Quantum kinetics of transport and gain in quantum cascade lasers: Looking for the essential principles of design

The complex interplay by tunneling and scattering in quantum cascade lasers is analyzed with nonequilibrium Green's functions. Data for a recently proposed simple structure based on only two wells per period is presented. This simple structure allows to visualize the relevance of the tunneling resonance very clearly. It is argued that a QCL design needs at least three levels per period, thus further simplifications of the design are not likely to occur for THz lasers. Finally the similarities and differences between quantum cascade lasers and superlattices are addressed.