A graphene-based material converts incoming terahertz pulses (from above) into visible light in an ultrafast and controllable manner—optimal for data transport in optical fibers. [Image: B. Schröder/HZDR]
Researchers in Germany, Spain and the United Kingdom have demonstrated the ultrafast and tunable conversion of terahertz waves to visible light, with the help of graphene (Nano Lett., doi: 10.1021/acs.nanolett.3c00507). The upconversion occurs due to a thermal radiation mechanism from the heated electron system of graphene.
The findings have potential applications for a wide range of terahertz photonics technologies, such as sixth-generation (6G) wireless communication systems.
To 6G and beyond
Terahertz radiation lies within the transition region between microwave and far infrared, sharing some properties of each. Recent years have seen an increased interest in terahertz radiation for security screening, astronomy, quality control and other fields. Most notably, high-speed, high-bandwidth wireless communication at the terahertz frequency band (0.1–10 THz) could push systems beyond 5G.
In current communication technologies, frequency conversion allows optical interconnects to provide a higher information-transmission capacity between the electronic devices of end users.
“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,” said study author Igor Ilyakov of the Institute of Radiation Physics at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, in a press release accompanying the research. “Imaging and sensing technologies could also benefit from such a mechanism.”
The power of graphene
Ilyakov and his colleagues discovered that the strong nonlinear response of graphene to terahertz light pulses produces the expected harmonics within the terahertz range, as well as visible and near-infrared light.
Ilyakov and his colleagues discovered that the strong nonlinear response of graphene to terahertz light pulses produces the expected harmonics within the terahertz range, as well as visible and near-infrared light. They studied three different samples of graphene: single-layer graphene, a few-layer graphene film and a grating−graphene metamaterial. After exciting the samples using single-cycle terahertz pulses, light conversion occurred very rapidly, on the sub-nanosecond timescale.
“Until now, this effect has been extremely inefficient, and the underlying physical mechanism unknown,” said study author Sergey Kovalev of the Institute of Radiation Physics at HZDR.
The researchers believe the process can be attributed to terahertz-induced thermal radiation. Incident terahertz radiation is absorbed by the mobile charge carriers in graphene, which rapidly exchange energy, leading to carrier heating and emission of photons in the visible spectrum. The use of a grating−graphene metamaterial resulted in an increase in the emitted power of visible light by two orders of magnitude.
The technology could serve as a way to convert signals from telecom photons to terahertz photons and vice versa. Kovalev and his colleagues add that the peak of the terahertz-induced thermal emission spectra occurs around 1 eV, which is close to the most important telecom bands.