Theory of light amplification in active fishnet metamaterials
  We establish a theory that traces light amplification in an active double-fishnet metamaterial back to its microscopic origins. Based on ab initio calculations of the light/plasmon fields we extract energy rates and conversion efficiencies associated with gain/loss channels directly from Poynting's theorem. We find that for the negative refactive index mode both radiative loss and gain outweigh resistive loss by more than a factor of two, opening a broad window of steady-state amplification (free of instabilities) accessible even when a gain reduction close to the metal is taken into account. 
Joachim M. Hamm, Sebastian Wuestner, Kosmas L. Tsakmakidis and Ortwin   Hess
physics.optics cond-mat.mes-hall cond-mat.mtrl-sci physics.comp-ph