Soldiers have to see through smoke, dust, fog, or other airborne obscurant and detect the presence of toxins or other chemicals on the front lines or in the field. In order to identify those chemicals, they employ infrared (IR) sensors and spectroscopy, which allow a particular color of light to shine at a specific frequency corresponding to each chemical.
To be able to identify each chemical would require a soldier to coat the goggle with an exclusive filter, enabling the chemical signature to pass through at a specific frequency (i.e., a particular color).
Scientists at the University of Illinois, however, have effectively developed a tunable infrared filter composed of graphene, which would allow a soldier to modify the frequency of a filter just by controlled mechanical deformation of the filter (i.e., graphene origami), and not by substituting the substance on the goggles used to filter a specific spectrum of colors.
The research is sponsored by the Air Force Office of Scientific Research, which is keen on sensors that are not only sensitive to various IR wavelengths but also mechanically controllable and tunable. The results are reported in a paper titled "Mechanically Reconfigurable Architectured Graphene for Tunable Plasmonic Resonances" in Light: Science & Applications.
This application is one of the many discoveries of "wonder material" graphene by SungWoo Nam, an Assistant Professor of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign.
Typically when you place graphene on a substrate, it is extremely transparent and absorbs only about three percent of light. At certain angles, you can see it. We use this versatility to make other structures like flexible and transparent sensors out of graphene.
SungWoo Nam, Assistant Professor
Since it is one-atom thin, graphene is typically used while flat. Nam's research team raised a question: what would occur if, through origami (paper-folding art), graphene was wrinkled? Could the properties of graphene be changed by altering its topography?
According to Nam, researchers haven't attempted this idea before with other conventional materials as they are brittle and cannot be bent without breaking. What's exclusive about graphene is that it is not only thin, but it is strong, meaning it will not break easily when bent.
Let's say we create graphene wrinkles by mechanical deformation. If you get a certain dimension, is there going be any changes in the way the light is going to be absorbed by the graphene? We wanted to link the dimensions of the wrinkled graphene to its optical absorption.
SungWoo Nam, Assistant Professor
Nam's team found out that truly, wrinkled graphene absorbs light in a different way depending on the dimensions and structure through plasmonic resonances, thus creating different colors. Furthermore, in contrast to paper, which cannot easily be flattened after crumpling or folding, graphene can be re-stretched to become flat and wrinkle-free again. Yet another feature is that the amount of light absorption can be changed by a factor of approximately 10.
"By changing the shape, you can absorb the light of a different frequency by controlling plasmonic resonance conditions," Pilgyu Kang, the paper’s first author and currently an Assistant Professor at Mechanical Engineering Department at George Mason University, stated. "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."
By selecting graphene as a filter for infrared goggles, users can turn a knob to mechanically compress and stretch the graphene. That allows for a modification of the light wavelength being absorbed. So as an example of its application, a soldier can thus easily alter the graphene filter to a preferred wavelength to match the type of chemical he/she is aiming to identify.
"In a conventional filter, once you make the filter, you are done," Nam concluded. "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."
This research is based on an international partnership with Dr. Kyoung-Ho Kim and Professor Hong-Gyu Park at Korea University and is supported by the AFOSR and National Science Foundation.