New metasurface shows different images depending on light source

The three-channel device is an advance with potential applications in anticounterfeiting, or to offer a new way of securely delivering encrypted information.

“Metasurfaces are artificial materials with tiny nanostructures that can be used to manipulate light,” said research team member Qi Dai. “In this work, we exploited both the size and orientation of the nanostructures to design a metasurface with three working modes.”

The team’s work is described in Optics Express. They also showed that 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,” Dai said in a statement. “Because this multi-functional metasurface features twofold safeguards, it could provide a simple but effective approach to fight against counterfeiting.”

Metasurface-based anticounterfeiting devices have been developed, but 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 are said to have 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 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.

“When our metasurface is employed for anticounterfeiting, the unwatermarked structural-colour nanoprinting could be easily observed using a camera on a smartphone,” said 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 at Wuhan University 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 continuum laser, iris, 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 480nm to about 650nm.”

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 the metasurfaces could be used for optical computing and biomedical sensing and are working on ways to mass produce the new material.