Advances in THz quantum cascade lasers: fulfilling the application potential

Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two and a half years ago, and, since then, their development has proceeded at a very rapid pace. Most of the research has focused on new concepts for the quantum design of the gain medium as well as on improving device structure by optimizing fabrication and waveguide technology. These efforts by various groups have led to maximum operating temperatures of about 140 K in pulsed mode, output powers of up to 50 mW, and lasing in continuous wave up to 93 K. Such advances are making THz QC lasers more and more appealing for applications in various fields like chemical sensing, astronomy' spectroscopy, and imaging. For their successful implementation, specific requirements have to be addressed, particularly concerning the spectral properties of the emission. Here we report some latest developments in this direction. We demonstrate the perfect control of laser design for the realization of devices with precisely defined emission frequency in the whole range from 2.3 THz to 4.8 THz. Additionally, single-mode THz lasers with distributed feedback resonators have been achieved and a new technique involving surface plasmon gratings has been developed to improve performances. The latter offers also the possibility of constructing distributed Bragg ',ratings as a replacement for high-reflection coatings. Finally, solutions allowing broad tuneability are examined, with preliminary results illustrating the viability of external cavity set-ups.