Hiding Images with a Plasmonic Sandwich

Images can be imprinted on top of the plasmonic sandwich, and aspects of the holes, such as size and depth, help dictate which infrared band the image can be seen in. (Source: UCF)

What is real is not always as it appears. Uni­versity of Central Florida researchers have found a way to hide infor­mation on materials and only make it visible to a person using the right tech. “We found we can create a surface where we pre­ferentially control absorption of light,” said Debashis Chanda, an associate professor in UCF’s Nano­Science Tech­nology Center who has developed the technique.

The trick is to put the infor­mation on a surface that is riddled with nanoscale patterns, which can fool the naked eye by reflecting only a solid color rather than the intended infor­mation. To get the intended infor­mation, a person must look through an infrared lens or camera tuned to the correct infrared band. And not only can infor­mation be hidden this way, the infor­mation can also be changed so that the secret messages invi­sible to the human eye can appear and reappear.

The appli­cations for this tech­nology could include anti-counter­feiting security, infrared tagging or infrared camou­flages where, for instance, the presence of a designer label could be confirmed with a look through an infrared camera. It also has mili­tary appli­cations, such as confirming which assets are friendly and which are enemy by tags on their surfaces that are only visible in a specific infrared band or by dynami­cally changing the infor­mation for infrared camou­flage.

The researchers demon­strated that they can hide images within the infrared spectrum while the same area appears as a solid color in the visible spectrum. To do so, they created a three-level, layered, plas­monic system that sandwiches a polymer layer imprin­ted with nanoscale holes, between a gold mirror at the bottom and a gold layer at the top with holes that match the polymer layer.

Images can be imprinted on top of the plas­monic sandwich, and aspects of the holes, such as size and depth, help dictate which infrared band the image can be seen in. Without looking through an in­frared 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.

Varying the pattern charac­teristics allows 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,” Chanda said.

Now, the researchers went a step further and developed a way to erase and display the image in selected infrared bands. They did this by adding a layer of phase change material vanadium dioxide within the plas­monic sandwich that dynami­cally changes the light reflection from the surface from 100 to 0 percent and back as the phase change is trig­gered. “It provides an addi­tional element of dynamic tuna­bility where the coded infor­mation is concealed or revealed to infrared cameras,” Chanda said. (Source: UCF)

Reference: D. Franklin et al.: Covert infrared image encoding through imprinted plasmonic cavities, Light: Sci. & App. 7, 93 (2018); DOI: 10.1038/s41377-018-0095-9

Link: College of Optics and Photonics, University of Central Florida, Orlando, USA

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