Extremely Sub-wavelength Planar Magnetic Metamaterials
  We present highly sub-wavelength magnetic metamaterials designed for operation at radio frequencies (RFs). A dual layer design consisting of independent planar spiral elements enables experimental demonstration of a unit cell size (a) that is ~ 700 times smaller than the resonant wavelength ({\lambda}0). Simulations indicate that utilization of a conductive via to connect spiral layers permits further optimization and we achieve a unit cell that is {\lambda}0/a ~ 2000. Magnetic metamaterials are characterized by a novel time domain method which permits determination of the complex magnetic response. Numerical simulations are performed to support experimental data and we find excellent agreement. These new designs make metamaterial low frequency experimental investigations practical and suggest their use for study of magneto-inductive waves, levitation, and further enable potential RF applications. 
Wen-Chen Chen, Christopher M. Bingham, Kelley M. Mak, Nicholas W.   Caira, and Willie J. Padilla
physics.optics cond-mat.mtrl-sci