Multiple Plasmonic Fano Resonances Revisited with Modified Transformation-Optics Theory

Plasmonic Fano resonances have recently attracted a great deal of research interest due to their sharp asymmetric profile, high sensitivity to the ambient material and low radiation damping. In this paper, we extend the plasmonic Fano resonances that resulted from the interference between bright and dark modes in a core-shell nanoparticle system for novel biosensing, optical switching applications, by using the theory of transformation optics. To this end, we consider the optical properties of dielectric-core-metallic-shell dimer and derive full analytical formulae for different geometric configurations. Our results demonstrated that breaking the geometrical symmetry of the structure, multiple Fano resonances arise owing to the near-field coupling of the bright and dark resonant modes. Electromagnetic induced transparency (EIT)-like effects are observed when the resonance frequencies of the bright and dark modes overlap. Strong dependence of the localized surface plasmon (LSP) on the geometry renders the multiple Fano resonances highly tunable in the proposed structure through independently altering the spectral profile of each nanoparticle, which provides much feasibility since most multiple Fano resonances reported in literature are results of collective plasmonic behavior and cannot be tuned independently. Furthermore, the figure of merit for refractive index sensing of the higher-order dark modes are predicted to be up to 36% greater than that of a single nanowire. These results make the proposed nanostructure attractive for many potential applications such as multiwavelength biosensing, switching and modulation.