2D Tunable Materials
Recent advances with graphene and other 2D tunable materials.
Tunable 2D materials like graphene and black phosphorous are opening a whole range of new technological applications, from driverless cars to 3D holographic imaging.
Over the last decade or so, researchers have discovered that by making certain materials into thin, two-dimensional sheets — sometimes only one or two atoms thick — the materials can acquire new properties and behaviors.
In particular, by engineering the normally static nanostructure of these materials in certain ways, researchers can create materials whose properties can be tuned in real time. Simply by adjusting the voltage, for example, researchers can change the material’s basic optical properties, potentially controlling the wave vector, wavelength, amplitude, phase, and polarization of light.
The goal is to do it in the context of all optical processes, from scattering and absorption to luminescence and thermal emission.
SPIE member Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science at the California Institute of Technology (USA), reported in a conference on engineered nanostructures at SPIE Photonics West earlier this year that he and his colleagues have used graphene to make a material with 100% optical absorption, something first proposed five years ago.
“This allows one to move from still-life daguerreotype nanophotonics to the film and television era,” Atwater said.
To force the graphene to interact strongly with light, the researchers sliced a monolayer of graphene into thin ribbons only 50 or 100 nm wide. These ribbons allow light to efficiently couple with surface plasmons — the collective excitation of electrons — in the graphene.
Surrounding the ribbons is gold film that funnels light to the graphene. Underneath is a Salisbury screen, which acts like a mirror that prevents light from escaping through the material, reflecting photons back into the graphene. The researchers designed the structure of this surface so that its impedance matches that of free space, which enables it to absorb all photons that come its way.
“That’s quite a dramatic result,” Atwater said.
By changing the voltage going through the graphene, the researchers can adjust how much it absorbs light. The graphene nanomaterial works in IR wavelengths, so tunability could lead to all sorts of devices for controlling thermal radiation. In essence, Atwater explained, you could turn a black body into a white body with a flick of a switch.
Covering a building with this kind of material could provide a new way to control heating and cooling, by adjusting whether the building absorbs or reflects heat.
“It’s like a coat of paint. I can change the color from black to white in the infrared,” Atwater said.
Caltech researchers have also used graphene to make a tunable phase modulator, paving the way toward beam steering devices that can reflect IR beams in any direction without the need for the slower, mechanically moving mirrors used in conventional beam steerers.
IR beam-steering devices would be essential for lidar systems in driverless cars, and Atwater’s group has already demonstrated a tunable phased array in the NIR that scans at megahertz frequencies.
Atwater said his team is also working on controlling polarization of light in graphene; developing devices for 3D holographic images; gauging potential applications for 2D black phosphorous; and developing new photovoltaic cells with sheets of materials such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2).
Cells based on MoS2 and WSe2 are extremely efficient, absorbing nearly 100% of light. Being so thin and light, they could be useful for wearable technologies, vehicles, and other applications where weight is an issue.
Topological insulators are another class of promising 2D semiconducting materials. The defining characteristic of these materials, Atwater said, is a correlation between spin and charge. As with black phosphorous, researchers are still exploring the properties of these materials.
–Marcus Woo is a freelance science journalist based in California.
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