A German-Spanish research team has used gold-covered graphene to generate terahertz pulses more effectively. This is key step for the next generation 6G radio systems as the technology could potentially be integrated into a CMOS chip process.
Researchers working on the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) accelerator found they could use graphene can act as a frequency multiplier. When the two-dimensional carbon graphene is irradiated with light pulses in the low terahertz frequency range, these are converted to higher frequencies. Until now, the problem has been that extremely strong input signals, which in turn could only be produced by a full-scale particle accelerator, were required to generate such terahertz pulses efficiently.
“This is obviously impractical for future technical applications,” said Jan-Christoph Deinert of the Institute of Radiation Physics at HZDR. “So, we looked for a material system that also works with a much less violent input, i.e., with lower field strengths.”
So the HZDR team worked with researchers at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), the Institute of Photonic Sciences (ICFO), the University of Bielefeld, TU Berlin and the Mainz-based Max Planck Institute for Polymer Research on a new approach, covering the graphene with tiny gold lamellae, or flakes.
“These act like antennas that significantly amplify the incoming terahertz radiation in graphene,” said project coordinator Klaas-Jan Tielrooij from ICN2. “As a result, we get very strong fields where the graphene is exposed between the lamellae. This allows us to generate terahertz pulses very efficiently.”
To test the idea, team members from ICN2 in Barcelona produced samples. First, they applied a single graphene layer to a glass carrier. On top, they deposited an ultra-thin insulating layer of aluminium oxide, followed by a lattice of gold strips.
The samples were then taken to the TELBE terahertz facility in Dresden-Rossendorf, where they were hit with light pulses in the low terahertz range (0.3 to 0.7 THz). During this process, the team used special detectors to analyse how effectively the graphene coated with gold lamellae can multiply the frequency of the incident radiation.
“It worked very well,” said Sergey Kovalev, who runs the TELBE accelerator facility at HZDR. “Compared to untreated graphene, much weaker input signals sufficed to produce a frequency-multiplied signal.”
The gold covered graphene requires 10 percent of the original field strength to produce the frequency multiplication, opening up the use of electrical inputs. The wider the individual lamellae and the smaller the areas of graphene that are left exposed, the more pronounced the effect. Initially, the team was able to triple the incoming frequencies. Later, they achieved ninefold increases in the input frequency.
“Our graphene-based metamaterial would be quite compatible with current semiconductor technology,” said Deinert. “In principle, it could be integrated into ordinary chips.”
As well as uses in 6G radio, terahertz signals are also used in quality control in industry and security scanners at airports to a wide variety of scientific applications in materials research.
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