Self-Scattering of Gold Nano-particles under Femtosecond Laser Ablation Conditions
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This thesis covers the interaction of a femtosecond pulsed laser (about 150 fs at FWHM) with spherical gold nano-particles (100 nm in diameter) in the abla- tion regime. The study is separated into an experimental part and a simulation part. For the experimental part, gold nano-particles are isolated by the drop cast- ing technique on a substrate. Scanning electron microscope data is used as a mapping system for our diffraction limited optical ablation setup. The self- scattering of the nano-particles as a function of fluence is investigated. We found the ablation threshold to be about 0.03 J/cm^2 and reached a maximum fluence of about 0.2 J/cm^2 . The self-scattering results from ablated nano-particles are inconclusive due to a high background contribution from the substrate. The simulation part covers the light scattering properties of gold nano- particles sized around the Rayleigh limit. Due to high electron temperatures reached (2/3 of the Fermi temperature for gold) in femtosecond laser-gold inter- action, we incorporate ab initio data by Lin and Zhigilei on the electron heat capacity into our model. We also include experimental data by Fourment et al. on the number of free conduction electrons per atom. Results show a decrease in reflectivity during irradiation at a fluence of 0.8 J/cm^2 . The model allows to go up to a fluence of 10 J/cm^2 , where we observe a decrease in reflectivity of about 10 %.