A step forward in making terahertz technology usable in the real world

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

A team of scientists from the Cavendish Laboratory, together with colleagues from the Universities of Augsburg (Germany) and Lancaster, have found 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 lies between microwaves and infrared radiation,” explains Prof. David Ritchie, head of Semiconductor Physics. Group at Cambridge University’s Cavendish Laboratory, “but there is currently a lack of sources and detectors of this type of radiation that would be cheap, efficient and easy to use. This hampers the widespread use of terahertz technology.”

The researchers of the Semiconductor Physics group, together with researchers from Pisa and Turin in Italy, were the first to demonstrate, in 2002, the operation of a terahertz laser, a quantum cascade laser. The group has since continued to research terahertz physics and technology, and currently studies and develops functional terahertz devices that incorporate metamaterials to form modulators, as well as new types of detectors.

If the lack of usable devices were resolved, terahertz radiation could have many useful applications in safety, 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 help distinguish medicines from illegal drugs and explosives and could be used to enable even faster wireless communications beyond state of the art.

So what is the recent discovery about? “We were developing a new type of terahertz detector,” says Dr. Wladislaw Michailow, Junior Research Fellow at Trinity College Cambridge, “but by measuring his performance, it turned out that he was showing a much stronger signal than one would expect in theory. 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 individual particles: photons. This interpretation, first proposed by Einstein, formed the basis of quantum mechanics and explained the photoelectric effect. This quantum photo-excitation is how light is detected by the cameras of our smartphones; it is also what generates electricity from the light in the solar cells.

The known photoelectric effect consists in 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 vacuum by photons in the ultraviolet or X-ray range, or released in 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 within highly conductive two-dimensional electron gases at much lower frequencies has not been understood so far,” explains Wladislaw, first author of the study, “but we have been able to demonstrate it experimentally.” The quantitative effect theory was developed by a colleague from the University of Augsburg, Germany, and the international team of researchers published the results in the journal Science advances.

The researchers called the phenomenon accordingly, a “photoelectric effect on the plane”. In the corresponding paper, the scientists describe several benefits of exploiting this effect for sensing terahertz. In particular, the magnitude of the photoresponse generated by the incident terahertz radiation from the “in-plane photoelectric effect” is much higher than that predicted by other mechanisms that have been known up to now to give rise to a terahertz photoresponse. Therefore, the scientists expect that this effect will enable the fabrication of terahertz detectors with substantially greater sensitivity.

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