MetasurfaceFuture of display: security-enhanced crypto-display using dual-mode metasurfaces
An introductory course about light will naturally impart findings about the continuous range of wavelengths or frequencies, and the results of a light wave striking an object. Whether the light is absorbed, reflected, or transmitted will depend on many factors regarding the frequencies of light and the optical properties of the object itself. This means that if the optical properties can be selectively controlled—for example, through nanofabrication—it would be possible to create objects with desired optical properties not found in nature. Similar in reasoning to metamaterials, metasurfaces are ultrathin artificial materials made of meta-atoms that dictate its optical properties.
Existing research of metasurfaces primarily focused on either reflective displays—displays that reflect ambient (white) light rather than emitting it—or holograms produced through transmission. However, collaborative research conducted by Professor Junsuk Rho from the Departments of Mechanical Engineering and Department of Chemical Engineering at Pohang University of Science and Technology has successfully demonstrated a ‘crypto-display’ that can concurrently show both meta-holograms under laser illumination and reflective images under white light. This achievement was published in the world renowned ACS Nano.
The research team fabricated a dual-mode metasurface with parallel double-nanorods that can control both the phase and spectral responses at the same time. The orientation of the ‘nanoantennas’ determines the phase distribution under a laser (hologram), while the size determines the reflection spectrum under white light. In other words, the team created a metasurface that independently and simultaneously displays different images depending on the source of illumination.
As the encrypted holographic image is invisible under white light, the metasurface’s potential for applications in the security industry is substantial. As the team was able to verify that both modes are independently controllable, it will be possible to create crypto-displays as effective security devices for everything from official documents and currencies to the latest high-end designer goods. Professor Rho expressed his enthusiasm of the achievement and remarked that, “Since the only way to display the holographic image is with a laser, there is no way of knowing if there is even an encrypted image, which consequently will make for a highly effective security device.”
This work was financially supported from the LGD-SNU Incubation program funded by LG Display and the National Research Foundation grants funded by the Ministry of Science and, ICT and future Planning of the Korean government.