We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides
clear-cut information on changes in charge location at different bias. Theoretical simulations
of the evolution of the gain/absorption spectrum for a λ∼ 7.4 μ m InGaAs/AlInAs/InP
quantum cascade laser have been compared with the experimental findings. Transfer of
electrons between the ground states in the active region and the states in the injector goes
hand in hand with a decrease of discrete intersubband absorption peaks and an increase of …