This chapter reviews the applications of terahertz (THz) quantum cascade lasers (QCLs). THz QCLs have come a long way since their first demonstration in 2002. Although still operating at or close to cryogenic temperatures, their applications have been multiplying steadily over the last decade, helped by the availability of compact commercial THz QCL systems and the growing adoption of the THz QCL technology in the THz scientific community. Currently, the key fields of THz QCL applications are imaging, spectroscopy and sensing.

Laser feedback interferometry, based on the self-mixing (SM) effect in quantum cascade lasers (QCLs), is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and SM is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. The SM phenomenon in QCLs has been exploited for a wide range of applications, including the measurement of internal laser characteristics, two- and three-dimensional imaging, materials analysis and near-field imaging. This chapter provides an overview of the SM effect in QCLs, and reviews the state of the art in sensing using this technique.