Theoretical and experimental analysis of the directionality of the electromagnetic scattering by magnetodielectric small spherical particles

Magnetodielectric small spheres present unusual electromagnetic scattering features, theoretically predicted a few decades ago by Kerker et al. (1). However, achieving such behavior has remained elusive, due to the non-magnetic character of natural optical materials or the difficulty in obtaining low-loss highly permeable magnetic materials in the gigahertz regime. Here we present unambiguous experimental evidence that a single low-loss dielectric subwavelength sphere of moderate refractive index (n≈4 like some semiconductors (Si, Ge) at near-infrared) and radius a<λ radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres. The measured far-field scattering radiation patterns (see Fig. 1(a)) and degree of linear polarization (3-9 GHz/33-100mm range) show that, by appropriately tuning the a/λ ratio, zero-backward ('Huygens' source) or almost zero-forward ('Huygens' reflector) radiated power can be obtained (2). Also, the near-field scattering distributions and their correlation with those measured in far-field, are numerically calculated and analyzed (see Fig. 1(b)). These Kerker scattering conditions (1) only depend on a/λ. Our results open new technological challenges from nano and micro-photonics to science and engineering of antennas, metamaterials and electromagnetic devices.