Non-magnetic negative-refraction systems for terahertz and far-infrared   frequencies
  We demonstrate that homogeneous naturally-occurring materials can form non-magnetic negative refractive index systems, and present specific realizations of the proposed approach for the THz and far-IR frequencies. The proposed structure operates away from resonance, thereby promising the capacity for low-loss devices. 
Leonid V. Alekseyev, Viktor A. Podolskiy, Evgenii E. Narimanov
physics.optics cond-mat.mtrl-sci