Self-Consistent Theory of the Gain Linewidth for Quantum Cascade Lasers
  The linewidth in intersubband transitions can be significantly reduced below the sum of the lifetime broadening for the involved states, if the scattering environment is similar for both states. This is studied within a nonequilibrium Green function approach here. We find that the effect is of particular relevance for a recent, relatively low doped, THz quantum cascade laser. 
F. Banit, S.-C. Lee, A. Knorr, and A. Wacker