Incredibly fast and tunable: Graphene-based terahertz-to-visible conversion
(Nanowerk News) A study conducted by a research team from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Catalan Institute of Nanoscience and Nanotechnology (ICN2), University of Exeter Center for Graphene Science, and TU Eindhoven showed that graphene-based materials can be used to efficiently converting high-frequency signals into visible light, and that this mechanism is extremely fast and adjustable, the team presented their findings at Nano Letters (“Ultrafast Tunable Terahertz-to-Visible Light Conversion via Thermal Radiation from Graphene Metamaterials”). These results pave the way for exciting applications in information and communication technologies in the future.
The ability to convert signals from one frequency regime to another is key to a variety of technologies, particularly in telecommunications, where, for example, data processed by electronic devices is often transmitted as optical signals via glass fibres. To enable much higher data transmission rates, future 6G wireless communication systems will need to extend carrier frequencies beyond 100 gigahertz into the terahertz range.
Terahertz waves are the part of the electromagnetic spectrum that lies between microwaves and infrared light. However, terahertz waves can only be used to transmit data wirelessly over a very limited range.
“Therefore, a fast and controllable mechanism is needed to convert terahertz waves into visible or infrared light, which can be transmitted through optical fiber. Imaging and sensing technologies can also benefit from such mechanisms,” said Dr. Igor Ilyakov from the Institute of Radiation Physics at the HZDR.
What has been missing so far are materials capable of changing the photon energy by a factor of about 1000. The team recently identified the strong nonlinear response of so-called Dirac quantum materials, for example graphene and topological insulators, to terahertz pulses of light. .
“This manifests itself in the highly efficient generation of high harmonics, i.e. light with multiples of the original laser frequency. These harmonics are still in the terahertz range, however, there were also the first observations of visible light emission from graphene at infrared and terahertz excitations,” recalls Dr. Sergey Kovalev of the Institute of Radiation Physics at the HZDR. “Until now, this effect was highly inefficient, and the underlying physical mechanisms were unknown.”
The new results provide a physical explanation for this mechanism and demonstrate how light emission can be greatly enhanced by using highly doped graphene or by using grating-graphene metamaterials – materials with adapted structures characterized by special optical, electrical or magnetic properties. The team also observed that the conversion occurs very quickly – on a sub-nanosecond timescale, and can be controlled by electrostatic channels.
“We consider the conversion of the frequency of light in graphene to a terahertz-induced thermal radiation mechanism, that is, charge carriers absorb electromagnetic energy from the terahertz incident field. The absorbed energy rapidly diffuses into the material, causing heating of the carrier; and finally this leads to the emission of photons in the visible spectrum, quite like the light emitted by a heated object,” explained Prof. Klaas-Jan Tielroij of ICN2’s Ultrafast Dynamics in Nanoscale Systems group and Eindhoven University of Technology.
The tunability and speed of conversion of terahertz light to visible light achieved in graphene-based materials has great potential to be applied in terahertz information and communication technology. The underlying ultrafast thermodynamic mechanisms can certainly have an impact on terahertz-to-telecommunications interconnections, as well as on any technology that requires ultrafast frequency signal conversion.
