Researchers have developed a metasurface device that can display three types of images depending on the illu­mination light. The three-channel device could be used as an anticounter­feiting measure or offer a new way to securely deliver encrypted information. “Metasurfaces are artificial materials with tiny nano­structures that can be used to manipulate light,” said research team member Qi Dai from Wuhan University in China. “In this work, we exploited both the size and orienta­tion of the nano­structures to design a metasurface with three working modes.”

The researchers also showed that depending on the light used, the metasurface would generate a holographic image or a structural-color nano­printing image with or without polari­zation-dependent watermarks. “Our tiny metasurface could be easily attached to currency, ID cards, credit cards, certificates, watches or rings for anticounter­feiting,” said Dai. “Because this multi-functional meta­surface features twofold safeguards, it could provide a simple but effective approach to fight against counterfeiting.”

Although other metasurface-based anticounter­feiting 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-color 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-color 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-color uses nano­structures with different geometric parameters to produce color by directly influencing the spectrum of transmitted or reflected light.

The unwater­marked structural-color 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 polarizer. 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-color nano­printing could be easily observed using a camera on a smartphone,” said Dai. “The watermarked pattern could encode information needed to provide authenti­fication since it only appears with the help of an optical polarizer. 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 visualized using an optical path consisting of a continuum laser, iris, lens, the sample and an optical screen.

“Our experiments showed that the watermarked structural-color nano­printing had high polari­zation sensi­tivity and created a clear visual with bright color effects,” said Dai. “We also found that the designed metasurface can create a holographic image over a broad wavelength range from 480 to about 650 nanometer.” 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. (Source: OSA)

Reference: N. Zhao et al.: Tri-channel metasurface for watermarked structural-color nanoprinting and holographic imaging, Opt. Exp. 30, 37554 (2022); DOI: 10.1364/OE.472789

Link: Electronic Information School, Wuhan University, Wuhan, China