Graphene has been used to convert gigahertz-frequency electronic signals into high-harmonic, terahertz-frequency signals with extremely high efficiency. The work exploited the nonlinear properties of graphene to achieve terahertz conversion and was done by researchers in Germany at the Helmholtz Centre Dresden-Rossendorf (HZDR), the University of Duisburg-Essen (UDE) and the Max Planck Institute for Polymer Research.

Most electronic devices we use today exploit the semiconducting properties of silicon-based materials to create high-frequency signals to maximize processing speeds. Calculations have suggested that graphene – a sheet of carbon just one atom thick –could be much better suited to this task. Indeed, theoretical studies suggest that graphene could deliver signals at frequencies thousands of times higher than those created by silicon. The predicted response arises from the highly efficient nonlinear interaction between light and matter which takes place in graphene, due to its unique electronic band structure. However, no previous studies had yet achieved this conversion in the lab.

To observe the effect, the German researchers, led by Hassan Hafez at UDE, used graphene containing a large number of mobile electrons that originate from the interaction between the graphene and the substrate onto which it was deposited. When excited by rapid, gigahertz-frequency electromagnetic pulses in ambient, room-temperature conditions, these mobile electrons rapidly shared their energy with bound electrons in the material.

Electron vaporization

This causes the overall system of electrons to undergo a process that is comparable to a heated fluid making the transition from an electronic “liquid” phase to a hot “vapour” in just trillionths of a second. In turn, the graphene’s conductivity undergoes strong, rapid changes and this drives the multiplication of the frequency of the original gigahertz pulses.

This highly efficient nonlinear response results in three distinct converted signals, with three, five and seven times the frequency of the original pulses – bringing them into the terahertz range. “We have now been able to provide the first direct proof of frequency multiplication from gigahertz to terahertz in a graphene monolayer and to generate electronic signals in the terahertz range with remarkable efficiency,” explains Michael Gensch at HZDR.

The team’s result reveals a more promising future for the use of graphene in electronic devices than ever before. UDE’s Dmitry Turchinovich says, “The nonlinear coefficients describing the efficiency of the generation of this third, fifth and seventh harmonic frequency were exceptionally high”. He adds, “Graphene is thus possibly the electronic material with the strongest nonlinearity known to date. The good agreement of the measured values with our thermodynamic model suggests that we will also be able to use it to predict the properties of ultrahigh-speed nanoelectronic devices made of graphene.”

The research is described in Nature.