Dear Colleagues,

The global challenges that characterize our time, from climate change modelling to pollution control, biomedical, and homeland security applications, are pushing towards the development of increasingly sensitive, accurate, and robust instrumentation able to detect tiny amounts of specific molecules in gaseous or liquid samples.

The mid-infrared region represents a first choice for the high-sensitivity detection of many simple molecules of environmental interest, since their fundamental ro-vibrational transitions lay there. This region is characterized by strong absorption even by organic compounds, which are of great interest for medicine and biology and many other scientific and technological branches.

During the past decade, advances in semiconductor laser manufacturing opened new doors for the development of robust and compact apparatuses for trace gas sensing in the mid-infrared. Over the years, quantum cascade lasers have become increasingly widespread, easier to find on the market, and with better performance in terms of threshold current, emitted power, tuning range, and stability. Novel comb-emitting devices have been demonstrated and started to be utilized and commercialized, opening the doors, for example, to fast multispecies detection with dual-comb techniques. At the same time, alternative devices like interband cascade lasers have become commercially available and are now being used in spectroscopy, demonstrating their potential thanks to their lower dissipation, interesting narrow linewidth, and possibility of operating at lower wavelengths in the 3–4 µm region. Finally, research is also pursuing the development of other semiconductor devices working in the 2.5–4 µm range, like VECSEL, with results that suggest interesting development prospects. 

Optical sensors characterized by record sensitivity and high selectivity have been reported based on almost all these sources, exploiting a variety of spectroscopic techniques. This Special Issue is dedicated to state-of-the-art optical sensors based such sources, QCLs and ICLs and other semiconductor lasers, working in the mid-infrared. Particular interest will be given to comb-based spectroscopies with QCLs and ICLs, as well as to the progress in the development of such innovative sources. Special attention will also be paid to recent progress in portable sensors, based on techniques as varied as photoacoustic and photothermal spectroscopy, cavity ring-down spectroscopy, frequency or amplitude modulation spectroscopy, techniques based on enhancement cavities or multipass cells, and so on. Contributions reporting emerging photonic technologies for mid-infrared sensors will find their place in this Issue, with particular attention paid to novel and improved sources, comb-emitting semiconductor lasers, novel and improved detectors, and innovative tools (optical resonators, electro-optic devices, waveguides) for the mid-infrared.

Dr. Simone Borri
Guest Editor

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• mid-IR spectroscopic techniques
• mid-IR semiconductor lasers
• QCL and ICL optical frequency combs
• portable optical sensors
• mid-IR waveguides
• mid-IR detectors
• dual comb spectroscopy
• emerging mid-IR materials and technology