We demonstrate that all-dielectric metasurfaces provide a powerful platform for highly efficient flat optical metadevices, owing to their strong electric and magnetic dipolar response accompanied with negligible losses. We demonstrate broadband almost reflectionless metasurfaces for highly efficient amplitude, phase, and polarization manipulation based on the generalized Huygens principle. In this approach we utilize the superposition of several electric and magnetic multipolar scattering contributions of the constituent meta-atoms to achieve destructive interference in reflection over a large spectral bandwidth. By employing this approach, we demonstrate reflectionless broadband half-wave plates, quarter-wave plates, and vector beam q-plates that can operate across several telecom bands.
Here we present a general approach for describing the physics of Fano resonances in nanoparticle oligomers. It is shown that the interference of nonorthogonal collective eigenmodes is a sufficient condition to produce Fano resonances. We then show that such nonorthogonality between eigenmodes also permits the existence of a new form circular dichroism in the absorption and scattering cross-sections, even when circular dichroism is forbidden in the extinction cross-section.