How does a quantum cascade laser generate terahertz radiation?

A quantum cascade laser (QCL) generates terahertz radiation by passing electrons through a series of precisely engineered quantum wells in a semiconductor superlattice (typically GaAs/AlGaAs), where each electron emits a terahertz photon as it transitions between designed energy subbands within the conduction band. Unlike conventional semiconductor lasers that rely on electron-hole recombination across the bandgap (which determines the emission wavelength), QCLs use intersubband transitions whose energy spacing is determined by the quantum well width, allowing the emission frequency to be engineered by design from mid-infrared down to about 1 THz. A single QCL structure contains 100-200 cascaded active modules, so one electron emits multiple photons as it cascades through the structure, providing the gain needed to overcome waveguide losses. Terahertz QCLs produce continuous-wave output power of 1-100 mW but currently require cryogenic cooling to operate, with the highest demonstrated operating temperature around 250 K for pulsed operation and about 130 K for CW at terahertz frequencies.