Tunable split-ring resonators for nonlinear negative-index metamaterials
  We study experimentally the dynamic tunability and self-induced nonlinearity of split-ring resonators incorporating variable capacitance diodes. We demonstrate that the eigenfrequencies of the resonators can be tuned over a wide frequency range, and significantly, we show that the self-induced nonlinear effects observed in the varactor-loaded split-ring resonator structures can appear at relatively low power levels. 
Ilya V. Shadrivov, Steven K. Morrison, and Yuri S. Kivshar