  Frequency combs have revolutionized time and frequency metrology and in
recent years, new frequency comb lasers that are highly compact or even on-chip
have been demonstrated in the mid-infrared and THz regions of the
electromagnetic spectrum. The emerging technologies include electrically pumped
quantum and interband cascade semiconductor devices, as well as high-quality
factor microresonators. In this guest editorial, the authors summarize recent
advances in the field, the potential for rapid broadband spectroscopy, as well
as the challenges and prospects for use in molecular gas sensing.
