According to the Encyclopaedia Britannica, the "middle infrared" region covers the portion of the electromagnetic spectrum between 400 and 4000 wave numbers, which corresponds to the wavelength range 2.5-25 gin. This range is of particular interest for many applications, especially for spectroscopy, since the electromagnetic frequencies involved coincide with frequencies of the internal vibrational motion of most molecules (the "molecular fingerprint" region). In our book, we define the "mid-infrared" more loosely to cover a slightly broader range starting from ~ 2 gm wavelength. On the longer wavelength end, we even included two chapters on terahertz wave generation, reflecting the fact that as terahertz waves are pushed further to higher frequencies, they converge with the long-wavelength infrared domain (for example, a frequency of 10 THz corresponds to a wavelength of 30 ~tm).
The development of solid-state laser sources in the mid-infrared opens unprecedented possibilities in spectroscopy~ as compared to traditionally used Fourier-transform spectrometers. The obvious advantages of lasers are directionality, coherence, narrow linewidth or small pulse duration, and high spectral brightness. Compact and efficient nfid-infrared laser sources can serve advantageously for remote light detection and ranging (LIDAR) (down to parts-per-billion in volume) of many trace gases and vapors that are important in pollution detection, and atmospheric chemistry. Other opportunities for fixed-wavelength and tunable mid-infrared laser sources include medical applications (e.g. microsurgery, dentistry, keratectomy or non-invasive diagnostics by means of breath analysis), ultrasensitive detection of drugs and explosives (down to one part per trillion) using photoacoustie or cavity ringdown spectroscopy, and free-space communications.