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| a terahertz detector after fabrication. Credit: Wladislaw Michailow |
Researchers have discovered in two-dimensional conductive systems a new
effect that promises improved performance of terahertz detectors.
A team of scientists at the Cavendish Laboratory, together with colleagues
at the Universities of Augsburg (Germany) and Lancaster, has found a new
physical effect when two-dimensional electron systems are exposed to
terahertz waves.
First of all, what are terahertz waves? "We communicate using mobile phones
that transmit microwave radiation and use infrared cameras for night vision.
Terahertz is the type of electromagnetic radiation that lies in-between
microwave and infrared radiation," explains Prof. David Ritchie, Head of the
Semiconductor Physics Group at the Cavendish Laboratory of the University of
Cambridge, "but at the moment, there is a lack of sources and detectors of
this type of radiation that 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 Torino 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 to research terahertz physics and
technology and currently investigates and develops functional terahertz
devices incorporating metamaterials to form modulators, as well as new types
of detectors.
If the lack of usable devices were solved, terahertz radiation could have
many useful applications in security, materials science, communications, and
medicine. For example, terahertz waves allow the imaging of cancerous tissue
that couldn't be seen with the naked eye. They can be employed in new
generations of safe and fast airport scanners that make it possible to
distinguish medicines from illegal drugs and explosives, and they could be
used to enable even faster wireless communications beyond the
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 when measuring its performance, it turned
out that it showed a much stronger signal than should be theoretically
expected. So we came up with a new explanation."
This explanation, as the scientists say, lies in the way how 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 cameras in 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 vacuum by
photons in the ultraviolet or X-ray range, or released into a dielectric in
the mid-infrared to visible range. The novelty is 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 prove this experimentally." The quantitative theory of
the effect was developed by a colleague from the University of Augsburg,
Germany, and the international team of researchers published their findings
in the journal Science Advances.
The researchers named the phenomenon accordingly, an "in-plane photoelectric
effect." In the corresponding paper, the scientists describe several
benefits of exploiting this effect for terahertz detection. In particular,
the magnitude of photoresponse that is generated by incident terahertz
radiation by the "in-plane photoelectric effect" is much higher than
expected from other mechanisms that have been heretofore known to give rise
to a terahertz photoresponse. Thus, the scientists expect that this effect
will enable fabrication of terahertz detectors with substantially higher
sensitivity.
"This brings us one step closer to making terahertz technology usable in the
real world," concludes Prof Ritchie.
Reference:
Wladislaw Michailow et al, An in-plane photoelectric effect in
two-dimensional electron systems for terahertz detection, Science Advances
(2022).
DOI: 10.1126/sciadv.abi8398
Tags:
Physics