Over the past decade, the mid-infrared (mid-IR) spectral range covering wavelengths from ${\sim}{2}\;{\rm{\unicode{x00B5}{\rm m}}}$ up to THz has become a region of increasing interest due a wide variety of emerging scientific and technological applications. The region covers important atmospheric windows and is rich in spectroscopic fingerprints of numerous molecules. Many harmful gases, toxic agents, air, water, and soil pollutants, components of human breath, biological tissue, and several explosive and narcotic agents have strong absorption fingerprints in this region. This makes the mid-IR and THz spectrum highly important for a variety of applications, including environmental monitoring, atmospheric transmission, non-invasive disease diagnosis and therapy through breath analysis, minimally invasive human surgery, food quality control and safety, imaging, and security and defense. Timely progress in mid-IR technology is, therefore, vital for future development of modern diagnostic tools for novel environmental, chemical, biological, medical, and security applications.

A critical component in the advancement of mid-IR science and technology is the development of novel laser light sources with continually evolving performance capabilities with regard to wavelength coverage, optical power and efficiency, temporal, spectral, and spatial quality, and output stability, in increasingly compact, practical, portable, miniaturized design, in all time scales from the continuous-wave to the ultrafast femtosecond domain. This, in turn, relies on progress in material science and the availability of new crystalline solid-state and fiber laser gain media, novel semiconductor materials and structures, and new nonlinear materials for the mid-IR. The steady progress in mid-IR materials and laser sources over the recent years has paved the way for the emergence of new applications in a multitude of scientific and technological fields.

This feature issue of Journal of the Optical Society of America B is focused on mid-IR science and technology, from materials to laser sources and applications. It follows the OSA Topical Meeting on Mid-Infrared Coherent Sources (MICS), which was originally scheduled for 26–28 March 2020 in Prague, Czech Republic, but was held as a virtual conference on 16–20 November 2020. The feature includes some of the recent developments in mid-IR nonlinear materials, laser gain media, solid-state lasers, semiconductor and quantum cascade lasers, nonlinear frequency conversion sources, parametric devices, THz technology, and application of semiconductor lasers in mid-IR spectroscopy and trace gas sensing. It is hoped that this feature issue will provide further impetus for the advancement of mid-IR science and technology, a topic which has witnessed increasing research interest and remarkable growth over the past decade, with significant impact on a wide range of multidisciplinary applications.