Fundamental limitations to gain enhancement in periodic media and   waveguides
  A common strategy to compensate for losses in optical nanostructures is to add gain material in the system. By exploiting slow-light effects it is expected that the gain may be enhanced beyond its bulk value. Here we show that this route cannot be followed uncritically: inclusion of gain inevitably modifies the underlying dispersion law, and thereby may degrade the slow-light properties underlying the device operation and the anticipated gain enhancement itself. This degradation is generic; we demonstrate it for three different systems of current interest (coupled resonator optical waveguides, Bragg stacks, and photonic crystal waveguides). Nevertheless, a small amount of added gain may be beneficial. 
Jure Grgi\'c, Johan Raunkj{\ae}r Ott, Fengwen Wang, Ole Sigmund,   Antti-Pekka Jauho, Jesper M{\o}rk, and N. Asger Mortensen
physics.optics cond-mat.mes-hall