Active Negative Index Metamaterial Powered by an Electron Beam
  A novel active negative index metamaterial that derives its gain from an electron beam is intro- duced. The metamaterial consists of a stack of equidistant parallel metal plates perforated by a periodic array of holes shaped as complementary split-ring resonators. It is shown that this structure supports a negative-index transverse magnetic electromagnetic mode that can resonantly interact with a relativistic electron beam. Such metamaterial can be used as a coherent radiation source or a particle accelerator. 
M. A. Shapiro, S. Trendafilov, Y. Urzhumov, A. Alu, R. J. Temkin and   G. Shvets
physics.optics cond-mat.mtrl-sci