  Material electromagnetic duality symmetry requires a system to have equal
electric and magnetic responses. Electromagnetic duality enables
technologically important effects like artificial optical activity and zero
back-scattering, is a requirement for metamaterials in transformation optics,
Huygens wave-front control, and maximal electromagnetic chirality, and appears
in topological photonic systems. Intrinsically dual materials that meet the
duality conditions at the level of the constitutive relations do not exist in
many frequency bands. Nevertheless, discrete objects like metallic helices and
homogeneous dielectric spheres can be engineered to approximate the dual
behavior. The discrete objects can then be used as building blocks with the
objective of obtaining composite systems with high duality symmetry. Here, we
exploit the extra degrees of freedom of a core-shell dielectric sphere to
obtain a particle whose duality symmetry is more than one order of magnitude
better than previously reported non-magnetic objects. We show that the
improvement is transferred onto the duality symmetry of composite objects when
the core-shell particle is used as a building block instead of homogeneous
spheres.
