"Invisible" images created with off-the-shelf inkjet printers
Using inks created from artificial materials that exhibit properties not found in nature (aka metamaterials) and commercially-available inkjet printers, researchers at the University of Utah have developed a way to create images only visible using polarized sub-millimeter electromagnetic waves. The researchers believe their new technique could be used to secretly conceal information in images that look normal to the naked eye, which could be useful in anti-counterfeiting or product authentication.
Building on previous research by the University of Utah team, where they developed a relatively uncomplicated way to use a retail inkjet printer to print inks created with silver and carbon available from online specialty retailers, the researchers experimented with methods to print images with varying conductivities. This is ordinarily difficult to achieve due to the fact that different conductivities usually requires different types of metal at different points in the image. Using standard multi-layer printing, this would be a long and expensive process as each metal layer would need to be applied in a many-step procedure.
"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 Professor Ajay Nahata of the University of Utah. "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."
In verifying that their process was able to encode data in the printed symbols, the researchers produced three 72 by 72 pixel QR codes of different types, all with varying conductivities corresponding to different grayscales and colors. Individual pixels in the printed image consisted of the same pattern of rods, but all of these were different in conductivity, so that the three images each had overlapping QR codes equating to each of one RGB (red, green, blue) color channel.
As each pixel contains four conductivities that could each correspond to a different color, up to 64 different colors are visible in each image. However, the researchers say it is likely more colors would be possible with refinements to the printing process. Each of the QR codes could then be read using a different polarization of terahertz radiation to illuminate them.
"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," says Nahata. "That means that when we print a QR code, we see the QR code and not any blurring or bleeding of colors."
The current printing resolution achievable with the cheap (less than $US60) printers used is around 100 microns. According to the researchers, higher-quality, off-the-shelf printers could get down to about a 20-micron resolution. The researchers also believe that even though their QR codes are somewhat simple in the information they carry, the process could be employed to encode information within much more detailed images on larger scales.
Though terahertz electromagnetic radiation is used to reveal the the information embedded in the images due to the compatibility of the wavelength with images produced by ink-jet printers, the researchers are aiming to develop ways to use the technique at visible wavelengths. This will require bespoke new printers to be developed, however, that can print smaller rods to create images with greater resolutions.
The research team is also investigating the development of other methods that could further secure the embedded information by developing inks that first need to be exposed to heat or light of a particular frequency before the encoded data was made visible using sub-millimeter frequency radiation.
The results of this research were recently published in the journal Optica.