Spheres of maximum electromagnetic chirality

The search for objects that yield maximum electromagnetic chirality in their emitted wavefield has garnered significant attention in recent years. However, achieving such maximum chirality is challenging, as it typically requires complex chiral metamaterials. Here we demonstrate that chiral spheres can yield maximum chirality in their emitted wavefield. Specifically, we analytically find the spectral trajectories at which chiral spheres become optically transparent to a given helicity of the incident field, while for its opposite helicity, they behave as dual objects, i.e., on scattering, they preserve helicity. Since chiral spheres behave as dual objects at the first Kerker condition of zero backscattering, we significantly simplify this condition in terms of a Riccati-Bessel function. Importantly, all our results are exact and applicable regardless of the multipolar order, refractive index contrast, optical size, and intrinsic chirality of the chiral sphere. Thus, our exact findings can serve as building blocks for designing novel metasurfaces or metamaterials with maximum electromagnetic chirality properties.