Topological insulators conduct electricity in a special way and hold the
promise of novel circuits and faster mobile communications. Under the
leadership of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a research
team from Germany, Spain and Russia questioned a fundamental property of
this new class of materials: How exactly do the electrons in the material
respond when they are "startled" by short pulses of so-called terahertz
radiation? The results could herald faster mobile data communications or
high-sensitivity detector systems for exploring distant worlds in years to
come, the team reports in npj Quantum Materials.
Topological insulators are a recent class of materials with a special
quantum property: On their surface, they can conduct electricity almost
loss-free while their interior functions as an insulator—no current can flow
there. This opens up interesting prospects: Topological insulators could
form the basis for high-efficiency electronic components, which makes them
an interesting research field for physicists.
But a number of fundamental questions are still unanswered. What happens,
for example, when you give the electrons in the material a "nudge" using
specific electromagnetic waves—so-called terahertz radiation—thus generating
an excited state? One thing is clear: The electrons want to rid themselves
of the energy boost forced upon them as quickly as possible, such as by
heating up the crystal lattice surrounding them. In the case of topological
insulators, however, it was previously unclear whether getting rid of this
energy happened faster in the conducting surface than in the insulating
core. "So far, we simply didn't have the appropriate experiments to find
out," explains study leader Dr. Sergey Kovalev from the Institute of
Radiation Physics at 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, he and his international team developed an
ingenious test set-up: Intensive terahertz pulses hit a sample and excite
the electrons. Immediately after, laser flashes illuminate the material and
register how the sample responds to the terahertz stimulation. In a second
test series, special detectors measure to what extent the sample exhibits an
unusual non-linear effect and multiplies the frequency of the terahertz
pulses applied. 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.
Rapid energy transfer
The decisive thing was that the international team did not only 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 behavior 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, the "inner" electrons took
approximately 10 times as long, that is, a few picoseconds. "Topological
insulators are highly complex systems. The theory is anything but easy to
understand," says Michael Gensch, former head of the TELBE facility at HZDR
and now head of department in the Institute of Optical Sensor Systems at the
German Aerospace Center (DLR) and professor at TU Berlin. "Our results can
help decide which of the theoretical ideas hold true."
Highly effective multiplication
But the experiment also augurs well for interesting developments in digital
communication like WLAN and mobile communications. Today, technologies such
as 5G function in the gigahertz range. If we could harness higher
frequencies in the terahertz range, significantly more data could be
transmitted by a single radio channel, whereby 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 realized that, under certain
conditions, graphene—a two-dimensional, super-thin carbon material—can act
as an efficient frequency multiplier. It is able to convert 300 gigahertz
radiation into frequencies of some terahertz. The problem is that when the
applied radiation is extremely intensive, there is a significant drop in the
efficiency of the graphene. Topological insulators, on the other hand, even
function with the most intensive stimulation, the new study discovered.
"This might mean it's possible to multiply frequencies from a few terahertz
to several dozen terahertz," surmises 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."
If such a development comes about, the new quantum materials could be used
in a much wider frequency range than with graphene. "At DLR, we are
interested in using quantum materials of this kind in high-performance
heterodyne receivers for astronomy, especially in space telescopes," Gensch
explains.
Reference:
S. Kovalev et al, Terahertz signatures of ultrafast Dirac fermion relaxation
at the surface of topological insulators, npj Quantum Materials (2021).
DOI: 10.1038/s41535-021-00384-9
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