Terahertz Quantum-Cascade Lasers: From Design to Applications

We report on the development and the application of high-performance terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures. These lasers with emission frequencies between 2.6 and 4.7 THz are based on a hybrid design, which is preferred for continuous-wave applications. For the design of the active regions, we employ an efficient Fourier-transform-based model, which also allows for the simulation of heterostructures with gradual interfaces. Since the inherent interface width is on the same order as the thickness of the layers in the active region, the use of nominally binary AlAs barriers results in an effective Al content up to $x$ $=$ 0.6 for the tallest barriers. For practical applications, Fabry–Pérot lasers based on single-plasmon waveguides are fabricated. Single-mode operation is in most cases achieved by using short cavities. In particular, GaAs/AlAs THz QCLs show a sufficiently high wall plug efficiency so that they can be operated in miniature mechanical cryocoolers. Currently, high-performance THz QCLs are used for commercial continuous-wave, table-top THz systems, local oscillators in 3.5- and 4.7-THz heterodyne spectrometers, and absorption spectrometers for the determination of the density of atomic oxygen in plasmas.