This chapter reviews typical waveguide and active region designs for quantum cascade lasing in the terahertz (THz) frequency range. Operating principles are analyzed in details with special attention paid to the most recent developments with the state-of-the-art device performance. The maximum operation temperature of THz QCL is still the main obstacle for its wide employment in applications, although it has been lifted to 250 K, allowing cryogenic-free THz coherent radiation for potentially portable applications. Optimization of various limiting factors in the most advanced resonant-phonon designs or the combined designs with scattering-assisted injection scheme could be promising for further breakthroughs in achieving higher temperature operations. The discussions in this chapter mainly focus on the matured GaAs/AlGaAs material system, but the design strategies can be applied to THz QCLs utilizing other material systems, which may overcome the main challenges of the GaAs/AlGaAs material system and achieve better performance in the future.

Quantum cascade lasers are based on Intersubband transitions between quantum confined states in semiconductor heterostructures. The origin of these states is briefly described in this chapter starting with linear combination of atomic orbitals and then proceeding to the k.P theory. The relations between the interband and Intersubband transitions including their oscillator strength and selection rules are established. It is shown that “giant” Intersubband dipole owes its existence to the confinement induced band mixing. Aside from the radiative Intersubband transitions investigated in this chapter, nonradiative transitions also play important roles in QCL operation, hence most relevant of these processes: electron phonon, electron-electron, interface roughness and alloy disorder are also described in detail.

General structure of quantum cascade lasers: resonant tunnelling, minigap and miniband. Gain coefficient. Rate equations and threshold conditions. Output power, slope efficiency and wall-plug efficiency. Applications of quantum cascade lasers.