6. Environment

Electromagnetic investigation techniques, from ultraviolet to terahertz, are the only ones that enable remote physico-chemical analysis of gases. These techniques are used to detect or monitor the presence of gases or pollutants in the atmosphere, and to analyze the chemical composition of the upper layers of the atmosphere, for example to determine the ozone concentration in the stratosphere. Several of these techniques are well-established and highly effective, such as LIDAR for the detection of sulfur compounds. Terahertz technology is therefore only of interest for molecules that do not show strong signatures in other spectral domains, particularly infrared. This is the case for volatile organic compounds (VOCs). In general, gases made up of polyatomic molecules with a non-zero dipole transition moment show an absorption spectrum in the terahertz range with numerous lines, due to the...