Coexistence of Scattering Enhancement and Suppression by Plasmonic Cavity Modes in Loaded Dimer Gap-Antennas

TL;DR

This paper demonstrates how carefully designed plasmonic dimer antennas can exhibit both enhanced and suppressed scattering by manipulating cavity modes and their interaction with antenna modes, with potential applications in sensing and cloaking.

Contribution

It introduces a novel design of loaded dimer gap-antennas that enables control over scattering via cavity mode and antenna mode interactions, including Fano and EIT-like effects.

Findings

Loaded gap layers excite toroidal moments and induce Fano/EIT-like spectra.

Resonance tuning achieved by adjusting geometry and permittivity.

Modal overlap and phase relationships are crucial for scattering control.

Abstract

Plasmonic nanoantenna is of promising applications in optical sensing, single-molecular detection, and enhancement of optical nonlinear effect, surface optical spectroscopy, photochemistry, photoemission, photovoltaics, 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 (e.g., the radial order n and the azimuthal quantum number m ). 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 for these phenomena, showing that the modal overlap and phase relationship between the fundamental moments of different order play a crucial role. A classical coupled oscillator model is proposed which clearly reproduces the coexistence of scattering enhancement and suppression. 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.