Recent advances and future prospects of THz quantum cascade lasers

The development of quantum cascade lasers (QCLs) operating at terahertz frequencies is proceeding at a rapid pace. Improvements have been obtained both in output power and operating temperatures, thanks to a combination of quantum design and fabrication methods, while the range of lasing frequencies has been extended below 1 THz. Distributed feedback devices have been fabricated with a variety of techniques for the purpose of achieving predictable single-mode emission, and external cavity set-ups have been implemented for tunability. THz QCLs have also become an important test-bed for new device concepts like non-periodic resonators, low volume microcavity lasers, and phase-matched non-linear devices that generate a THz signal on a near-IR carrier. Future developments will likely include optical circuits with photonic crystals, random lasers, new electro-mechanical tuning mechanisms, and possibly quantum dot lasers, further expanding the range of possible applications in many research areas.