QCL‐based frequency metrology from the mid‐infrared to the THz range: a review

Quantum cascade lasers (QCLs) are becoming a key tool for plenty of applications, from the mid‐infrared (mid‐IR) to the THz range. Progress in related areas, such as the development of ultra‐low‐loss crystalline microresonators, optical frequency standards, and optical fiber networks for time and frequency dissemination, is paving the way for unprecedented applications in many fields. For most demanding applications, a thorough control of QCLs emission must be achieved. In the last few years, QCLs’ unique spectral features have been unveiled, while multifrequency QCLs have been demonstrated. Ultra‐narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra‐high sensitivity spectroscopy to infrared/THz metrology. A review of the present status of research in this field is presented, with a view of perspectives and future applications.