THz Quantum Cascade Lasers

Abstract THz quantum cascaded lasers (QCL) are high power solid-state electrically driven coherent light sources that span 1–5 THz (4–20 meV, 60–300 µm). The lasing energies and lifetimes are designed through bandgap engineering, making control of alloy composition, layer thickness, and carrier concentration of significant importance. To achieve low-loss waveguides at these long wavelengths, active regions are 5–30 µm-thick and are composed of hundreds of repeating unit cells. Since their introduction, THz QCLs have achieved watt-level output power and progressed from 4K to 199K pulsed T max . The main types of active regions, growth challenges, growth asymmetries, and several materials systems are discussed. The growth of monolayer thick barriers, hundreds of identical active regions, and total structure thickness push MBE to its limits.