A graphene waveguide and splitter. |
Two Univ. of Pennsylvania
engineers have proposed the possibility of two-dimensional metamaterials. These
one-atom-thick metamaterials could be achieved by controlling the conductivity
of sheets of graphene, which is a single layer of carbon atoms.
Professor Nader Engheta
and graduate student Ashkan Vakil, both of the Department of Electrical and
Systems Engineering in Penn’s School
of Engineering and
Applied Science, published their theoretical research in Science.
The study of
metamaterials is an interdisciplinary field of science and engineering that has
grown considerably in recent years. It is premised on the idea that materials
can be designed so that their overall wave qualities rely not only upon the
material they are made of but also on the pattern, shape, and size of
irregularities, known as “inclusions,” or “meta-molecules” that are embedded
within host media.
“By designing the
properties of the inclusions, as well as their shapes and density, you achieve
in the bulk property something that may be unusual and not readily available in
nature,” Engheta said.
These unusual properties
generally have to do with manipulating electromagnetic (EM) or acoustic waves;
in this case, it is EM waves in the infrared spectrum. Changing the shape,
speed and direction of these kinds of waves is a subfield of metamaterials
known as “transformation optics” and may find applications in everything from
telecommunications to imaging to signal processing.
Engheta and Vakil’s
research shows how transformation optics might now be achieved using graphene,
a lattice of carbon a single atom thick.
Researchers, including
many at Penn, have devoted considerable effort into developing new ways to
manufacture and manipulate graphene, as its unprecedented conductivity would
have many applications in the field of electronics. Engheta and Vakil’s
interest in graphene, however, is due to its capability to transport and guide
EM waves in addition to electrical charges and the fact that its conductivity
can be easily altered.
Applying direct voltage
to a sheet of graphene, by way of ground plate running parallel to the sheet,
changes how conductive the graphene is to EM waves. Varying the voltage or the
distance between the ground plate and the graphene alters the conductivity, “just like tuning a knob,” Engheta said.
“This allows you to
change the conductivity of different segments of a single sheet of graphene
differently from each other,” he said. “And if you can do that, you can navigate
and manipulate a wave with those segments. In other words, you can do
transformation optics using graphene.”
In this marriage between
graphene and metamaterials, the different regions of conductivity on the
effectively two-dimensional, one-atom-thick sheet function as the physical
inclusions present in three-dimensional versions.
The examples Engheta and
Vakil have demonstrated with computer models include a sheet of graphene with
two areas that have different conductivities, one that can support a wave, and
one that cannot. The boundary between the two areas acts as a wall, capable of
reflecting a guided EM wave on the graphene much like one would in a three
dimensional space.
Another example involves
three regions, one that can support a wave surrounded by two that cannot. This
produces a “waveguide,” which functions like a one-atom-thick fiber optic
cable. A third example builds on the waveguide, adding another non-supporting region
to split the waveguide into two.
“We can ‘tame’ the wave
so that it moves and bends however we like,” Engheta said. “Rather than playing
around with the boundary between two media, we’re thinking about changes of
conductivity across a single sheet of graphene.”
Other applications
include lensing and the ability to do “flatland” Fourier transforms, a
fundamental aspect of signal processing that is found in nearly every piece of
technology with audio or visual components.
“This will pave the way
to the thinnest optical devices imaginable,” Engheta said. “You can’t have
anything thinner than one atom.”
