Laser-based Mid-infrared Sources and Applications

1Mid‐IR Spectral Range

1.1 Definition of the Mid‐IR

Infrared radiation was unknown before the year 1800 when Friedrich Wilhelm Herschel – a German‐born musician, who moved to England to work as a music band conductor, but later became obsessed with astronomy and eventually landed the position of the King's Astronomer − discovered infrared radiation. He made this finding while exploring sunlight, dispersed into its colors by a glass prism, with the aid of a liquid thermometer with a blackened bulb to absorb radiation (a prototype of a modern microbolometer). His experimentation led to the conclusion that there must be an invisible form of light beyond the visible spectrum [1].

Further experiments showed that this invisible radiation is electromagnetic radiation with a lower frequency than the red in the visible spectrum. Modern science further divides the infrared spectral region into near‐infrared, mid‐infrared, and far‐infrared.

According to the Encyclopedia Britannica, the “middle infrared” (mid‐infrared or mid‐IR) region of the electromagnetic spectrum covers, in wavelength, the portion between 2.5 and 50 μm (6–120 THz in frequency or 200–4000 cm⁻¹ in wavenumbers).¹ (The wavenumber is the inverse of the vacuum wavelength, λ, expressed in cm⁻¹; it is also equal to the optical frequency divided by the speed of light, ν/c.)

However, the definitions of the “mid‐IR” vary substantially in the technical literature, depending on a field‐specific community. For example, the detector‐based ...