Breakthrough brings us closer to real-world terahertz technologies

Body Scanner Security Concept
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Terahertz technology could enable advanced scanners for security, medicine and materials science. It could also enable much faster wireless communication devices than is currently possible.

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

A recent physical discovery in two-dimensional conductive systems enables a new type of terahertz detector. Terahertz frequencies, which lie between microwaves and infrared on the spectrum of electromagnetic radiation, could enable faster, safer and more efficient imaging technologies, as well as much faster wireless telecommunications. A lack of effective real-world devices has hampered these developments, but this new breakthrough brings us one step closer to these advanced technologies.

A new physical effect when two-dimensional electronic systems are exposed to terahertz waves has been discovered by a team of scientists from the Cavendish Laboratory with colleagues from the universities of Augsburg (Germany) and Lancaster.

“The fact that such effects can exist in highly conductive two-dimensional electron gases at much lower frequencies has not been understood until now, but we have been able to prove it experimentally.” — Wladislaw Michailow

To begin with, 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,” says Professor David Ritchie, head of the semiconductor physics group at Cambridge University’s Cavendish Laboratory, “but for 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, associated 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.

Wladislaw Michailow Terahertz Detector

Wladislaw Michailow showing a device in the clean room and a terahertz detector after fabrication. 1 credit

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 have 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 basis of quantum mechanics and could explain 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. “The fact 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 from the University of Augsburg, Germany, and the international team of researchers recently published their findings in the reputable journal Scientists progress.

The researchers called the phenomenon accordingly, as 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.

Reference: “An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection” by Wladislaw Michailow, Peter Spencer, Nikita W. Almond, Stephen J. Kindness, Robert Wallis, Thomas A. Mitchell, Riccardo Degl’ Innocenti, Sergey A. Mikhailov, Harvey E. Beere and David A. Ritchie, April 15, 2022, Scientists progress.
DOI: 10.1126/sciadv.abi8398

The work was supported by the EPSRC HyperTerahertz projects (no. EP/P021859/1) and grant no. EP/S019383/1, The University of Cambridge Schiff Foundation, Trinity College Cambridge, and the European Union’s Horizon 2020 research and innovation program Graphene Core 3 (Grant No. 881603).

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