Invisible Ink for Hidden Images

A new technique can be used to hide multiple images in a printed array of rods with varying conductivities. Depending on the polarization of the terahertz radiation, different concealed images appear. (Source: A. Nahata, University of Utah)

A new technique can be used to hide multiple images in a printed array of rods with varying conductivities. Depending on the polarization of the terahertz radiation, different concealed images appear. (Source: A. Nahata, University of Utah)

Researchers have developed a way to use commercial inkjet printers and readily available ink to print hidden images that are only visible when illu­minated with appro­priately polarized waves in the terahertz region of the electro­magnetic spectrum. The inex­pensive method could be used as a type of invisible ink to hide infor­mation in otherwise normal-looking images, making it possible to distin­guish between authentic and counter­feit items, for example.

“We used silver and carbon ink to print an image consisting of small rods that are about a millimeter long and a couple of hundred microns wide,” said Ajay Nahata from the University of Utah, leader of the research team. “We found that changing the fraction of silver and carbon in each rod changes the conduc­tivity in each rod just slightly, but visually, you can’t see this modification. Passing terahertz radiation at the correct frequency and polari­zation through the array allows extraction of information encoded into the conduc­tivity.”

The researchers demonstrated their new method to hide image information in an array of printed rods that all look nearly identical. They used the technique to conceal both grayscale and 64-color QR codes, and even embedded two QR codes into a single image, with each code viewable using a different polarization. To the naked eye the images look like an array of identical looking lines, but when viewed with terahertz radiation, the embedded QR code image becomes apparent. “Our very easy-to-use method can print complex patterns of rods with varying conduc­tivity,” said Nahata. “This cannot easily be done even using a multimillion dollar nanofabri­cation facility. An added benefit to our technique is that it can performed very inex­pensively.”

The new technique allows printing of different shapes that form a type of metamaterial. Although there is a great deal of interest in mani­pulating meta­materials to better control the propa­gation of light, most techniques require expensive litho­graphy equipment found in nanofabri­cation facilities to pattern the material in a way that produces desired properties. Nahata and his colleagues pre­viously developed a simple method to use an off-the-shelf inkjet printer to apply inks made with silver and carbon, which can be purchased from specialty stores online. They wanted to see if their ink-jet printing technique could create various conduc­tivities, a parameter that is typically difficult to modify because it requires changing the type of metal applied at each spatial location. To do this using standard litho­graphy would be time consuming and expensive because each metal would have to be applied in a separate process.

“As we were printing these rods we saw that, in many cases, we couldn’t visually tell the difference between different conduc­tivities,” said Nahata. “That led to the idea of using this to encode an image without the need for standard encryption approaches.” To see if they could use the method to encode infor­mation, the researchers printed three types of QR codes, each 72 by 72 pixels. For one QR code they used arrays of rods to create nine different conduc­tivities, each coding for one gray level. When they imaged this QR code with terahertz illu­mination, only 2.7 percent of the rods gave values that were different from what was designed. The researchers also used rods printed in a cross formation to create two separate QR codes that could each be read with a different polari­zation of terahertz radiation.

The team then created a color QR code by using non-overlapping rods of three different lengths to create each pixel. Each pixel in the image contained the same pattern of rods but varied in conduc­tivity. By arranging the rods in a way that minimized errors, the researchers created three overlapping QR codes corres­ponding to RGB color channels. Because each pixel contained four different conduc­tivities that could each correspond to a color, a total of 64 colors was observed in the final image. The researchers said they could likely achieve even more than 64 colors with improve­ments in the printing process.

“We have created the capa­bility to fabricate structures that can have adjacent cells, or pixels, with very different conduc­tivities and shown that the conductivity can be read with high fidelity,” said Nahata. “That means that when we print a QR code, we see the QR code and not any blurring or bleeding of colors.” With the very inex­pensive printers used in the paper, the technique can produce images with a resolution of about 100 microns. With somewhat more expensive but still commer­cially available printers, 20-micron resolution should be achievable. Although the researchers used QR codes that are relatively simple and small, the technique could be used to embed information into more complex and detailed images using a larger canvas. (Source: OSA)

Reference: A. Chanana et al.: Hiding Multi-level Multi-color Images in Terahertz Metasurfaces, Optica 3, 1466 (2016); DOI: 10.1364/optica.3.001466

Link: Dept. of Electrical and Computer Engineering, University of Utah, Salt Lake City, USA

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