18 septembre 2019

[Optic] – Graphene key to terahertz space telescopes

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Terahertz heteronyne detection with Graphene - Codex International

Researchers from Chalmers University of Technology, Gothenburg, Sweden, have demonstrated a new type of optical detector made from graphene, which they say could “revolutionize the sensors used in next-generation space telescopes.” The findings have recently been published in Nature Astronomy.

Beyond superconductors, there are few materials that can fulfill the requirements needed for making ultra-sensitive and fast terahertz (THz) detectors for astronomy. Chalmers researchers have shown that engineered graphene adds a new material paradigm for THz heterodyne detection.
“Graphene might be the only known material that remains an excellent conductor of electricity and heat even when having, effectively, no electrons,” commented Samuel Lara-Avila, assistant professor at the Quantum Device Physics Laboratory and lead author of the paper.
“We have reached a near zero-electron scenario in graphene, also known as the Dirac Point, by assembling electron-accepting molecules on its surface. Our results show that graphene is an exceptionally good material for THz heterodyne detection when doped to achieve the Dirac point.”
Graphene functionality
The Chalmers group’s experimental demonstration involves heterodyne detection, in which two signals are combined, or mixed, using the graphene-based technique. One signal is a high intensity wave at a known terahertz frequency, generated by a local source (such as a local oscillator). The second is a faint terahertz signal that mimics the waves emanating from space.
Graphene has the ability to mix these signals and to produce an output wave at a much lower gigahertz (GHz) frequency, known as the intermediate frequency, which can then be analyzed with standard, low-noise gigahertz electronics. The higher the intermediate frequency can be, the higher bandwidth the detector is said to have, required to accurately identify motions inside the celestial objects of interest.
Sergey Cherednichenko, professor at Chalmers’ Terahertz and Millimetre Wave Laboratory (TML) and co-author of the Nature Astronomy paper, commented, “According to our theoretical model, this graphene terhertz detector has the potential to reach quantum-limited operation for the important 1-5 THz spectral range. Moreover, the bandwidth can exceed 20 GHz, larger than 5 GHz that today’s state-of-the-art technology has to offer.”
Low power demand
Another important feature of the graphene terhertz detector is the extremely low power needed for the local oscillator to achieve reliable detection of faint terhertz signals, which are several orders of magnitude lower than superconductor detectors require. The team say that this capability “could enable quantum-limited terahertz coherent detector arrays – hence opening the door to 3D imaging of the universe.”
Elvire De Beck, astronomer at the Department of Space, Earth and Environment, who did not take part in the research, explained the possible implications for practical astronomy: “This graphene-based technology has enormous potential for future space missions that aim at, for example, revealing how water, carbon, oxygen and life itself came to earth.
« A lightweight, power-effective 3D imager that is quantum-limited at terahertz frequencies is crucial for such ambitious tasks. But, up until now, terahertz 3D imagers have simply not been available.”
Sergey Kubatkin, professor at the Quantum Device Physics Laboratory and co-author of the paper, added, “The core of this detector is its system of graphene and molecular assemblies. This is in itself a novel composite 2D material that deserves deeper investigation from a fundamental point of view, as it displays a whole new regime of charge/heat transport governed by quantum-mechanical effects.”

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