Electronic temperatures and electron-lattice relaxation in THz QCLs

The electronic distribution in quantum cascade lasers (QCLs) arises from the detailed balance between the injection and the energy relaxation rates, i.e. interand intra-subband electron-electron, electron–LO phonon, electron-impurity, and interface roughness scattering. In mid-infrared QCLs the electron-electron and the electron-phonon interactions lead to a non-equilibrium thermalized distribution that can be described in terms of a single electronic temperature Te, higher than the lattice one (TL) at injected currents close to the laser threshold. In terahertz QCLs the photon energy is smaller than the longitudinal-optical (LO) phonon energy ELO, and hence the electron-LO phonon scattering between the radiative subbands is energetically forbidden at low Te. Also, the electronelectron scattering is strongly reduced with respect to the mid-IR case due to the large reduction (more than one order of magnitude) in the electron doping density. Therefore, the actual electronic distribution in THz QCLs may considerably vary with respect to mid-IR QCLs, and could even not correspond to the same electronic temperature among the different subbands. Assessing this electronic distribution is fundamental for the detailed modeling of THz QCLs and the design of new quantum structures to be used for devices with improved thermal, optical and temperature performance. At the present stage of development one important limit for the thermal performance of THz QCLs is the rather high value of the thermal resistance (15-25 K/W). In turn, this is determined by the following factors: i) the large number of interfaces (1500-2000); ii) the large active region thickness (> 12 μm); iii) the low thermal conductivity of the Al0.15Ga0.85As alloys; iv) the thermal coupling between the active region and the heat sink related with the waveguide and mounting configurations. To address these topics, we have exploited an experimental approach based on the analysis of micro-probe band-to-band photoluminescence experiments carried out on THz QCLs operating in CW mode.We have extracted simultaneously the subband electronic temperatures, the electronlattice energy relaxation times and the population inversion both below and well above the laser threshold in a set of THz QCLs based on resonant-phonon (RP) [1] and bound-to-continuum (BTC) [2] schemes and fabricated with different optical waveguide configurations. The role of conduction band offset will be also discussed [3]. This information will be used to propose optimized quantum designs and waveguide structures for the extension of THz QCLs at higher operating temperatures and longer wavelengths. Finally, the fabrication of BTC QCLs emitting 75 mW peak power at 2.85 THz, based on MBE wafers acquired by a commercial provider, will be reported.