Researchers from Helmholtz-Zentrum Dresden-Rossendorf, Catalan Institute of Nanoscience and Nanotechnology (ICN2) and the University of Exeter have demonstrated that the properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. The results show that graphene-based materials can be used to efficiently convert high-frequency signals into visible light.
The team showed a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. They also demonstrated that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. These recent results could provide a route towards novel functionalities of optoelectronic technologies in the THz regime.
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.
Terahertz waves are a part of the electromagnetic spectrum that lies between microwaves and infrared light. However, terahertz waves can only be used to transport data wirelessly over very limited distances.
The team explained that a fast and controllable mechanism to convert terahertz waves into visible or infrared light will be required, which can be transported via optical fibers. This would require 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 so-called Dirac quantum materials, e.g. graphene and topological insulators, to terahertz light pulses.
According to the scientists, 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. They said, however, that 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 Prof. 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 terahertz 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.