The chapter reviews long wavelength mid-infrared quantum cascade lasers (QCLs) emitting between 15 and 28 μm. Historically, 15 μm was a border wavelength above which the QCL performances dramatically degraded, which was partly due to an increase in optical losses in the devices with approaching the Reststrahlen band. This intrinsic limitation caused by multi-phonon absorption sets forbidden or favorable spectral areas depending on the employed materials. The chapter considers specific properties of long wavelength mid-infrared QCLs based on different materials, as well as more general issues related to the QCL design in this long-wavelength frontier of the mid-infrared. The discussed results are presented in the chronological order for each QCL material system, which allows the reader to follow the advances in the field.

Semiconductor lasers: rate equations and threshold conditions for laser action. Confinement factor. Temperature dependence of the threshold current: characteristic temperature. Output power: external quantum efficiency and slope efficiency. Quantum well lasers. General structures of semiconductor lasers. Spectral and spatial characteristics of diode laser emission.