Metasurface-driven optically variable devices for advanced anti-counterfeiting and security

With the increasing threat of sophisticated counterfeits, what is a security device that can combat against them? It has been known that a technology that has never existed before can be used in security devices, but difficult to compare their strengths since no standards were in place to measure their performance. To this, a Korean research team that leads the world on optical anti-counterfeiting technology using metasurface – a completely novel material that freely controls light – has presented the standard.

A POSTECH research team led by Professor Junsuk Rho (Department of Mechanical Engineering and Chemical Engineering), Ph.D. candidates Chunghwan Jung, Jaehyuck Jang (Department of Chemical Engineering), Gyeongtae Kim, and Minsu Jeong (Department of Mechanical Engineering) have summarized and published various optically variable devices (OVDs) according to their information capacity and security level. An OVD, mainly used for anti-counterfeiting, is a tinfoil-like sticker that changes shape and color depending on the angle of incidence.

The findings from this study were published as the front cover paper and research highlight in the first November issue of Chemical Reviews (impact factor of 60.622), the top journal in the field of nanotechnology. Chemical Reviews, published by the American Chemical Society, is an academic journal that presents research trends and prospects by leading scientists in the field and is recognized as having as much influence as Nature and Science.

The rainbow hologram sticker is a typical OVD that changes color depending on the angle and is widely used to identify counterfeit bills and for corporate security labels. However, the risk of counterfeiting remains since it is simple to make and can only store little information.

Various devices, including color conversion inks that change colors when moved and fluorescent inks that emit light when irradiated with ultraviolet light, have been developed. But their inability to encrypt complex information has hindered their wide applicability.

To this, Professor Rho’s team focused on the metasurface which controls light by arranging nanostructures – that are one-thousandth the thickness of a human hair strand – in a desired shape.

The metasurface-based optical device can store more than 100 times more information than the conventional rainbow hologram sticker and can selectively display a desired image according to the angle, color, and polarization.

Encrypted information can never be decrypted without the correct optical decryption. By hierarchically encrypting information, it is possible to separately store images that can be seen without special equipment for the untrained eye and images that can only be viewed with specific analysis equipment by security professionals

This device is easy to store thanks to its very small size and can be mass-produced at low cost. The metasurface technology has infinite potential to expand into new areas of application including optical forgery and counterfeit devices as well as ultra-thin lenses and lidars.

“Security labels made using metasurfaces are difficult to replicate and are highly secure since they cannot be decrypted without specific settings,” explained Professor Junsuk Rho. “This study is significant in that it systematically summarized the core principles and applications of metasurface-based security devices led by a Korean research team.”

This study was conducted with the grant from the Samsung Research Funding & Incubation Center for Future Technology.