Today's diffractive laminates such as Surys' DID optical security markers found across millions of banknotes and ID documents are based on nanostructured resonant waveguide gratings (RWGs). So far, the markers' colour changes (when seen at different angles) can't be reproduced by counterfeiters, but being visible, fine imitations of these visual elements will trick the unwary consumer.
Stepping up the game in mass-producible anti-counterfeit measures, Swiss researchers from CSEM SA and EPFL Lausanne have come up with a novel way to use RWGs, pairing them to yield new optical properties. Moving away from specular reflection that gives a visible image under daylight, from any angle, the researchers coupled RWGs of different periods in such a way that the direction of the out-coupled light differs from the direction of the incoming light and can be tuned so as to be invisible except under a narrow viewpoint under a strong white light.
Publishing their findings in ACS Photonics under the titled "Color-Selective and Versatile Light Steering with up-scalable Subwavelength Planar Optics", the researchers put their theory in practice, crafting patterns of nanostructures that remain essentially transparent (invisible to the eye) but that can be revealed on a smartphone's screen when flashed by the camera's white LED.