Micro-tags for art: covert visible and infrared images using gap plasmons in native aluminum oxide

Artwork often needs to be verified for authenticity during transactions. Electronic tags and labels are easily spotted and hence counterfeited. A covert and effective tag needs to be: (1) microscopic, (2) camouflaged into the surroundings. and (3) contain multiple sets of information. Here, we developed aluminum (Al) nanostructures that resonate across the ultraviolet (UV) to infrared (IR) spectra for use in micro-tags with varying colors of similar brightness and containing two sets of information in the visible and IR. Native Al2O3 on Al films was measured to be similar to 4-7 nm thick, enabling resonances to be supported by Al disks with diameters merely similar to 1/6th of the wavelength at the fundamental mode. Through accurate modeling of the nanostructures and high-resolution electron-beam lithography, we designed and fabricated a printed micro-tag on silicon. This micro-tag requires image processing to extract a quick response (QR) code in the visible, and 1.2 mu m IR illumination (or visible light darkficld imaging) to ex tract a covert barcode. The compact and multi-spectral encoding of prints demonstrated here is particularly suited for discreet tagging of art and high-value merchandise. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement