  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.
Specially, 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 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 one 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.
