Hot electron effects and nanoscale heat transfer in Terahertz quantum cascade lasers

We show that the internal quantum efficiency and the wall-plug efficiency of quantum cascade lasers (QCL) are correlated with the hot-electron cooling associated with photon emission. The experimental procedure for the assessment of these key device parameters is based on micro-probe photoluminescence (PL) that allows high resolution measurements of the electronic and local lattice temperatures in operating QCLs. By using a terahertz QCL as a prototype we demonstrate that the electronic distributions are Fermi-Dirac functions characterized by temperatures significantly larger than the lattice one. The lattice temperature is in turn well above the one of the heat sink bath. Combining the above observation with time-resolved PL experiments we assessed the characteristic time constants controlling the heating and cooling processes of terahertz QCLs that are limited by the presence of a high density of interfaces that causes phonon interference effects. The correlation between the above constants, the thermal diffusivities and the diffusion lengths have been extracted from the comparison with the outcome of a transient heat diffusion model