A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance

Abstract

The lack of symmetry between electric and magnetic charges, a fundamental consequence of the small value of the fine-structure constant, is directly related to the weakness of magnetic effects in optical materials. Properly tailored plasmonic nanoclusters have been proposed recently to induce artificial optical magnetism based on the principle that magnetic effects are indistinguishable from specific forms of spatial dispersion of permittivity at optical frequencies. In a different context, plasmonic Fano resonances have generated a great deal of interest, particularly for use in sensing applications that benefit from sharp spectral features and extreme field localization. In the absence of natural magnetism, optical Fano resonances have so far been based on purely electric effects. In this Letter, we demonstrate that a subwavelength plasmonic metamolecule consisting of four closely spaced gold nanoparticles supports a strong magnetic response coupled to a broad electric resonance. Small structural asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Our findings are supported by excellent agreement with simulations and analytical calculations, and represent an important step towards the quest for artificial magnetism and negative refractive index metamaterials at optical frequencies.

Fano-like resonance emerging from magnetic and electric plasmon mode coupling in small arrays of gold particles

“…The design of Fano resonances involving both electric and magnetic type modes appears as an effective way to enhance the magnetic response of plasmonic structures39. In this way, 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 structure40.…”

Giant Magnetic Field Enhancement in Hybridized MIM Structures

“…As a result, the magnetic plasmon resonance appears in the optical-NIR range of the spectrum [2,3]. Several structures have been proposed in the literature to demonstrate magnetic hot spots including nanorings [4], MIM nanosandwiches [5][6][7] and nanoparticle clusters [8,9]. The MIM structure (nanosandwich) consists of two metallic strips separated by a dielectric layer.…”

Target decoupling in coupled systems resistant to random perturbation

“…Consider the ‘hiding’ (or ‘decoupling’) of some elements inside densely packed coupled optical systems 5, 7–11 . Transformation optics in this scenario provides the severely intricate solution even for the approximated case 12 : the coating of target elements with spatially-varying, highly anisotropic metamaterials of extreme material parameters (effective permittivity ~0), which derives the ‘microscopic’ removal of the coupling to the target elements.…”