Electronic temperatures of terahertz quantum cascade active regions with phonon scattering assisted injection and extraction scheme

We measured the lattice and subband electronic temperatures of terahertz quantum cascade devices based on the optical phonon-scattering assisted active region scheme. While the electronic temperature of the injector state (j = 4) significantly increases by Delta T = T-e(4) - T-L similar to 40 K, in analogy with the reported values in resonant phonon scheme (Delta T similar to 70-110 K), both the laser levels (j = 2,3) remain much colder with respect to the latter (by a factor of 3-5) and share the same electronic temperature of the ground level (j = 1). The electronic population ratio n(2)/n(1) shows that the optical phonon scattering efficiently depopulates the lower laser level (j = 2) up to an electronic temperature T-e similar to 180 K.