They are considered to be highly interesting materials for the electronics of the future. Topological insulators conduct electricity in a special way and promise new types of electronic circuits. An international team of researchers, with the participation of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), has now clarified a fundamental property of this new class of materials – how do the electrons in the material react when they are 'excited' with short pulses of terahertz-frequency radiation? The results are not only important for the fundamental understanding of these novel quantum materials but could also allow faster mobile data communications in the future or be used in highly sensitive detector systems for the investigation of distant planets. The study was published in the journal NPJ Quantum Materials on 19 October 2021.
"The properties offer promising prospects for the future," emphasised Anke Kaysser-Pyzalla, Chair of the DLR Executive Board. "Topological insulators can be the basis for highly efficient electronic components and ensure faster mobile data transmission in the future."
Topological insulators are a very recent class of materials that have a special quantum property. On their surface, they can conduct electricity almost without loss, while their interior functions as an insulator – no current can flow there. Looking to the future, this opens up interesting possibilities. Topological insulators could form the basis for high-efficiency electronic components, which makes them an interesting object of research for physicists. "At DLR, we are for instance very interested in using quantum materials of this kind in high-performance heterodyne receivers for astronomy, especially in space telescopes," says Michael Gensch, Head of the Department Terahertz and Laser Spectroscopy at the DLR Institute of Optical Sensor Systems and a professor at the TU Berlin Institute for Optics and Atomic Physics.
However, a number of fundamental questions remain unanswered. What happens, for example, when the electrons in the material are given a 'nudge' using specific electromagnetic waves – terahertz-frequency radiation – thus moving them into an excited state? One thing is clear – the electrons want to rid themselves of the energy boost forced upon them as quickly as possible, for example by heating up the crystal lattice surrounding them. In the case of topological insulators, however, it was previously unclear whether releasing this energy happened faster on the conducting surface than in the insulating core. "Previously, there were no suitable experiments to determine this," explains study leader Sergey Kovalev from the Institute of Radiation Physics at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). "Up to now, at room temperature, it was extremely difficult to differentiate the surface reaction from that in the interior of the material."
In order to overcome this hurdle, Kovalev's international team developed a sophisticated experimental setup. Intense terahertz-frequency pulses hit a sample and excite the electrons. Immediately afterwards, laser flashes illuminate the material and record how the sample responds to the terahertz stimulation. In a second series of experiments, special detectors measure the extent to which the sample exhibits an unusual non-linear effect and multiplies the frequency of the applied terahertz pulses. Kovalev and his colleagues conducted these experiments using the TELBE terahertz light source at HZDR's ELBE Center for High-Power Radiation Sources. Researchers from the Catalan Institute of Nanoscience and Nanotechnology in Barcelona, Bielefeld University, the German Aerospace Center (DLR), the Technical University of Berlin, and Lomonosov University and the Kotelnikov Institute of Radio Engineering and Electronics in Moscow were involved.
The decisive factor was that the international team did not just investigate a single material. Instead, the Russian project partners produced three different topological insulators with different, precisely determined properties. In one case, only the electrons on the surface could directly absorb the terahertz pulses. In the others, the electrons were mainly excited in the interior of the sample. "By comparing these three experiments we were able to differentiate precisely between the behaviour of the surface and the interior of the material," Kovalev explains. "And it emerged that the electrons in the surface became excited significantly faster than those in the interior of the material." Apparently, they were able to transfer their energy to the crystal lattice immediately.
Put into figures, while the surface electrons reverted to their original energetic state in a few hundred femtoseconds, it took the 'inner' electrons approximately 10 times as long, that is, a few picoseconds. "Topological insulators are highly complex systems. The theory is anything but easy to understand," emphasises Michael Gensch "Our results can help decide which of the theoretical ideas are correct."
The experiment also promises interesting developments in digital communications such as WLAN and mobile communications. Today, technologies such as 5G function in the gigahertz range. If higher frequencies in the terahertz range could be used, significantly more data could be transmitted over a single radio channel. Frequency multipliers could play an important role; they are able to translate relatively low radio frequencies into significantly higher ones.
Some time ago, the research team had already realised that, under certain conditions, graphene – two-dimensional, super thin carbon – can act as an efficient frequency multiplier. It is able to convert 300 gigahertz radiation into frequencies of a few terahertz. The problem is that when the applied radiation is extremely intense, there is a significant drop in the efficiency of the graphene. Topological insulators, on the other hand, still function with the most intensive excitation, the new study discovered. "This might mean it's possible to multiply frequencies from a few terahertz to several dozen terahertz," believes HZDR physicist Jan-Christoph Deinert, who heads the TELBE team together with Sergey Kovalev. "At the moment, there is no end in sight when it comes to topological insulators." This means that the new quantum materials could be used in a much wider frequency range than graphene, for example.