Passive plasmonic metasurfaces enable image multiplexing by displaying different images when altering the conditions of observation. Under white light, three-image multiplexing with polarization selective switching has been recently demonstrated using femtosecond-laser-processed random plasmonic metasurfaces. Here, the implementation of image multiplexing is extended, thanks to a color search algorithm, to various observation modes compatible with naked-eye observation under incoherent white light and to four-image multiplexing under polarized light. The laser-processed random plasmonic metasurfaces enabling image multiplexing exhibit self-organized patterns that can diffract light or induce dichroism through hybridization between the localized surface plasmon resonance of metallic nanoparticles and a lattice resonance. Improved spatial resolution makes the image quality compatible with commercial use in secured documents, as well as the processing time and cost thanks to the use of a nanosecond laser. This high speed and flexible laser process, based on energy efficient nanoparticle reshaping and self-organization, produces centimeter-scale customized tamper-proof images at low cost, which can serve as overt security features.

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Laser can be an effective tool to modify materials at the nanoscale in order to achieve desired optical properties. When dealing with metal-dielectric nanocomposite thin films, different mechanisms can be triggered by laser on large areas to control the statistical properties of these materials. Nanoparticles can be reshaped, resized and ordered according to self-organization mechanisms that set over micrometer wide areas. The dielectric crystal phase and film thickness can be changed upon laser-induced temperature rise. These mechanisms lead to changes in the optical properties of the films. Here, we investigate the structural changes that a Ag:TiO2 nanocomposite thin film undergoes under nanosecond laser scanning and their resulting optical properties. We especially focus on the color properties in different modes of observation such as reflection and diffraction. The colors originate from combination of absorption by the localized surface plasmon resonance of metallic nanoparticles, diffraction by the nanoparticles assemblies and interference between the incident, reflected and guided waves, the latter being excited by scattering on the nanoparticles. The morphological characterizations unveil the role of nanoparticle size, density and arrangement on the transition from a diffractive to a dichroic behavior. A full color image is also drawn to demonstrate the potential of the technique in industrial applications ranging from design, coloration to information storage and data security.