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.