Hot Electrons in THz Quantum Cascade Lasers

We compare the electrical power dependence of the lattice temperature and the electronic temperature of GaAs/AlxGa1-xAs THz quantum cascade lasers (QCLs) with different active region schemes, as extracted by the analysis of microprobe band-to-band photoluminescence experiments. Thermalized non-equilibrium distributions are found in all classes of QCLs. While in the case of bound-to-continuum structures all subbands share the same temperature, the upper laser level of active regions based on the resonant-phonon scheme heats up by Delta T similar to 100 K with respect to lower energy levels. The comparison among samples with different Al mole fractions show that the use of smaller x values leads to larger electronic temperatures.