Quantum material boosts terahertz frequencies
Topological insulators are a still young class of materials with a special quantum property: on their surface they can conduct electricity almost without loss, whereas their interior acts as an insulator. This promises interesting perspectives for component development: topological insulators could serve as the basis for highly efficient electronic components. This makes them an interesting field of research in physics.
First, however, some fundamental questions remain open: For example, what happens when the electrons in the material are energetically excited with terahertz radiation? One thing is clear: the electrons want to get rid of the forcibly missed energy boost as quickly as possible, for example by heating up the crystal lattice around them. But in the case of topological insulators, it was previously questionable whether this release of energy happens faster in the conducting surface than in the insulating core. A research team from Germany, Spain and Russia led by the Helmholtz Centre Dresden-Rossendorf (HZDR) has investigated this question. “To determine this, there was previously a lack of suitable experiments,” explains study leader Dr Sergey Kovalev from the Institute of Radiation Physics at the HZDR. “Until now, it has been extremely difficult to distinguish between the reaction of the surface and that of the interior of the material at room temperature.”
To overcome this hurdle, the team developed a sophisticated experimental setup: Intense terahertz pulses hit the sample and excite the electrons. Immediately afterwards, laser flashes illuminate the material and record how the sample reacts to the terahertz stimulus. In a second series of experiments, special detectors measure the extent to which the sample shows an unusual non-linear effect and multiplies the frequency of the incoming terahertz pulses. Kovalev conducted these experiments at the TELBE terahertz light source in the HZDR’s Centre for High Power Radiation Sources. Researchers from the Catalan Institute for Nanosciences and Nanotechnology in Barcelona, the University of Bielefeld, the German Aerospace Center (DLR), the TU Berlin as well as the Lomonosov University and the Kotelnikov Institute for Radio Engineering and Electronics in Moscow were involved.
For the tests, the Russian project partners produced three different topological insulators with different, precisely matched properties: in one, only the electrons on the surface could directly absorb the energy of the terahertz pulses; in the others, mainly electrons inside the samples were excited. The comparison of these three experiments made it possible to distinguish precisely between the behaviour of the surface and that of the interior of the material.
It turned out that the electrons in the surface excited much faster than those inside the material. The presumed reason: they were able to transfer their energy immediately to the crystal lattice of the material. In numbers: While the surface electrons returned to their original energetic state after a few hundred femtoseconds, this took around ten times as long for the “inner” electrons, i.e. a few picoseconds.
In addition to a better understanding of such materials, the experiment also opens up interesting perspectives for digital communication, such as WLAN and mobile radio. The most advanced radio technologies, such as 5G, operate in the gigahertz range today. If higher frequencies in the terahertz range could be used, significantly more data could be transmitted over a radio channel. Frequency multipliers could play an important role here: They are able to translate relatively low radio frequencies into significantly higher ones.
Some time ago, the research team had already realised that graphene – two-dimensional, super-thin carbon – can serve as an efficient frequency multiplier under certain conditions. Graphene layers can convert a 300-gigahertz radiation into frequencies of a few terahertz. The problem: If the incoming radiation is extremely intense, graphene loses a lot of efficiency. Topological insulators, on the other hand, still work even with the most intense excitation, according to the result of the current study. “This could make it possible to multiply frequencies from a few terahertz to several dozen terahertz,” believes HZDR physicist Dr Jan-Christoph Deinert, who heads the TELBE team together with Kovalev. “So far, we don’t see an end there for topological insulators.” This means that the new quantum materials could be used in a much wider frequency range than graphene, for example. The researchers already have one possible application in mind: “At DLR, we are very interested in using such quantum materials in powerful heterodyne receivers for astronomy, especially in space telescopes,” explains Michael Gensch, head of department at DLR’s Institute of Optical Sensor Systems.
More information: email@example.com