Unconventional Fano effect and off-resonance field enhancement in plasmonic coated spheres

We investigate light scattering by coated spheres composed of a dispersive plasmonic core and a dielectric shell. By writing the absorption cross section in terms of the internal electromagnetic fields, we demonstrate it is an observable sensitive to interferences that ultimately lead to the Fano effect. In particular, we show that unconventional Fano resonances, recently discovered for homogeneous spheres with large dielectric permittivities, can also occur for metallic spheres coated with single dielectric layers. These resonances arise from the interference between two electromagnetic modes with the same multipole moment inside the shell and not from interactions between the various plasmon modes of different layers of the particle. In contrast to the case of homogeneous spheres, unconventional Fano resonances in coated spheres exist even in the Rayleigh limit. These resonances can induce an off-resonance field enhancement, which is approximately 1 order of magnitude larger than the one achieved with conventional Fano resonances. We find that unconventional and conventional Fano resonances can occur at the same input frequency provided the dispersive core has a negative refraction index. This leads to an optimal field enhancement inside the particle, a result that could be useful for potential applications in plasmonics.