In this chapter, we overview recent developments of a simulation framework capable of capturing the highly nonequilibrium physics of the strongly coupled electron and phonon systems in quantum cascade lasers (QCLs). In midinfrared (mid-IR) devices, both electronic and optical phonon systems are largely semiclassical and described by coupled Boltzmann transport equations, which we solve using an efficient stochastic technique known as ensemble Monte Carlo. The optical phonon system is strongly coupled to acoustic phonons, the dominant carriers of heat, whose dynamics and thermal transport throughout the whole device are described via a global heat-diffusion solver. We discuss the roles of nonequilibrium optical phonons in QCLs at the level of a single stage , anisotropic thermal transport of acoustic phonons in QCLs, outline the algorithm for multiscale electrothermal simulation, and present data for a mid-IR QCL based on this framework.

Quantum cascade lasers provide a variety of challenges to theory, which are outlined in this review: (i) The choice of basis states is discussed, where energy eigenstates are badly defined if the full periodic structure is considered. (ii) The tunneling through barriers requires a treatment of quantum coherences, which sets particular demands on the formulation of the quantum kinetic equations. Here the advantages and disadvantages of different approaches such as rate equations, Monte-Carlo simulations, different density matrix approaches, and Green’s functions methods are addressed. (iii) The evaluation of gain is detailed, where broadening is of utmost importance. (iv) An overview regarding the electrical instabilities of the extended structure due to domain formation is given, which strongly affect the overall performance

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.

This chapter provides an overview of a class terahertz quantum cascade lasers based upon amplifying electromagnetic metasurfaces. The metasurface comprises two-dimensional arrays of sub-wavelength surface radiating antenna elements, in which the antennas are loaded with the quantum cascade laser gain material. Several types devices are described: (a) vertical-external-cavity surface-emitting-lasers (VECSELs) in which the amplifying metasurface is paired with external optics to form a laser cavity; (b) monolithic metasurface lasers in which the metasurface array self-oscillates in a coherent supermode; and (c) metasurfaces which operate below threshold as free-space terahertz amplifiers. The metasurface approach allows the realization of large-area radiating apertures while preserving the sub-wavelength sized of the individual metallic waveguide antenna elements. This has resulted in significantly improved performance and functionality in many categories, including lasers with high-quality beam patterns, high-efficiency lasers with scalable output powers, broadband spectral tunability of single-mode emission, and free-space amplification of terahertz beams.

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.