Graphene technology enables clock rates in the terahertz range
For some time now, the electronics industry has been striving to develop components for switching frequencies in the terahertz range. Graphene, which has a high electrical conductivity and is compatible with all existing electronic technologies, is regarded as a promising material and potential successor to silicon. In particular, it has long been theoretically predicted that graphene could be a very efficient “nonlinear” electronic material, i.e. a material that can convert an applied alternating electromagnetic field into fields of much higher frequency. However, all experimental efforts over the last ten years to prove this effect in graphene have been unsuccessful.
Now scientists have demonstrated that frequencies can be transposed from the GHz to the THz range – and even with high energetic efficiency. The researchers demonstrated this effect by means of a graphene monolayer, explains Michael Gensch, whose research group is investigating ultra-short time physics at the Helmholtz Centre HZDR in Dresden. And not only that: Their cooperation partners around profesor Dmitry Turchinovich, experimental physicist at the University of Duisburg-Essen (UDE), have succeeded in describing the measurements quantitatively with the help of a simple model based on basic physical principles of thermodynamics.
The experimental proof was successful with the help of a trick: The researchers used graphene, which contains numerous free electrons due to its special production – exactly one layer of carbon atoms is deposited on a special substrate – and the interaction with the substrate and the ambient air. When these moving electrons are excited by an alternating field, they rapidly share their energy with the other electrons in the graphene, which react like a heated liquid: From an electronic “liquid”, an electronic “vapour” in the graphene is figuratively speaking. The change between the “liquid” and “vapor” phases takes place within a trillionth of a second and causes particularly rapid and strong changes in conductivity. This is the basic building block for efficient frequency multiplication.
The researchers are thus paving the way for ultrafast nanoelectronics based on graphene: “We were not only able to experimentally demonstrate a long-predicted effect in graphene for the first time, but at the same time to describe it quantitatively well,” emphasizes Turchinovich. “A few years ago, we began to study the fundamental physical mechanisms of the electronic nonlinearity of graphene. However, our laboratory light sources were not sufficient for the actual detection and quantification of frequency multiplication. For this we needed experimental possibilities that are currently only available at the TELBE facility”.
The scientists used electromagnetic pulses with frequencies between 300 and 680 gigahertz and converted them in the graph into pulses with three, five and seven times the frequency, thus transposing them into the terahertz frequency range. “The nonlinear coefficients describing the efficiency of the generation of this third, fifth and seventh harmonic frequency were exceptionally high,” explains Turchinovich. “Graphene may thus be the electronic material with the highest nonlinearity known to date. The good agreement of the measured values with our thermodynamic model gives us hope that we can also use it to predict the properties of nanoelectronic components made of graphene.” Science regards this discovery as groundbreaking. The work demonstrates that carbon-based electronics can operate efficiently at ultra-fast rates. It also makes ultrafast hybrid components made of graphene and traditional semiconductors conceivable.
The experiment was carried out at the novel terahertz radiation source TELBE, which is based on a superconducting accelerator. Its pulse rate, which was a hundred times higher than that of laser-based terahertz sources, made the measurement accuracy required for the investigation of graphs possible in the first place. A data processing method developed as part of the EU project EUCALL allows the researchers to use the measurement data for each of the 100,000 light pulses per second. In terms of measurement technology, the project was at the limit of what is currently feasible.
The first authors of the article are the two young scientists Hassan A. Hafez (UDE/MPI-P) and Sergey Kovalev (HZDR). The researchers present their results in the scientific journal “Nature”.
Further information: www.hzdr.de
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