Metallo-dielectric core-shell nanospheres as building blocks for optical   3D isotropic negative-index metamaterials
  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. 
R. Paniagua-Dom\'inguez, F. L\'opez-Tejeira, R. Marqu\'es, J. A.   S\'anchez-Gil
physics.optics cond-mat.other