Quantum-cascade lasers based on the GaAs/(Al,Ga)As materials system are very useful radiation sources for many applications in the terahertz (THz) spectral region. This spectral region ranges typically from 0.1 to 10 THz bridging the electronics-based microwave region with the optics-based infrared region.
Due to their high emission powers and narrow line widths in continuous-wave operation, THz quantum-cascade lasers are excellent emitters of electromagnetic radiation for high-resolution spectroscopy. The THz region is of great current interest for spectroscopic applications, since rotational states of many molecules, impurity transitions in semiconductors, and fine-structure transitions in atoms as well as ions can result in THz absorption or emission. Currently, quantum-cascade lasers are unrivaled for local oscillators at frequencies above 3 THz in heterodyne detectors. Furthermore, the absolute density of oxygen atoms in a plasma can be determined by high-resolution absorption spectroscopy based on a tunable THz quantum-cascade laser operating in a Stirling cooler. The practical operating temperature of the lasers is determined by an output power of at least 1 mW in continuous-wave operation. The key challenges are the increase of the practical operating temperature and the wall plug efficiency of THz quantum-cascade lasers.