Terahertz quantum cascade lasers based on intra-well optical transitions

The possibility to achieve laser action at different THz frequencies employing the so-called vertical transition will be discussed. The intra-well or vertical optical transition presents a number of features that makes it an ideal candidate for a systematic study of resonant tunnelling and dephasing in quantum cascade lasers. Laser action at 3.5 THz with very low threshold employing a single quantum well design [1] is presented: the population inversion is achieved via the interplay between the resonant tunnelling extraction from the lower state of the lasing transition and the long lifetime of the upper state. The active region design reduces essentially to one quantum well and can be studied with a model based on a density matrix approach. The model accounts for the relevant mechanisms in transport and laser emission and yields values for the relevant lifetimes in agreement with experimental results. The same intra-well system is also the object of an extensive study in presence of a strong magnetic field that modifies dramatically the intersubband relaxation processes [3,4]. This strategy, is employed to obtain laser action at 3.4 THz, 1.9 THz, 1.66 THz and 1.39 THz (82, 159, 180 and 215 μm). Due to the very low energy of the emitted photon (hν=7.9, 7 and 6 meV) the devices can be investigated in the regime of very strong magnetic confinement, where the cyclotron energy hωc exceeds several times the photon energy. This allows the observation of extremely long intersubband lifetimes via a progressive quenching of the nonradiative transitions in consequence of in-plane Landau quantization and electron wavefunction localization. The laser action exploiting vertical transitions in a single quantum well can be obtained also by injecting the carriers successively in the excited states of the well, with the possibility to obtain multiwavelength emission [4,5]. We will present the latest measurements and analysis on these devices.