Abstract : Terahertz or Far Infrared radiation extends from 100 GHz to 10 THz. This frequency range was considered
to be a "spectral gap" due to the considerable technological difficulties in producing a tunable source with reasonable emission powers. Two THz spectrometers have been developed and exploited. The first uses THz pulses and is based on Time Domain Spectroscopy. It is capable of providing a spectrum which extends from few GHz to 1.2 THz within few minutes and has a spectral resolution in the order of few GHz. The second instrument generates continous wave THz by photomixing and can reach up to 3 THz with a spectral purity of 5 MHz. After presenting the analytical descriptions and experimentally characterizations of the instruments, we focus on the monitoring and detection of atmospheric pollutants, illustrated by the analysis
of cigarette smoke which represents an example of hostile medium. This study reveals the presences of the hydrogen cyanide (HCN), carbon monoxide (CO) and formaldehyde (H2CO) and allows their concentrations to be measured. This analysis is verified by comparison with standard chemical methods. The fundamentalpart of this work is the study of the gas response after a subpicosecond excitation : the free induction decay of carbonyl sulphide that consists of a series of uniformly spaced subpicosecond commensurate transients. A theoretical model is used to understand the classical phenomena. The quasi exponential decrease of the reemitted pulses is linked to the spectral width of the rotational transition while the anharmonicity influences the pulse shape due to the propagation, dispersion and centrifugal distortion.