Scientists Spin Graphene and Gold into Terahertz Waves

A German-Spanish research team has developed a graphene-based approach for increasing the efficiency with which the scientists were able generate terahertz pulses. In the system, the researchers coated a graphene sheet with a metallic lamellar structure — specifically, gold lamellae.

Scientists currently use accelerator devices, which are often complex, and large lasers to generate terahertz waves. The capabilities of the new material system make it compatible to existing semiconductor technology in actively transitioning from gigahertz to terahertz frequencies with increased efficiency over current sources and converters that perform that transition.

Though graphene is a known frequency multiplier (when light pulses in the low terahertz frequency range [0.3 to 0.7 THz] irradiate the 2D carbon material, they convert to higher frequencies), the efficient generation of terahertz pulses had previously relied on extremely strong input signals. The particle accelerators operating at full scale or large laser systems that have been needed to reliably generate such signals made the method incompatible for many applications. These include telecommunications (5G and beyond).

Ultrathin gold lamellae drastically amplify the incoming terahertz pulses (red) in the underlying graphene layer, enabling efficient frequency multiplication. Courtesy of HZDR/Werkstatt X.

To develop a material system with a drastically reduced strength of field, the researchers coated the graphene with gold lamellae; functioning much like antennas, the gold lamellae amplify incoming terahertz radiation in graphene. In a physical system, that characteristic delivers very strong fields at the point where the graphene is exposed to the lamellae, said Klaas-Jan Tielrooji of the Catalan Institute of Nanoscience and Nanotechnology (ICN2).

ICN2 researchers collaborated with those from the Institute of Radiation Physics at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Institute of Photonic Sciences (ICFO), the University of Bielefeld, TU Berlin, and the Max Planck Institute for Polymer Research, in Mainz.

The team tested its concept by applying a graphene layer to a glass carrier and subsequently vapor-depositing an ultrathin layer of aluminum oxide over the graphene for insulation. The researchers then added a gold strip lattice. Light pulses in the low terahertz range hit the materials, multiplying the frequency of the incident radiation and enabling the team to detect and analyze the effectiveness of the process.

“Compared to untreated graphene, much weaker input signals sufficed to produce a frequency-multiplied signal,” said Sergey Kovalev, who is responsible for the TELBE terahertz facility at HZDR. One-tenth of the field strength originally needed to produce a frequency-multiplied signal was enough to allow the researchers to observe frequency multiplication. Once converted, the power of the pulses was more than 1000× stronger than in systems using other methods.

The researchers reported that expanding the width of the lamellae and lessening the coverage area of the exposed graphene layer enhanced the process and its effect. Team members additionally demonstrated the ability to achieve an up-to-ninefold frequency input increase.

The new material raises the possibility to transition from gigahertz to terahertz with purely electrical signals — meaning with significantly reduced effort. It could be integrated onto chips, said Jan-Christoph Deinert of the Institute of Radiation Physics HZDR and primary author of the study describing the research.

The researchers said the terahertz range, and hence their system, support applications in materials research and sensors and detectors.

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.0c08106).