Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers

We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers sQCLsd operating at 2.8 THz, 3.2 THz, and 3.8 THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one s90 K–100 Kd, the upper radiative state heats up to ,200 K. From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-latti