Schematic illustration of the tri-functional metasurface integrating a color image, hologram, and luminescence image by controlling the amplitude, phase, and luminescence of light.
Schematic illustration of the tri-functional metasurface integrating a color image, hologram, and luminescence image by controlling the amplitude, phase, and luminescence of light.

Nanoparticles’ ability to interact with light in unique ways is the ideal basis for optical anti-counterfeiting tags to provide security against fraudulent copying. Tags need to be flat, compact, robust, easy to read from any angle, work in reflection (as most items are not transparent) and combine multiple optical effects for added security. Typically, multiple optically variable devices (OVDs) that manipulate light in different ways must be used. Now researchers have designed a metasurface that simultaneously controls three different optical responses – phase, amplitude, and luminescence – to combine the effects of multiple OVDs in a single device [Daqiqeh Rezaei et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.11.010].

“We fabricated a gap-plasmon based metasurface comprising a thin dielectric sandwiched between a metallic mirror and metallic nanopatterns to control three properties [of light],” explains first author of the study, Soroosh Daqiqeh Rezaei.

Together with colleagues at Singapore University of Technology and Design, the Institute of Materials Research and Engineering at A*STAR, the National University of Singapore, Macquarie University in Australia, and Ferdowsi University of Mashhad in Iran, the researchers used electron beam lithography to create a geometric array of anisotropic cross-shaped nanostructures. The metasurface controls the amplitude of light to produce a color image and the phase to create a hologram. To add another layer of security, the team incorporated a layer of upconversion nanoparticles (NaYF4:Yb/Er), which absorb two or more photons and emit a shorter wavelength photon, in the dielectric gap of the metasurface. Under near-infrared illumination, the upconversion nanoparticles produce a luminescent image that serves as a unique optical ‘fingerprint’, since the particles are not distributed identically in each device. The inclusion of upconversion nanoparticles adds a physically unclonable function (PUF) to OVDs.

“Counterfeiting is a growing issue that causes many problems from monetary loss for manufacturers to lives taken by counterfeit medicines,” points out Daqiqeh Rezaei. “With significant advancement of metasurfaces in recent years, we are able to integrate several optical security features into a single optical metasurface, making it extremely difficult to counterfeit.”

The OVD combines overt, covert, and forensic security features in the form of a color image (or microprint), which can be viewed in white light, a hologram, and a luminescence image, which can only be seen under red and near-infrared laser illumination. The researchers believe the new metasurface will enable more effective anticounterfeiting tags that combine multiple optical features in a single device.