Quantum cascade lasers (QCLs) emitting in the 4-10 micron wavelength range are treated with emphasis on key issues not covered in previous books on QCLs. The foremost issue discussed: what does it take to achieve continuous-wave (CW) operation to multi-watt powers in a highly efficient manner, is of interest to a wide range of applications. A comprehensive review of the temperature dependence of the electro-optical characteristics of QCLs is presented by including elastic scattering and carrier-leakage triggered by elastic and inelastic scattering, thus accounting for all mechanisms behind the device internal efficiency. Maximizing the CW wall-plug efficiency via conduction-band and elastic-scattering engineering, and photon-induced carrier transport is treated in detail. Then coherent-power scaling is discussed for both one- and two-dimensional (2-D) structures with emphasis on the optimal solution: high-index-contrast (HC) photonic-crystal (PC) lasers. Grating-coupled surface-emitting lasers are also treated with emphasis on those needed for 2-D HC-PC lasers; that is, devices most likely to operate in diffraction-limited, single-lobe beam pattern to multi-watt CW output powers

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.