A phonon scattering assisted injection and extraction based terahertz   quantum cascade laser
  A novel lasing scheme for terahertz quantum cascade lasers, based on consecutive phonon-photon-phonon emissions per module, is proposed and experimentally demonstrated. The charge transport of the proposed structure is modeled using a rate equation formalism. An optimization code based on a genetic algorithm was developed to find a four-well design in the $\mathrm{GaAs/Al_{0.25}Ga_{0.75}As}$ material system that maximizes the product of population inversion and oscillator strength at 150 K. The fabricated devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K. The electrical characteristics display no sign of differential resistance drop at lasing threshold, which suggests - thanks to the rate equation model - a slow depopulation rate of the lower lasing state, a hypothesis confirmed by non-equilibrium Green's function calculations. 
E. Dupont, S. Fathololoumi, Z.R. Wasilewski, G. Aers, S. R.   Laframboise, M. Lindskog, A. Wacker, D. Ban, and H. C. Liu
cond-mat.mes-hall