Metasurface Helps to Conquer Counterfeiting

Researchers at Pohang University of Science and Technology (POSTECH) have developed an anticounterfeiting and tampering prevention system using ultraviolet (UV) and visible light. The technology uses metasurfaces — it in fact resolves a long-standing issue of the UV light operation of metasurfaces — and is expected to have applications in various industrial sectors, according to the researchers.

Led by Junsuk Rho, a professor in the departments of Mechanical and Chemical Engineering, and Ph.D. candidate Joohoon Kim, the team developed an optical encryption platform that works simultaneously in the visible and UV regions.

Illustration of the dual-band vectorial metahologram that works in ultraviolet and visible light. POSTECH researchers developed an anticounterfeiting system using two light bands. The technology uses stacked metasurfaces composed of subwavelength structures. Courtesy of POSTECH.

For the metasurface in the design to work, it must be composed of subwavelength structures — smaller than the wavelength of light. However, the wavelength of UV light is very short, making it difficult to fabricate a suitable structure. Additionally, silicon — which is frequently used to create metausrfaces — has a propensity to absorb UV light.

To overcome these issues, the team adjusted the physical properties of silicon nitride to reduce its tendency to absorb UV light. Using this material, the team fabricated a metahologram in which an image clearly appears when it is irradiated with UV light. Using the electron beam lithography overlay method, the team combined the two metaholograms that work in the UV and visible regions to create an anticounterfeiting device that displays a unique product number.

When a UV or visible laser is irradiated on the device, images with disparate polarization states are displayed, respectively. The hologram that appears when a visible light laser is irradiated acts as a key, and when this key information is input into the ultraviolet polarizer and irradiated with UV light, certain numbers disappear. These numbers are unique.

The system is difficult to decrypt because it uses the invisible ultraviolet light and reduces the chances of exposing the password or code that can confirm forgery or falsification. In addition, by stacking two metasurfaces, the number of images and information that can be stored is markedly increased.

“We were able to create a higher performing optical encryption system utilizing the invisible ultraviolet characteristics,” Rho said. “This study will serve as the basis for opening up the metasurface research to the ultraviolet regime, from being confined to the long wavelength region.”

According to Rho, he and the team are reviewing the technology with relevant organizations for use in future security applications such as paper bills, passports, and more.

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.1c10100).