Carrier dynamics investigation in quantum cascade lasers using Mid-IR ultrafast pulses

Quantum cascade lasers (QCLs) are unipolar devices composed of repeated stack of semiconductor multiple quantum well heterostructures utilizing intersubband transitions and resonant tunneling. In this paper, 120 fs Mid-IR pulses are used to investigate the nature of carrier transport through the quantum wells and barriers of a pulse biased, room temperature operating ultrastrong coupling design QCL. Despite the low average power of Mid-IR pulses, we managed to efficiently couple these pulses into the QCL waveguide so as to observe distinct phenomena by varying the pump and probe's power. Biased just below the threshold, we observed a strong gain depletion dip at t=0 which is mainly caused by the depletion of electrons from the upper lasing state mainly by stimulated emission. Ultrafast gain recovery within the first 200 fs was observed. This is mainly attributed to phonon scattering and electrons resonantly tunneling through a much thinner injector barrier, which overcomes the interface-roughness-induced detuning of resonant tunneling. Electron transport through the injector region contributes to a slower gain recovery lifetime of 2-3 ps.