Terahertz Electromagnetic Waves. Principles and Technics

Terahertz (THz) electromagnetic waves (EM) correspond to the spectral range between the far infrared and microwave frequencies. Although explored since the initial work of Rubens in the early 20th century, technical difficulties have long held back studies and technological development at these frequencies. This can be explained simply by physical reasons. In the optical and infrared domains, the incident electromagnetic wave induces molecular dipoles in matter (displacement current), and the electromagnetic response of matter is translated by the notions of permittivity and therefore refractive index. Waves are detected by absorption and, in particular, by carrier photogeneration. In the microwave domain, the predominant electromagnetic response is that of free electrons (conduction currents), and many devices are made of metal to facilitate the flow of these conduction currents. For example, the wave is detected by currents induced in antennas. In the terahertz range, conduction and displacement currents are of the same order of magnitude, and optical and microwave techniques lose efficiency. As a result, terahertz radiation sources are less powerful, compact or easy to use than optical and microwave sources. Similarly, terahertz detection is made more difficult by the low energy of terahertz photons, which is typically 5 to 10 times lower than thermal energy at room temperature. Finally, the earth's atmosphere (at sea level and under normal conditions: 20°C, 50% humidity) is not very transparent beyond 1 terahertz: attenuation in excess of 1 dB/m, with numerous peaks of high absorption (of the order of 20 dB/m) due to water vapor resonances. Nevertheless, the study of the terahertz domain was revitalized and facilitated at the end of the 1980s thanks to the emergence of new techniques and technologies, first optoelectronic, then based on the frequency upgrading of electronic components or the development of new nanometric components. This research effort is stimulated, beyond academic research, by the many applications that have been identified.