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.
Understanding the time-dependent states of plasmonic metasurfaces under intense laser irradiation is crucial to optimize the laser processing parameters and reach specific optical properties. In this work, the physico-chemical mechanisms of laser-induced shape transformation of Ag nanoparticle ensemble embedded in thin TiO2 layers is investigated in details. The combination of high-repetition-rate pump-probe microscopy in transmission configuration with ex situ scanning electron microscopy reveals the variety of mechanisms that contribute to the reshaping of near-coalescence Ag nanoparticles with broad size and complex shape distributions into an ensemble of spherical particles. The description of the mechanisms on multiple time scales from the ultrafast electron dynamics to the much slower thermal processes triggered by the accumulation of 800 pulses at high repetition rate reveals different pulse number regimes. Owing to the high-repetition-rate data acquisition, the effect of the heat accumulation on the nanoparticle reshaping kinetics is demonstrated. The observed structural changes are discussed in the scope of the in situ transmission spectra. The presented results are intended to help improving applications of such laser-processed plasmonic metasurfaces in security printing, data storage and digital fabrication of nanoscale electronics.
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