Detectors of terahertz electromagnetic 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 in developing applications that are nevertheless promising. In the case of detectors, we are looking for compact, easy-to-use, low-cost devices with high sensitivity and dynamic range, as well as the ability to manufacture detector arrays for imaging.

To understand the difficulties involved in designing and producing such high-performance detectors, we need to return to the physical basis of electromagnetic radiation detection. This radiation, absorbed by the material illuminated in the detector, generates either a rise in the temperature of the material, or transitions between the energy levels of the material's atoms/molecules, or the transfer of each photon's energy to the material's free charges. In the case of detector heating, the THz beams used in most application studies are not very powerful, and the temperature rise is minimal. In the case of energy transfer from THz photons to free or bound charges in the illuminated material, this energy is less than or of the order of the energy of the thermal quantum (24 meV) at room temperature. This prevents "optical" detectors, usually semiconductor-based, from operating efficiently in the THz range, since they require an empty conduction band (or excited level) and a populated valence band (or fundamental level). For this reason, detectors used in the visible and infrared range lose their effectiveness at THz frequencies. On the microwave side, receivers are based on the principle that free electrons in the metal forming the receiving antenna are accelerated by the Coulomb force induced by the electromagnetic field coupled to the antenna. The resulting electric current is read by an electronic device. These receiving 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.

This article begins with an historical overview of the invention and development of THz detectors, which have gradually progressed in an attempt to escape the constraints just listed. Bolometers have made spectacular progress, thanks for example to the introduction of superconductors, and since the 2000s have practically reached the quantum limits of detection. Another major advance came in the late 1980s with the development of optoelectronic...