ZnO quantum cascade lasers for room temperature operation

The main obstacle towards widespread industrial adoption of THz quantum cascade lasers is the requirement of cryogenic cooling. Room-temperature operation using the conventional GaAs/AlGaAs material system is inherently limited by the optical phonon energy (ELO=36 meV) being close to the laser transition energy (~10-20 meV). In contrast, the ZnO/ZnMgO material system has a significantly higher ELO of 72 meV, pushing the theoretical high-temperature limit far above room temperature. At the same time, ZnO comes with it's own challenges, such as significantly broadened energy levels and short upper laser state lifetime. In the present talk, these considerations will be discussed in the light of non equilibrium Green’s function modelling, which is necessary to correctly treat the strong electron-phonon scattering. In addition, design schemes suitable for m-plane (avoiding internal fields) ZnO QCLs will be presented and analysed, providing pathways towards room temperature THz QCLs.