Superthin graphene transforms optics

U. PENNSYLVANIA (US) — By controlling the conductivity of sheets of graphene, engineers say they can create two-dimensional, one-atom thick metamaterials.

The study of metamaterials is based 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,” says Nader Engheta, professor of electrical and systems engineering and bioengineering at the University of Pennsylvania.

The research is published in the journal Science.

The unusual properties generally have to do with manipulating electromagnetic (EM) or acoustic waves; but 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 telecommunications, imaging, and signal processing.

Researchers have devoted considerable effort into developing new ways to manufacture and manipulate graphene, because its unprecedented conductivity could have wide reaching applications in the field of electronics.

The current research focuses on graphene’s capability to transport and guide EM waves along with 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, Engheta says. Varying the voltage or the distance between the ground plate and the graphene alters the conductivity, “just like tuning a knob.

“This allows you to change the conductivity of different segments of a single sheet of graphene differently from each other,” he says. “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 the 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.

Engheta and Vakil have demonstrated with computer models a sheet of graphene with two areas that have different conductivities—one that can support a wave, and one that can’t. 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 can’t. 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 says. “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 says. “You can’t have anything thinner than one atom.”

The study was funded by the U.S. Air Force Office of Scientific Research.

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