Resonances in light scattering from nonequilibrium dipoles pairs

Abstract

We consider the light scattering from a pair of point-like electrical dipoles. Whenever the polarizability of each dipole violates the optical theorem, the response of the pair (both in far-field and near-field) exhibits exact resonances as a function of the frequency and the inter-dipole distance. This polarizability is consistent with causality and the crossing condition (i.e., a real field generates a real response). Hence, the emergence of the resonances requires nonequilibrium conditions, e.g., corresponding to active dipoles. Within our approach (classical optics, monochromatic incident field, point-like dipoles), the exact resonances can be infinite. The resonances also appear in the equilibrium domain, where the optical theorem is valid. In that domain, they are finite, but can produce large amplification factors; e.g., for a pair of metallic nanoparticles under Drude's model, the single-particle plasmonic resonance can be amplified $\sim 10^{2}$ times. But the global maximization of the scattering can still be achieved by violating the optical theorem. Our results for one electric and one magnetic dipole show how resonances can amplify a weak magnetic response of a single dipole to the incident field. We also discuss an anti-resonance (dark-state) effect present in the two-dipole scattering.