Three-dimensional metamaterials with an ultra-high effective refractive   index over broad bandwidth
  The authors introduce a general mechanism, based on electrostatic and magnetostatic considerations, for designing three-dimensional isotopic metamaterials that possess an enhanced refractive index over an extremely large frequency range. The mechanism allows nearly independent control of effective electric permittivity and magnetic permeability without the use of resonant elements. 
Jonghwa Shin, Jung-Tsung Shen, Shanhui Fan