IR Image Encoding and Camouflage System Uses Plasmonics to Hide Images from Naked Eye

Using a plasmonic nanostructure, researchers at the University of Central Florida (UCF) have devised a way to hide information on the surface of a material, so that it is only visible through an IR lens or camera that is tuned to the correct IR band.

In a Light: Science & Applications study, the researchers demonstrated a method for hiding images within the IR spectrum, so that while visible in the IR, the same area appears as a solid color in the visible spectrum. To do so, they created a dielectric layer patterned with regularly spaced nanosize holes, sandwiched between a reflective metallic mirror and a thin upper gold layer with holes corresponding to the middle layer’s discs. They encoded images onto the surface of the plasmonic sandwich using spun coat films of thermoplastic. They showed that by changing the diameters and depths of the holes, they could change how different wavelengths of light react with the materials.

Images can be imprinted on top of the plasmonic sandwich, and aspects of the holes, such as size and depth, help dictate which IR band the image can be seen in. Without looking through an IR camera tuned to the right band, the top of the device looks like a solid color, such as a yellow square, thanks to the unique properties that can be achieved with materials at the nanoscale. Courtesy of University of Central Florida.

The team was able to tune the device’s parameters in a way that made the surface appear as a uniform block of color unless viewed through an IR camera over a specific band. Varying the pattern characteristics allowed the researchers to control the electron plasma resonance, or the electric energy, created when light hits the device.

“So by controlling this electron plasma resonance, we can actually control which color of light or which band of light is absorbed and reflected,” said professor Debashis Chanda. “We found that we can create a surface where we preferentially control absorption of light.”

Researchers led by Debashis Chanda (r), an associate professor in the University of Central Florida’s NanoScience Technology Center, demonstrated that they can hide images within the IR spectrum while the same area appears as a solid color in the visible spectrum. Courtesy of UCF/Karen Norum.

In a different study, published in ACS Photonics, the researchers showed that their IR camouflage system could be used to erase and then display an image in selected IR bands. The team added a layer of phase change material (vanadium dioxide, or VO₂) to its plasmonic sandwich. The VO₂ layer dynamically changed the light reflecting from the surface from 100 to 0 percent, and back, when the phase change was triggered.

“It provides an additional element of dynamic tunability where the coded information is concealed or revealed to infrared cameras,” Chanda said.

Applications for this technology could include anticounterfeiting security, IR tagging, and/or IR camouflage where, for example, the presence of a designer label could be confirmed by viewing the tag through an IR camera. The IR camouflage technique could also be used for defense — for example, to confirm which assets are friendly and which are not, according to tags on their surfaces that are only visible in a specific IR band. The information on such tags could be dynamically changed for IR camouflage.

The research was published in Light: Science & Applications (https://doi.org/10.1038/s41377-018-0095-9) and in ACS Photonics (https://pubs.acs.org/doi/10.1021/acsphotonics.8b00972).

Now you see it, now you don’t. The University of Central Florida Pegasus symbol, visible in infrared, disappears as it enters the visible light spectrum that the human eye sees. UCF NanoScience Technology Center associate professor Debashis Chanda led a team of researchers who developed a technique using nanotechnology to preferentially control absorption of light to hide images and information in plain sight. Further, in the infrared domain the image can appear, disappear, or change based on external stimuli such as voltage. The technology has potential applications in anticounterfeiting, security, and camouflage. Courtesy of University of Central Florida.