Negative free carrier absorption in terahertz quantum cascade lasers
  We analyze the peculiar case where the free carrier absorption arising from LO phonon absorption-assisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm$^{-1}$'s at 200K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime. 
C. Ndebeka-Bandou, M. R\"osch, K. Ohtani, M. Beck, J. Faist
cond-mat.mes-hall