An optical encryption device that prevents forgery via a double-lock mechanism using visible and ultraviolet (UV) lights has been developed. The technology is driven by metasurfaces that freely manipulate the properties of light and has overcome the challenge of using metasurfaces within the UV regions.
A research team led by Professor Junsuk Rho (ME and CE) and Kim Joo-hoon (ME Ph.D. Candidate, advisor Prof. Junsuk Rho) has developed an encryption device system that operates simultaneously in both visible and UV regions and published it in ACS Nano, an international journal under American Chemical Society. Enhanced forgery prevention in documents is expected using this system.
To make use of a metasurface, its subwavelength-structure must be smaller than the wavelength of electromagnetic rays. Therefore, it was hard to create a structure suitable for UV rays due to its short wavelength. Additionally, substances such as silicon, which are readily used on metasurfaces, were limited by their high absorption of UV rays.
The research team handled this issue by controlling the physical properties of silicon nitride to reduce its high absorptive property to UV light. Using this material, the team created a metahologram that presents a clear image when a UV ray is projected. Then, using an electron beam lithography overlay method, the team combined two metaholograms that operate in UV and visible regions to create a forgery-preventing encryption device that displays a unique product number.
When UV or visible rays are projected on the device, hologram images with different polarization states appear. The image that appears when visible light is projected serves as the key to the lock. When this “key” information is inputted into the UV polarizer, certain numbers disappear. These numbers become the unique product number.
This encryption system is difficult to decipher as it uses invisible UV rays and hence reduces the likelihood of the unique product number or password exposure. Also, by stacking two metasurfaces, the number of images and information that can be stored increases greatly.
Prof. Rho said, “We have developed a highly enhanced optical encryption system using the invisible characteristics of UV rays.” He continued, “This research will serve as a basis for expanding the metasurface research, which was limited to wavelengths longer than that of visible light, to the UV light region.” He also added, “The concept is currently in review with related organizations for future security technology.”
Meanwhile, the research was funded by the Samsung Research Funding & Incubation Center for Future Technology.