A study carried out by a research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Catalan Institute of Nanoscience and Nano­technology (ICN2), University of Exeter Centre for Graphene Science, and TU Eindhoven demons­trates that graphene-based materials can be used to efficiently convert high-frequency signals into visible light, and that this mechanism is ultrafast and tunable. These outcomes open the path to exciting appli­cations in near-future information and communi­cation technologies.

The ability to convert signals from one frequency regime to another is key to various technologies, in particular in telecommuni­cations, where, for example, data processed by electronic devices are often transmitted as optical signals through glass fibers. To enable significantly higher data trans­mission rates future 6G wireless communication systems will need to extend the carrier frequency above 100 gigahertz up to the terahertz range. However, terahertz waves can only be used to transport data wirelessly over very limited distances. “Therefore, a fast and controllable mechanism to convert terahertz waves into visible or infrared light will be required, which can be transported via optical fibers. Imaging and sensing techno­logies could also benefit from such a mechanism,” says Igor Ilyakov of the Institute of Radiation Physics at HZDR.

What is missing so far is a material that is capable of upcon­verting photon energies by a factor of about 1000. The team has only recently identified the strong nonlinear response of Dirac quantum materials, e.g. graphene and topological insulators, to terahertz light pulses. “This manifests in the highly efficient generation of high harmonics, that is, light with a multiple of the original laser frequency. These harmonics are still within the terahertz range, however, there were also first obser­vations of visible light emission from graphene upon infrared and terahertz excitation,” recalls Sergey Kovalev of the Institute of Radiation Physics at HZDR. “Until now, this effect has been extremely inefficient, and the underlying physical mechanism unknown.”

The new results provide a physical explanation for this mechanism and show how the light emission can be strongly enhanced by using highly doped graphene or by using a grating-graphene meta­material – a material with a tailored structure charac­terized by special optical, electrical or magnetic properties. The team also observed that the conversion occurs very rapidly – on the sub-nano­second time scale, and that it can be controlled by electrostatic gating.

“We ascribe the light frequency conversion in graphene to a terahertz-induced thermal radiation mechanism, that is, the charge carriers absorb electro­magnetic energy from the incident terahertz field. The absorbed energy rapidly distributes in the material, leading to carrier heating; and finally this leads to emission of photons in the visible spectrum, quite like light emitted by any heated object,” explains Klaas-Jan Tielrooij of ICN2's Ultrafast Dynamics in Nanoscale Systems group and Eindhoven University of Technology.

The tunability and speed of the terahertz-to-visible light conversion achieved in graphene-based materials has great potential for application in information and communi­cation technologies. The underlying ultrafast thermo­dynamic mechanism could certainly produce an impact on terahertz-to-telecom inter­connects, as well as in any techno­logy that requires ultrafast frequency conversion of signals. (Source: HZDR)

Reference: I. Ilyakov et al.: Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials, Nano Lett. 23, 3872 (2023); DOI: 10.1021/acs.nanolett.3c00507

Link: Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany