Fano type transparency and other multimode interference effects in all-dielectric nanoshells

Abstract: Recently, the coupling of two different modes of a homogeneous plasmonic particle and their sharply varying spectra were elucidated as Fano resonances; an 'interference' of two spatially orthogonal modes driving each other. On the other hand, the scattering (and extinction) cross-section of a non-absorbing dielectric particle is always the sum of the cross-sections of all mode numbers; and this rules out any such Fano type interference between two different mode numbers. So delectric particles exhibit an interference structure in their extinction spectra only if it manifests in the individual modes describing the scattered field of the particle. We show that in a all-dielectric core-shell particle such strong interferences in multiple mode numbers can be attained, and notably even as a spectral region of transparency and directional scattering of incident light. Here interference between the complementary normal modes of the nanoshell and core regions can be realized for each mode number, resulting in a sharp interference structure in the extinction of the particle. This manifests as spectral regions of minimal/maximal interaction with the incident electromagnetic field. Such spectral properties are significant for many applications where the non-radiative losses of plasmonic structures are a liability. Note that this behaviour is useful for optical antennas, cloaking materials and a quantum mechanical interpretation of this classical effect may be signicant for single-photon based applications.