One step closer to using terahertz technology in the real world

Researchers have discovered a new effect in two-dimensional conductive systems that promises improved performance of terahertz detectors.

A team of scientists from the Cavendish laboratory, in collaboration with colleagues from the universities of Augsburg (Germany) and Lancaster, have discovered a new physical effect when two-dimensional electronic systems are exposed to terahertz waves.

First of all, what are terahertz waves? “We communicate using cell phones that transmit microwave radiation and use infrared cameras for night vision. Terahertz is the type of electromagnetic radiation that falls between microwaves and infrared radiation,” explains Professor David Ritchie, head of the semiconductor physics group at Cambridge University’s Cavendish Laboratory, “but at the moment there is a lack of sources and detectors of this type of radiation which would be cheap, efficient and easy to use. This hinders the widespread use of terahertz technology.”

Researchers from the Semiconductor Physics Group, together with researchers from Pisa and Turin in Italy, were the first to demonstrate, in 2002, the operation of a laser at terahertz frequencies, a quantum cascade laser. Since then, the group has continued research in terahertz physics and technology and is currently investigating and developing functional terahertz devices incorporating metamaterials to form modulators, as well as new types of detectors.

If the lack of usable devices were addressed, terahertz radiation could have many useful applications in security, materials science, communications, and medicine. For example, terahertz waves allow imaging of cancerous tissue that could not be seen with the naked eye. They can be used in new generations of safe and fast airport scanners that distinguish drugs from illegal drugs and explosives, and they could be used to enable even faster wireless communications across the state. art.

So, what is the recent discovery about? “We were developing a new type of terahertz detector,” says Dr Wladislaw Michailow, a junior researcher at Trinity College Cambridge, “but when measuring its performance, it turned out to show a signal much stronger than we should theoretically expect. So we found a new explanation.”

This explanation, as scientists say, lies in the way light interacts with matter. At high frequencies, matter absorbs light in the form of single particles, photons. This interpretation, first proposed by Einstein, formed the foundation of quantum mechanics and explained the photoelectric effect. This quantum photoexcitation is how light is detected by the cameras of our smartphones; it is also what generates electricity from light in solar cells.

The well-known photoelectric effect consists of the release of electrons from a conductive material – a metal or a semiconductor – by incident photons. In the three-dimensional case, electrons can be expelled into a vacuum by photons in the ultraviolet or X-ray range, or released into a dielectric in the mid-infrared to visible range. The novelty lies in the discovery of a quantum photoexcitation process in the terahertz range, similar to the photoelectric effect. “That such effects can exist in highly conductive two-dimensional electron gases at much lower frequencies has not been understood until now,” says Wladislaw, first author of the study, “but we have been able to prove it experimentally. The quantitative theory of the effect was developed by a colleague at the University of Augsburg, Germany, and the international team of researchers published their findings in the journal Scientists progress.

The researchers named the phenomenon accordingly, an “in-plane photoelectric effect”. In the corresponding article, the scientists describe several advantages of exploiting this effect for terahertz detection. In particular, the magnitude of the photoresponse that is generated by the incident terahertz radiation by the “in-plane photoelectric effect” is much higher than expected from other mechanisms hitherto known to give rise to a terahertz photoresponse. . Thus, scientists expect this effect to allow the fabrication of terahertz detectors with significantly higher sensitivity.

“This brings us one step closer to using terahertz technology in the real world,” Prof Ritchie concludes.