Graphene brings tera from space

https://www.materialstoday.com/carbon/news/graphene-brings-tera-from-space/

A new detector made from graphene could be used in a terahertz-based astronomical sensor, according to research published in Nature Astronomy. [Lara-Avila, S. et al., Nature Astron. (2019); DOI: 10.1038/s41550-019-0843-7]. Terahertz observations of star- and planet-forming regions are off limits to conventional ground-based astronomy because of atmospheric attenuation. But, orbiting and airborne sub-millimetre-wavelength telescopes could work around the haze given suitable detectors.

"We have reached a near zero-electron scenario in uniformly doped graphene, the Dirac point, by assembling electron-accepting molecules on its surface," explains Samuel Lara-Avila of Chalmers University of Technology in Sweden. "Our results show that graphene is an exceptionally good material for terahertz heterodyne detection when doped to the Dirac point", he adds.

In this setup, two signals are mixed using graphene. One is a high-intensity wave at a given terahertz frequency, generated by a local oscillator. The second is a weaker terahertz signal that emulates radiation from space at those frequencies. The mixed wave emerging from the graphene device generates an output wave at a much lower frequency in the gigahertz range. This is the intermediate frequency and can be analyzed with standard low-noise gigahertz electronics. The higher the team can push the intermediate frequency, the greater the bandwidth that can be reported by the sensor and so the more detailed the information from the internal motions of an astronomical object being observed can be.

"According to our theoretical model, this graphene terahertz detector has a potential to reach quantum-limited operation for the important 1-5 THz spectral range," explains team member Sergey Cherednichenko. "Moreover, the bandwidth can exceed 20 GHz, which is greater than the 5 GHz that state-of-the-art technology has to offer."

The device is extremely low power at the local oscillator and can detect faint tewrahertz signals several orders of magnitude weaker than those available to devices that use superconductor-based detectors. This, the team suggests, might allow quantum-limited terahertz coherent detector arrays to be built. This they add could open the door to a new way to carry out three-dimensional imaging of the universe. There is enormous potential for future space missions that could investigate water, carbon, and oxygen on other planets and the moons in orbit around them.

"The core of the terahertz detector is the system of graphene and molecular assemblies. This is in itself a novel composite two-dimensional 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," adds team leader Sergey Kubatkin.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.

Graphene sets the stage for the next generation of THz astronomy detectors

The image depicts a schematic of terahertz (THz) heterodyne detection with graphene. In this, two THz waves (red) are coupled into graphene, where they are combined or mixed. One of the waves is a high intensity signal generated by a local THz light source (i.e. a local oscillator), at a known THz frequency. The other signal is a faint THz wave that mimics the waves coming from space. CREDIT Photographer / Source Hans He

https://www.eurekalert.org/multimedia/pub/210925.php
Researchers from Chalmers University of Technology have demonstrated a detector made from graphene that could revolutionize the sensors used in next-generation space telescopes. The findings were recently published in the scientific journal 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/heat even when having, effectively, no electrons. We have reached a near zero-electron scenario in graphene, also called 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 the Dirac point", says Samuel Lara-Avila, assistant professor at the Quantum Device Physics Laboratory and lead author of the paper.

In detail, the experimental demonstration involves heterodyne detection, in which two signals are combined, or mixed, using graphene. One signal is a high intensity wave at a known THz frequency, generated by a local source (i.e. a local oscillator). The second is a faint THz signal that mimics the waves coming from space. Graphene mixes these signals and then produces an output wave at a much lower gigahertz (GHz) frequency, called the intermediate frequency, that can 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.

Sergey Cherednichenko, professor at the Terahertz and Millimetre Wave Laboratory and co-author of the paper, says:

"According to our theoretical model, this graphene THz detector has a 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 the state of the artstate-of-the-art technology has to offer."

Another crucial aspect for the graphene THz detector is the extremely low power needed for the local oscillator to achieve a trustable detection of faint THz signals, few orders of magnitude lower than superconductors require. This could enable quantum-limited THz 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, explains the possible implications for practical astronomy:

"This graphene-based technology has enormous potential for future space missions that aim at e.g. unveiling 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, at the moment, THz 3D imagers are simply not available".

Sergey Kubatkin, professor at the Quantum Device Physics Laboratory and co-author of the paper, explains:

"The core of the THz detector is the 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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Read the full paper, "Towards quantum-limited coherent detection of terahertz waves in charge-neutral graphene" in Nature Astronomy

Watch an experimental demonstration with prof Sergey Cherednichenko here (YouTube)

For more information, contact:

Samuel Lara-Avila, assistant professor at the Quantum Device Physics Laboratory,
Department of Microtechnology and Nanoscience - MC2,
Chalmers University of Technology, Sweden
samuel.lara@chalmers.se