MEPhI: Method of focusing terahertz radiation

2024: Introducing a way to focus terahertz radiation

In the international research laboratory "Radiation of charged particles" of the Institute of Nanotechnology in Electronics, Spintronics and Photonics NRNU MEPhI have developed a way to focus terahertz radiation. The university announced this on May 7, 2024. This type of radiation is used to study the internal structure of objects and the processes taking place in them, including in industry, in the field of security systems, in medicine and biology. For example, the ability to focus radiation can be extremely important when terahertz radiation is used to diagnose skin conditions or changes in blood composition. On the one hand, such radiation is tried to focus as best as possible in order to reduce damage to tissues or to see the structure of the sample in a small area of ​ ​ space. On the other hand, focusing radiation leads to its amplification at a specific point, which makes it possible to examine weaker signals with this radiation.

Physical Review

When electrons move a short distance from the periodic lattice at a speed close to the speed of light in a vacuum, Smith-Parcell radiation is excited. This radiation is named after two scientists who first observed an interesting phenomenon in 1953: when electrons moved over a metal lattice, a sharp luminous color line appeared above its surface.

In the international research laboratory "Radiation of charged particles" NRNU MEPhI, theoretically examine the properties of Smith-Parsell radiation from photonic crystals - artificial media similar to ordinary crystals, in the nodes of which instead of atoms there are relatively large objects - nanoparticles, microparticles, holes, resonators of various shapes.

Physical Review

To generate Smith-Parsell radiation, two-dimensional photonic crystals are especially interesting - arrays of particles located in the same plane, forming a rectangular lattice.

NRNU MEPhI graduate student Damir Garaev showed how a source based on Smith-Parcell radiation from a two-dimensional photonic crystal can be improved.

Typically, the radiation from electrons is measured far from the source, i.e. in the far zone. The radiation field in this zone has already "formed" and separated from the electron field, which rapidly decreases with distance. Therefore, in the far zone, the Smith-Parcell emission frequency spectra have pronounced and rather narrow maxima, and all intensity is concentrated near individual directions, which is very convenient for the subsequent use of this radiation. However, if the electron speed is high, then the far zone is very far from the lattice - up to several meters. In practice, placing the detector at such distances is inconvenient. The solution is to detect radiation close to the target, i.e. in the near or pre-wave zones. But the radiation field here has not yet managed to "form" and separate from the electron field, therefore, the spectra are more similar to noise: there are no pronounced maxima in them, the distributions are quite wide, and there are no distinguished directions of propagation in space - the radiation goes almost in all directions with the same intensity.

Damir Garaev constructed the Smith-Parcell theory of radiation from two-dimensional photonic crystals, which describes the properties of radiation at any distance from the lattice to the detector. He also calculated exactly how to arrange the particles of the photon crystal on the plane so that the detector could be placed close to the lattice, and the radiation had such a spectrum and angular distribution as if the detector stood far away. It turned out that for this, the particles should not be in the nodes of the rectangular grid, but are located periodically on curved lines. Mathematically, the form of curvature is parabola and hyperbola. Radiation from such lattices is focused, and this leads to the suppression of the near zone effect (the disappearance of noisy spectra and the spread of radiation in space).

The obtained results will make it possible to create an effective radiation source, including sources of THz of the range, and will also allow controlling light in real time - its frequency and direction of propagation.