Surface Coloring\nand Plasmonic Information Encryption\nat 50000 dpi Enabled by Direct Femtosecond Laser Printing

Femtosecond (fs) laser pulses drive matter into a highly\nnonequilibrium\nstate, allowing precise sculpturing of irradiated surface sites with\nsophisticated nanomorphologies. Here, we used fs-laser patterning\nto create diverse plasmonic morphologies on the top Au layer of the\nmetal–insulator–metal sandwich. Mutual action of laser-driven\nthermomechanical effects and ultrafast solid-to-liquid transition\nallows control of the morphology resulting in pronounced surface\nreflectivity modulation, i.e., in a structural color effect. This\nenables template-free high-resolution color printing at a superior\nlateral resolution up to 50000 dots per inch and facile tunability\nof the color tone and saturation. Moreover, precise control over the\norientation of the printed nanostructures within subwavelength lattices\nallows modulation of their local plasmonic response encrypting the\noptical information within the colorful images. The hidden information\ncan be unveiled using a facile cross-polarized optical visualization\nscheme, rendering the proposed method with extra modalities combining\nhigh resolution information encryption, coloring, and security labeling.