The mid-infrared spectral range spanning wavelengths from about 2 μm up to THz has drawn a great deal of interest in the last few decades for both scientific and technological applications. This broad spectral domain includes important atmospheric windows and is rich in spectroscopic signatures of numerous molecules. Many harmful gases and air, water, and soil contaminants as well as components of human breath and biological tissue have strong absorption fingerprints in this region. This makes the mid-infrared region highly important for a variety of applications, including environmental monitoring, atmospheric transmission, breath analysis, minimally invasive surgery, food quality control, spectroscopy, and imaging. Timely progress in mid-infrared technology is, therefore, vital for the development of modern diagnostic tools for novel environmental, chemical, biological, and medical applications.

Today, mid-infrared technology efforts still mostly consist in the development of novel laser systems and parametric sources, with a special focus on wavelength tunability and several other crucial parameters, including temporal, spectral, and spatial quality. Moreover, a growing number of functionalities are being demonstrated in this spectral range, for instance, in optical processing and imaging, and for both classical and quantum sensing. Such developments rely on the progress in material science, aimed at developing not only new crystalline solid-state and fiber lasers, but also novel semiconductor structures on chip, new nonlinear crystals for wavelength conversion in the mid-infrared, and more exotic integrated structures.

This feature issue includes some of the most recent advances in mid-infrared science and technology, from materials to parametric sources and applications. These include the development of high-average-power nanosecond and picosecond optical parametric oscillators tunable near 2 μm, the generation of high-energy ultrashort pulses near 5 μm using optical parametric chirped pulse amplification, exploitation of intra-pulse difference-frequency generation for shaping and amplification of wavelength-tunable mid-infrared femtosecond pulses, and efficient supercontinuum generation in a fiber up to 2.3 μm. Beam processing is another functionality that can be usefully implemented at ever longer wavelengths, and the feature includes a report on the extension of the non-mechanical beam steering technology into the mid-wave infrared region with a mix of chalcogenide glass waveguides and liquid crystals transparent this spectral region. The feature also includes a study on a time-varying graphene metasurface that implements a time lens in the THz domain as well as on the optical constants of InGaAsP in the mid-infrared, where this popular laser-diode and ${\chi }^{(2)}$ material is far less known than in the near-infrared telecommunication band. The feature is completed by numerical investigation of mid-IR single-photon generation as well as sensing and spectroscopic characterization of a variant of toxic bisphenol in the mid-infrared.

We expect that this feature will provide a useful reference on mid-IR sources and applications, and will stimulate further research in this important research field.