Photon Mediation of Electron Transitions in Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are unipolar coherent light sources emitting in the terahertz and midinfrared portion of the electromagnetic spectrum. The active region of a QCL consist of periodic stacks of alternating semiconductor materials forming quantum wells which confine electrons in space and ensure they have desired discrete energy levels. Lasing is achieved by electron transition between these energy levels. In QCLs with the so-called diagonal design, this transition involves quantum tunneling between states with a small spatial overlap, separated by a potential barrier. This electron tunneling is mediated by photons present in the active region of the QCL and is referred to as photon-assisted (PA) tunneling. So far, theoretical studies dedicated to PA tunneling in QCLs have been limited to simple rate equations accompanied by empirical or phenomenological scattering rates. In this work, we present a quantum-transport study of PA tunneling in QCLs based on a Markovian master equation for the density matrix and investigate the effect of PA tunneling on electron dynamics and state lifetime.