Asymmetric parametric amplification in nonlinear left-handed   transmission lines
  We study parametric amplification in nonlinear left-handed transmission lines, which serve as model systems for nonlinear negative index metamaterials. We experimentally demonstrate amplification of a weak pump signal in three regimes: with the signal in the left-handed band, with the signal in the stop band, and with the signal at a defect frequency. In particular, we demonstrate the amplification of the incident wave by up to 15dB in the left-handed regime. 
David A. Powell, Ilya V. Shadrivov and Yuri S. Kivshar