Ultra-thin
gold lamellae drastically amplify the incoming terahertz pulses (red) in the
underlying graphene layer, enabling efficient frequency multiplication. Credit:
HZDR/Werkstatt X.
On the
electromagnetic spectrum, terahertz light is located between infrared radiation
and microwaves. It holds enormous potential for tomorrow's technologies: Among
other things, it might succeed 5G by enabling extremely fast mobile
communications connections and wireless networks. The bottleneck in the
transition from gigahertz to terahertz frequencies has been caused by
insufficiently efficient sources and converters. A German-Spanish research team
with the participation of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has
now developed a material system to generate terahertz pulses much more
effectively than before. It is based on graphene, i.e., a super-thin carbon
sheet, coated with a metallic lamellar structure. The research group presented
its results in the journal ACS Nano.
Some time
ago, a team of experts working on the HZDR accelerator ELBE were able to show
that graphene can act as a frequency multiplier: When the two-dimensional
carbon is irradiated with light pulses in the low terahertz frequency range,
these are converted to higher frequencies. Until now, the problem has been that
extremely strong input signals, which in turn could only be produced by a
full-scale particle accelerator, were required to generate such terahertz
pulses efficiently."This is obviously impractical for future technical
applications," explains the study's primary author Jan-Christoph Deinert
of the Institute of Radiation Physics at HZDR. "So, we looked for a
material system that also works with a much less violent input, i.e., with
lower field strengths."
For this
purpose, HZDR scientists, together with colleagues from the Catalan Institute
of Nanoscience and Nanotechnology (ICN2), the Institute of Photonic Sciences
(ICFO), the University of Bielefeld, TU Berlin and the Mainz-based Max Planck
Institute for Polymer Research, came up with a new idea: the frequency
conversion could be enhanced enormously by coating the graphene with tiny gold
lamellae, which possess a fascinating property: "They act like antennas
that significantly amplify the incoming terahertz radiation in graphene,"
explains project coordinator Klaas-Jan Tielrooij from ICN2. "As a result,
we get very strong fields where the graphene is exposed between the lamellae. This
allows us to generate terahertz pulses very efficiently."
Surprisingly
effective frequency multiplication
To test
the idea, team members from ICN2 in Barcelona produced samples: First, they
applied a single graphene layer to a glass carrier. On top, they vapor-deposited
an ultra-thin insulating layer of aluminum oxide, followed by a lattice of gold
strips. The samples were then taken to the TELBE terahertz facility in
Dresden-Rossendorf, where they were hit with light pulses in the low terahertz
range (0.3 to 0.7 THz). During this process, the experts used special detectors
to analyze how effectively the graphene coated with gold lamellae can multiply
the frequency of the incident radiation.
"It
worked very well," Sergey Kovalev is happy to report. He is responsible
for the TELBE facility at HZDR. "Compared to untreated graphene, much
weaker input signals sufficed to produce a frequency-multiplied signal."
Expressed in numbers, just one-tenth of the originally required field strength
was enough to observe the frequency multiplication. And at technologically
relevant low field strengths, the power of the converted terahertz pulses is
more than a thousand times stronger thanks to the new material system. The
wider the individual lamellae and the smaller the areas of graphene that are
left exposed, the more pronounced the phenomenon. Initially, the experts were
able to triple the incoming frequencies. Later, they attained even larger
effects—fivefold, sevenfold, and even ninefold increases in the input frequency.
Compatible
with chip technology
This
offers a very interesting prospect, because until now, scientists have needed
large, complex devices such as accelerators or large lasers to generate
terahertz waves. Thanks to the new material, it might also be possible to
achieve the leap from gigahertz to terahertz purely with electrical input
signals, i.e., with much less effort. "Our graphene-based metamaterial
would be quite compatible with current semiconductor technology," Deinert
emphasizes. "In principle, it could be integrated into ordinary
chips." He and his team have proven the feasibility of the new process—now
implementation in specific assemblies may become possible.
The
potential applications could be vast: Since terahertz waves have higher
frequencies than the gigahertz mobile communications frequencies used today,
they could be used to transmit significantly more wireless data—5G would become
6G. But the terahertz range is also of interest to other fields—from quality
control in industry and security scanners at airports to a wide variety of
scientific applications in materials research, for example.