Sources of terahertz electromagnétic radiations

In the electromagnetic spectrum, the terahertz (THz) range, also known as far infrared (FIR), lies between infrared and microwaves. Typically, it extends from wavelengths of around 30 μm to 3 mm, i.e. from around 100 GHz to 10 THz in terms of frequency, i.e. photons with energies of between 0.4 and 40 meV. This spectral position is at the root of numerous difficulties in developing high-performance sources and detectors, and therefore in carrying out studies at THz frequencies and developing promising applications. High-performance sources are devices that are efficient, compact, powerful, easy to use and, if possible, inexpensive.

To understand the difficulties involved in designing and producing such high-performance components, we need to go back to the physical basis of electromagnetic radiation emission and detection. This radiation is produced by accelerating electrical charges, such as those that make up an oscillating electric current. When it comes to radiation emitted or absorbed by an atom or molecule, the quantum equivalent of acceleration corresponds to a change in electronic energy, from an excited state to a less energetic state (for emission, and vice-versa for absorption). This latter phenomenon is predominant from X-rays to the infrared. To emit THz waves, the energy levels must be separated by a few meV. Unfortunately, thermal agitation (whose energy quantum is 24 meV at room temperature) similarly populates the energy levels required for THz emission, making THz radiation virtually impossible. This also prevents lasers from operating easily in the THz range, since they require an excited level that is more populated than the fundamental level, a population inversion easily destroyed by thermal agitation. As a result, visible and infrared sources lose their effectiveness at THz frequencies. On the microwave side, radiation is emitted by free electrons within a conductive material, i.e. by the electric current flowing, under the effect of an alternating voltage, in the conductor, which is shaped like an antenna for better coupling with the surrounding medium. These transmission systems lose their efficiency at THz frequencies, due to the lower efficiency of electronic components and resistors, as well as parasitic capacitances that limit their bandwidth.

Until the early 1990s, the lack of high-performance components limited THz research to the academic world, although some excellent results were obtained in gas spectroscopy and condensed matter physics. The advent of commercial lasers delivering femtosecond pulse trains revolutionized research in the far infrared. Thanks to antennas photo-excited by these laser pulses and to equivalent-time techniques, THz spectroscopy and imaging benches became accessible to most laboratories, operating simply...