New theory for detecting terahertz electromagnetic waves holds out hope for advances in computing and medicine

put and Vvacation are the bottoms of conduction bands in a metal and in vacuum; EF is the Fermi energy of the electrons in the metal. ϕ=Vvacation−EF>0 is the work function. (c) The dynamics of the photoexcitation process: at the normal incidence of the radiation, the electrons acquire a momentum px parallel to the surface, while to escape from the material they need a momentum component pz perpendicular to the material-vacuum interface (represented by the thick magenta line). Electrons can get the momentum pz after a few scattering events in the metal or under an oblique incidence of radiation. Credit : Physical examination B (2022). DOI: 10.1103/PhysRevB.106.075411″ width=”644″ height=”530″/>

The conventional (external) photoelectric effect in a conductive medium. (a) The geometry of a typical experiment. (b) The band structure and photon absorption process: Vmeet and Vvacuum are the bottoms of conduction bands in a metal and in vacuum; EF is the Fermi energy of the electrons in the metal. ϕ=Vvacuum−EF>0 is the work function. (c) The dynamics of the photoexcitation process: at a normal radiation incidence, the electrons acquire a momentum pX parallel to the surface, while to escape from the material they need a momentum component pz perpendicular to the material-vacuum interface (represented by the thick magenta line). Electrons can get momentum pz after a few diffusion events in the metal or under oblique incidence of radiation. Credit: Physical examination B (2022). DOI: 10.1103/PhysRevB.106.075411

The detection of electromagnetic waves in the terahertz frequency range remains a difficult problem. Researchers from the University of Cambridge, in collaboration with physicists from the University of Augsburg, have recently discovered a new physical effect that could change this. In a new study, scientists are now developing a theory explaining the underlying mechanism. Their discoveries make it possible to build small, inexpensive and highly sensitive terahertz detectors. These could be used, for example, in medical diagnostics, for contactless security checks or for faster wireless data transmission. The results of the new theory have been published in the journal Physical examination B.

When X-rays or UV rays fall on a metal surface, they expel electrons from the material. This “photoelectric effect” can form the basis of detectors detecting the presence of electromagnetic waves.

In a slightly modified form, a similar effect is used in digital camera recording chips or in solar cells. These react to visible and infrared light. However, its energy is significantly lower than that of UV radiation and is therefore insufficient to release electrons from the material. Instead, the radiation can alter the electrical properties of semiconductor structures, which are generally poor conductors. When exposed to light, however, they become conductive or can even generate voltages.

The energy of terahertz radiation is even lower than that of visible or infrared light. THz radiation generally does not provide enough energy even to excite electrons in semiconductors. There are currently several types of terahertz radiation detectors, but more efficient, inexpensive, and compact THz detectors are still needed. Therefore, researchers continue to search for alternative physical principles to detect terahertz radiation.

“Recently, together with colleagues from the UK, we discovered a new physical effect that allows the construction of very sensitive detectors,” explains Dr Sergey Mikhailov from the Institute of Physics at the University of Augsburg. “It is based on semiconductor materials with a two-dimensional electron gas – a thin conductive layer that forms under the surface of the semiconductor. Under certain conditions, a type of photoelectric effect can be observed even at frequencies terahertz in such a structure. When this semiconductor is illuminated by electromagnetic waves, a current is generated in the two-dimensional electron gas in a direction parallel to the surface of the semiconductor.”

In their current work, the researchers have developed a theory of this “in-plane photoelectric effect” that further explains the mechanism. Various predictions can be derived from their results. For example, depending on the effect, it should be possible to build detectors sensitive to the entire terahertz range (radiation of frequencies between 0.1 and 10 terahertz or wavelengths between 3 and 0.03 millimeters). “This is an area where any new detection mechanism is of great value,” says Mikhailov. Theoretically, it should also be possible to build detectors that react to very low radiation intensities.

These could be used in various applications. For example, skin cancer cells could easily be detected using terahertz radiation. Such detectors could also be used to find the smallest quantities of drugs or explosive material at security checkpoints. Additionally, terahertz waves oscillate back and forth faster than the electromagnetic radiation currently used in mobile communications. For this reason, they can transmit much more information in the same amount of time. The new detectors could therefore give a boost to the speed of mobile Internet.


One step closer to using terahertz technology in the real world


More information:
SA Mikhailov et al, Theory of in-plane photoelectric effect in two-dimensional electron systems, Physical examination B (2022). DOI: 10.1103/PhysRevB.106.075411

Provided by the University of Augsburg

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