The nature of the electron distribution functions in quantum cascade lasers

There has been much speculation about the nature of the electron distribution functions in quantum cascade lasers, in particular: Are they non equilibrium and if so, to what extent? In order to address this, detailed calculations of the intersubband and intrasubband electron-longitudinal phonon and electron-electron scattering rates in a real GaAs/Al_xGa_1-xAs device design [Sirtori et al., Appl. Phys. Lett. 73, 3486 (1998)] are presented. The results suggest that two important conclusions relating to the electron distribution functions in the quantum well subbands can be drawn: the electron distributions in both the active region and the injector subbands are thermalized, i.e., they can be described by Fermi-Dirac distribution functions and furthermore all the electron distributions have the same electron temperature.