Abstract
In this work we theoretically and experimentally analyze the resonant behavior of individual 3 × 3 gold particle oligomers illuminated under normal and oblique incidence. While this structure hosts both dipolar and quadrupolar electric and magnetic delocalized modes, only dipolar electric and quadrupolar magnetic modes remain at normal incidence. These modes couple into a strongly asymmetric spectral response typical of a Fano-like resonance. In the basis of the coupled mode theory, an analytical representation of the optical extinction in terms of singular functions is used to identify the hybrid modes emerging from the electric and magnetic mode coupling and to interpret the asymmetric line profiles. Especially, we demonstrate that the characteristic Fano line shape results from the spectral interference of a broad hybrid mode with a sharp one. This structure presents a special feature in which the electric field intensity is confined on different lines of the oligomer depending on the illumination wavelength relative to the Fano dip. This Fano-type resonance is experimentally observed performing extinction cross section measurements on arrays of gold nano-disks. The vanishing of the Fano dip when increasing the incidence angle is also experimentally observed in accordance with numerical simulations.
Highlights
The system[25,33]
A Fano resonance obtained via the interplay of dipolar electric and magnetic modes has been recently achieved on a gold nano-ring when geometrical asymmetries are induced in the structure[40]
We characterized the Fano-like resonance of a 3 × 3 array of gold nanoparticles illuminated at normal incidence, as resulting from the coupling between the dipolar electric and quadrupolar magnetic multipolar contributions to the total extinction of the structure
Summary
The system[25,33]. These two subclasses of hybrid modes are classified according to their ability to couple with plane waves. The design of Fano resonances involving both electric and magnetic type modes appears as an effective way to enhance the magnetic response of plasmonic structures[39]. We rely on the coupled mode theory to interpret the Fano-type resonance as resulting from a hybridization process between electric and magnetic resonant modes of the structure. To the best of our knowledge, Fano-type resonances on a single plasmonic oligomer under normal incidence have never been reported from the coupling between an electric and a magnetic mode. This result could lead to a revival of interest for this kind of structure, as building blocks for optical metamaterials
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