  Materials showing electromagnetic properties that are not attainable in
naturally occurring media, the so called metamaterials, have been lately, and
still are, among the most active fields in optical and materials physics and
engineering. Among those properties, one of the most attractive is the
sub-diffraction resolving capability predicted for media having index of
refraction of -1. Here we propose a fully 3D, isotropic metamaterial with
strong electric and magnetic responses in the optical regime, based on
spherical metallo-dielectric core-shell nanospheres. The magnetic response
stems from the lowest, magnetic-dipole resonance of the dielectric shell with
high refractive index, and can be tuned to coincide with the plasmon resonance
of the metal core, responsible for the electric response. Since the response
does not originate from coupling between structures, no particular periodic
arrangement needs to be imposed. Moreover, due to the geometry of the
constituents, the metamaterial is intrinsically isotropic and polarization
independent. It could be realized with current fabrication techniques with
materials such as Silver (core) and Silicon or Germanium (shell). For these
particular realistic designs, the metamaterials present negative index in the
range within 1.2-1.55 microns.
