Physics / mechanism
Quantum cascade lasers (QCLs) are unipolar semiconductor lasers emitting mid-infrared (3–25 µm) and THz radiation via intersubband transitions in coupled quantum wells — electrons cascade through a periodic active region stack (typically 20–70 stages), emitting one photon per stage. Wavelength is engineered by well/barrier thickness, not bandgap, decoupling emission from material composition. Dominant material systems: InGaAs/InAlAs on InP for mid-IR; GaAs/AlGaAs for THz. State-of-the-art CW room-temperature wall-plug efficiency ~20–25% (mid-IR), output powers >1 W CW in single-mode. Linewidths <1 kHz achievable with external cavities. THz QCLs still require cryogenic cooling (<200 K), the key unresolved constraint.
Competitive landscape
Mid-IR alternatives include interband cascade lasers (ICLs) — lower threshold current, better for <5 µm, but lower power ceiling. Lead-salt diode lasers predate QCLs but are largely displaced. OPOs and DFG sources cover the same spectral window with wider tunability but are bulky and alignment-sensitive. Supercontinuum sources (nonlinear fiber) offer broad coverage but poor brightness per mode. For THz: optically pumped THz lasers, IMPATT/Gunn oscillators, and photomixers compete at lower frequencies with lower coherence.
Companies using
Connected ideas
Sources
Frontier (open questions)
- To be added.