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Change of optical properties of silver surface due to laser structuring

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One of the main goals of jewelry is to give the product an aesthetic and artistic look. This can be achieved by changing its color. The promising methods of precious metal coloration is a one-step laser method of forming nanostructures with plasmonic properties. However, the lack of understanding of the mechanism of formation of color coatings remains an unresolved problem today. Surface structures are usually considered as a set of individual spherical nanoparticles. But to fully understand the physicochemical processes taking place, it is necessary to consider nanoparticles in aggregate as agglomerates of these nanoparticles on the surface. The silver samples of 99.99 % purity were selected for the study. Laser exposure was carried out in air using a system based on an ytterbium fiber laser with nanosecond pulse duration. The silver surface was processed by line-by-line scanning along one and two axes with a focused laser beam with the diameter d0 = 50 μm. Optical and scanning electron microscopy were used to characterize the silver surface before and after laser treatment. In this work, the effect of some laser exposure parameters, such as laser pulse energy and pulse repetition rate, on the optical properties of silver surface were investigated. The focus of this work is on generated laser-modified surface nanostructures and the character of their change when going from single axis scanning to line scanning. It is shown that surface topography changes are also observed in the region outside the immediate treatment zone. The uneven distribution of nanostructural elements on the surface of the treated area is registered, which causes the irregularity of the observed surface color at the microlevel. Based on the analysis of the obtained data, a hypothesis of nanostructure formation is proposed. Under laser exposure, individual spherical-shaped nanoparticles are initially formed on the silver surface. Then with increasing temperature their concentration increases significantly. This leads to their adhesion and formation of irregularly shaped clusters of agglomerated nanoparticles. The obtained new data on the process of formation of surface nanostructures allow us to expand the understanding of the ongoing processes, as well as to approach the integration of the method of direct laser coloring of silver in the jewelry industry.

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