Turns Out That 3 Into 1 Will Go

For many companies involved in authentication, the technologies they deploy are aimed at preventing cyber and online attacks. But, as this report demonstrates, research into authenticating physical goods still continues apace.

Metasurfaces are artificial materials with tiny nanostructures that can be engineered to manipulate light. Researchers from Wuhan University in China have developed a metasurface device that can display three types of images depending on the illumination light. The three-channel device could be used for authenticating goods or offer a new way to securely deliver encrypted information.

‘In this work, we exploited both the size and orientation of the nanostructures to design a metasurface with three working modes,’ said research team member Qi Dai from Wuhan University The researchers describe the new device in ‘Optics Express ¹’. They also showed that depending on the light used, the metasurface would generate a holographic image or a structural-colour nanoprinting image with or without polarisation- dependent watermarks.

‘Our tiny metasurface could be easily attached to currency, ID cards, credit cards, certificates, watches or rings for anticounterfeiting,’ said Qi Dai. ‘Because this multi-functional metasurface features twofold safeguards, it could provide a simple but effective approach to fight against counterfeiting.’ 

A three-in-one device 

Although other metasurface-based authentication devices have been developed, the hidden information is usually retrieved either on the surface or via a far-field holographic image.

To create a more secure three-channel metasurface, the researchers merged watermarked structural-colour nanoprinting with holographic imaging into a device, which is made of tiny nanobricks arranged on a transparent substrate.

By carefully engineering the sizes and orientations of the nanobricks, the researchers developed a way to create structural-colour images that appear on the surface of the device, as well as a holographic image that appears in the far-field. Instead of relying on inks or dyes, structural colour uses nanostructures with different geometric parameters to produce colour by directly influencing the spectrum of transmitted or reflected light.

The unwatermarked structural colour nanoprinting image can be readily observed under natural light illumination while the same image covered with a watermark pattern can be decoded only with an optical polariser. The holographic image in the third channel can only be viewed under coherent laser light.

Additional security

‘When our metasurface is employed for anticounterfeiting, the unwatermarked structural-colour nanoprinting could be easily observed using a camera on a smartphone,’ said Qi Dai. ‘The watermarked pattern could encode information needed to provide authentication since it only appears with the help of an optical polariser. The holographic image, which might be reconstructed with a laser pointer, could be used as a second layer of security.’

To demonstrate the new metasurface device, the researchers fabricated a sample using standard electron beam lithography. The watermarked and unwatermarked nanoprinting images were observed using an optical microscope, while the holographic images were visualised using an optical path consisting of a continuous laser, lens, the sample and an optical screen.

‘Our experiments showed that the watermarked structural-colour nanoprinting had high polarisation sensitivity and created a clear visual with bright colour effects,’ said Dai. ‘We also found that the designed metasurface can create a holographic image over a broad wavelength range from 480 nm to about 650 nm.’

The researchers plan to combine their new multi-functional metasurfaces with other materials such as liquid crystal and black phosphorus to achieve dynamic and more versatile control of light. They also want to explore how they can mass produce the new metasurface material.

1 - https://opg.optica.org/oe/fulltext.cfm?uri=oe-30-21-37554&id=507366.