The use of metal nanostructures to produce colour has recently attracted a great deal of interest. This interest is motivated by colours that can last a long time and that can be rendered down to the diffraction limit, and by processes that avoid the use of inks, paints or pigments for environmental, health or other reasons. The central idea consists of forming metal nanostructures which exhibit plasmon resonances in the visible such that the spectrum of reflected light renders a desired colour. We describe a single-step laser-writing process that produces a full palette of colours on bulk metal objects. The colours are rendered through spectral subtraction of incident white light. Surface plasmons on networks of metal nanoparticles created by laser ablation play a central role in the colour rendition. The plasmonic nature of the colours are studied via large-scale finite-difference time-domain simulations based on the statistical analysis of the nanoparticle distribution. The process is demonstrated on Ag, Au, Cu and Al surfaces, and on minted Ag coins targeting the collectibles market. We also discuss the use of these coloured surfaces in plasmonic assisted photochemistry and their passivation for day-to-day use. Reactions on silver that are normally driven by UV light exposure are demonstrated to occur in the visible spectrum.
We show the angle-independent coloring of metals in air arising from nanoparticle distributions on metal surfaces created via picosecond laser processing. Each of the colors is linked to a unique total accumulated fluence, rendering the process compatible with industry. We report the coating of the colored metal surfaces using atomic layer deposition which is shown to preserve colors and provide mechanical and chemical protection Laser bursts are composed of closely time-spaced pulses separated by 12.8 ns. The coloring of silver using burst versus non-burst is shown to increase the Chroma, or color saturation, by 50% and broaden the color Lightness range by up to 60%. The increase in Chroma and Lightness are accompanied by the creation of 3 kinds of different laser-induced periodic surface structures (LIPSS). One of these structures is measured to be 10 times the wavelength of light and are not yet explained by conventional theories. Two temperature model simulations of laser bursts interacting with the metal surface show a significant increase in the electron-phonon coupling responsible for the well-defined LIPSS observed on the surface of silver. Finite-difference time-domain simulations of nanoparticles distributed on the high-spatial frequency LIPSS (HSFL) explain the increase in color saturation (i.e. Chroma of the colors) by the enhanced absorption and enriched plasmon resonances.