Fano-type spectra and other interference effects in an all-dielectric nanoshell

Large spectral variations in the extinction of nanostructures are important for many applications like optical antennas, cloaking materials, optical circuits and others. Such features in extinction spectra are typically attained using plasmonic particles and high-index or complex nanostructures. In this work, we show that large interference structures can manifest even in the extinction spectra of a single dielectric particle with a low relative refractive index ($<2$), and notably even as a spectral region of transparency and directional scattering of incident light. This is observed when the core and shell resonances of a layered spherical particle have overlaps with each other in the energy spectrum. Interference between the core and shell modes results in larger spectral features in magnitudes of the scattering constants a _n, b _n i.e. a strong enhancement of what is traditionally called the ripple structure in the extinction spectra of a dielectric particle. This effect is in addition to any possible overlap of electric resonances of two different mode numbers or the overlap of an electric and magnetic resonance, which have been shown to result in sharp asymmetric (Fano) resonances in the forward or backscattering spectra of a high-index or metallic particle. (paper)