Tunability of Subradiant Dipolar and Fano-Type Plasmon Resonances in Metallic Ring/Disk Cavities: Implications for Nanoscale Optical Sensing

Abstract

Plasmonic nanocavities consisting of the concentric arrangement of a disk and a ring sustain both subradiant and superradiant dipolar plasmon modes with large associated field enhancements and high refractive index sensitivities. In structures with broken symmetry, additionally a highly tunable Fano interference feature appears, which can be explained with a simple analytical harmonic oscillator model. The spectral tunability of these resonances from the visible to the mid-infrared is investigated, highlighting a potential for applications in surface enhanced spectroscopies.

Defect-induced activation of symmetry forbidden infrared resonances in individual metallic nanorods

“…18 In addition, dark modes are observed in spectroscopic studies of complex multimer structures. [19][20][21][22] In this paper we report on the appearance of dipole-forbidden second-order modes ͑l =2͒ in the IR spectra of some single gold nanorods under normal incident radiation and we identify the origin of this effect.…”

Mechanisms of Fano Resonances in Coupled Plasmonic Systems

“…A central issue in this design is the spectral engineering of the resonances via controlled hybridization of the available modes. However, this is difficult in systems where higher order modes are excited in the spectral range of interest 12,26,27 or when the modes are very complex and spatially extend over a large part of the nanostructure. 28,29 A small variation of the geometries, like what can occur during the nanofabrication process, can drastically change the resonance line shape and wavelength.…”

Refractive Index Sensing with Subradiant Modes: A Framework To Reduce Losses in Plasmonic Nanostructures

“…In the following, the weakly radiating mode will be referred to as the subradiant mode. 13 The subradiant mode has therefore two different radiative loss channels, in addition to the nonradiative loss channel: one (intrinsic) from its dipole moment and the other (extrinsic) from its coupling to the radiative mode. These two channels contribute differently to the resonance line shape, in particular, to the modulation damping.…”

“…11,12 Plasmonic modes with a long radiative lifetime, subradiant modes, possess a strong spectral dispersion which further improves the limit of detection and makes them very attractive for sensing applications. 13,14 When subradiant modes are coupled to radiation or to a radiative plasmonic mode (extrinsic radiative channel), their optical spectrum carries an asymmetric line shape characteristic of Fano resonances. 14À17 Overall, subradiant modes have three different loss channels: Ohmic losses, the extrinsic coupling to radiation, and possibly their weak intrinsic dipole moment.…”

“…14À16,18 Among Fano-resonant systems, several designs have been proposed, such as dolmens, 18À20 nanocrosses, 21 plasmonic oligomers, 22À24 and ring-disk nanocavities. 13,25 Interactions with the substrate have also been used to tailor the near-field distribution for increased performances. 26,27 The high electromagnetic field enhancement generated from their excitation also serves as efficient platforms for surface-enhanced spectroscopy.…”