Surface plasmon amplification by stimulated emission of radiation in   hyperbolic metamaterials
  We study theoretically and numerically spasing conditions and optical dynamics of a composite hyperbolic metamaterial interacting with gain molecules. By combining Maxwell-Bloch equations with Green's function approach, we calculate lasing frequency and threshold population inversion for various gain density in the gain layer. We demonstrate high level of enhancement for photonic density of states in spacious spectral range provided by hyperbolic metastructures. We find that direct dipole-dipole interactions between molecules in the gain layer has a negligible effect on spasing conditions. We identify a region of parameters in which spasing can occur considering these effects. 
Vitaliy N. Pustovit, Augustine M. Urbas and David E. Zelmon
cond-mat.mes-hall