and transverse gap SPP cavity modes (CMs)
By analyzing the near field characteristics, we confirm that the narrow peak is related to the CM11 resonance while the Fano dip corresponds to the CM10 resonance
Summary
Plasmonic nanoparticles and their assemblies are well-known optical nanoantennas, and have been intensively studied in nanophotonics due to the fascinating optical properties originated from localized surface plasmon resonance (LSPR)[1,2]. The nanogap of a PDA provides the feeding port to excite the AM or to tune the equivalent circuit property[10,13,16,26] In such plasmonic nanostructures, the solid metallic parts and their inverse counterparts (e.g., the dielectric gap layer embedded between the metals) play significant roles. The toroidal dipole response could yield interesting consequences such as the formation of anapole[36], toroidal induced transparency[37,38,39], and enhanced optical scattering force[40] In this context, one would expect that a PDA may support both AM and CMs in the same band and it shall be of great interest to explore the couplings between them. The toroidal dipolar CM violently competes with the AM in the near field, yielding Fano resonance or EIT-like features in the spectrum
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