On the battlefield, soldiers use infrared (IR) sensors and spectroscopy to see through smoke and dust in order to detect the presence of toxins or other chemicals. Goggles that employ IR technology allow a specific color of light to shine at a frequency that corresponds to a specific chemical. That same specificity represents a limit to the technology, however; a unique filter coating is needed to identify any given chemical.
But researchers at the University of Illinois College of Engineering have circumvented that limitation by developing a tunable infrared filter from graphene. By employing “graphene origami” — a technique to alter the topography of flat, single-atom-thick graphene through controlled mechanical deformation — the filter’s frequency can be changed with the turn of a knob.
"Typically when you place graphene on a substrate, it is extremely transparent and absorbs only about three percent of light," explains SungWoo Nam, an assistant professor of mechanical science and engineering. "At certain angles, you can see it. We use this versatility to make other structures like flexible and transparent sensors out of graphene."
Unlike conventional materials, Nam adds, graphene is not only thin but resilient, and can be bent without breaking. It can also be re-stretched after folding to return to a flat, wrinkle-free state. In their research, Nam’s team discovered that, thanks to its plasmonic resonances, wrinkled graphene absorbs light differently depending on its structure and dimensions. This produces different colors. The material’s light absorption also can be altered by a factor of approximately 10.
"By changing the shape, you can absorb the light of a different frequency by controlling plasmonic resonance conditions," says Pilgyu Kang, first author of the paper and now an assistant professor in the mechanical engineering department at George Mason University “And by mechanically controlling the height and wavelength of the graphene wrinkles, I can excite different surface plasmons and thus absorb different frequency. At the end of the day, you get a tunable filter."
"In a conventional filter, once you make the filter, you are done," Nam summarized. "No matter the size, there is one unique light wavelength. With graphene, depending on how much you stretch and release, you can communicate in different light wavelengths."
You can read a preview of the research, soon to be published in the journal Light: Science & Applications.