Quantum cascade lasers

Abstract The quantum cascade laser (QCL) is a unipolar semiconductor device, in which electronic transitions between the subbands confined in a heterostructure conduction band lead to stimulated light emission. This process stands in contrast with diode lasers, in which photons arise from electronic transitions from the lowest-energy conduction band to the highest-energy valence band. The result is that, while diode lasers emit with photon energy similar to the bandgap energy, QCLs can emit with any photon energy that can be designed within the semiconductor heterostructure. Emission wavelengths for QCLs have been reported from 2.6 to 250 μm. This chapter summarizes some aspects of the fundamental physics of QCLs, an overview of the fabrication process, a review of a new paradigm for power and brightness based on broad-area QCLs, and a description of the physics and optics of external-cavity tunable QCLs