Imprinted micro-tags hidden in a painting, card, or banknote and showing different motifs when inspected with visible or near-infrared light are appealing for anticounterfeiting applications. This is because the information carried by them cannot be read with the naked eye, and because replicating them requires state-of-the-art fabrication tools. In this context, Ray Jia Hong Ng and coworkers report the fabrication of micro-tags where each few-micrometer pixel consists of an array of aluminium nanostructures on a surface-oxidized aluminium film. The few-nanometer native aluminium oxide layer separating the nanostructures from the film enables gap plasmon resonances. The wavelength of such resonances can be tuned in the visible and near-infrared by properly choosing the nanostructure size and array pitch. This enables achieving pixels with the desired visible color and near-infrared brightness, with a low visible/near-infrared crosstalk. By using four types of pixels with two different visible colors and two different near-infrared brightnesses, the authors designed a 240-micrometer × 240-micrometer micro-tag showing a quick-response code when illuminated with visible light and a barcode when illuminated with near-infrared light.
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