  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.
