Unidirectional scattering by nanoparticles near substrates: generalized Kerker conditions.

Starting from a general description of light scattering by a nanoparticle in homogeneous surroundings and situated near a substrate, we outline the connection to multipole expansion of scattered light and derive conditions and limits on achievable half-space scattering asymmetry, including the possibility of unidirectional scattering along the propagation direction of the incident light (i.e., generalized Kerker conditions). As a way of realizing strongly asymmetric scattering, we perform a parametric study of the optical properties of disk-shaped gap-surface plasmon (GSP) resonators, consisting of a glass spacer sandwiched between two gold disks, with numerical calculations that corroborate the conditions derived from the multipole expansion. Finally, we present proof-of-principle experiments of asymmetric scattering by GSP-resonators on a glass substrate.