Unveiling dipolar spectral regimes of large dielectric Mie spheres from helicity conservation

Abstract

Controlling the helicity and directionality of the scattered light by dielectric particles is paramount to a variety of phenomenology of interest in all-dielectric optics and photonics. In this paper, we demonstrate that the helicity of an incoming beam is not preserved when several multipoles contribute to the scattering. This inhibits the usual link between duality restoration and zero optical backscattering condition. In addition, we show that the helicity conservation in the scattering by high-refractive index dielectric Mie spheres can be used as a probe of pure-multipolar spectral regions, particularly of dipolar nature, beyond its presumed spectral interval. This finding reveals that the dipolar behavior is not necessarily limited to small particles, in contrast to the current understanding. Interestingly, our formalism allows us to demonstrate that the optimum forward light scattering condition, predicted for a particular nanosphere, is fulfilled for an infinite number of refractive indexes in these dipolar regimes.