Electron-electron scattering in far-infrared quantum cascade lasers

A large depolarization shift indicates a strong nonequilibrium intersubband electron-electron scattering (Gamma) in tunneling structure. For a far-infrared subband separation Delta similar to 10 meV, the rate Gamma scales with the upper-subband occupation and is never significantly reduced by screening. Despite this nonradiative decay a finite population inversion can be maintained. Finally, the applied bias changes the wave-function symmetry so as to cause a dramatic variation of the electron-electron scattering rate.