Pulsed laser ablation studied using digital holography

Sammanfattning: Pulsed digital holographic interferometry has been used to study the plume and the shock wave generated in the laser ablation process on different targets under atmospheric air pressure. A pulsed Nd-YAG laser system (pulse duration 12 ns) has been used both for ablating the material (wavelength 1064 nm) and for measurement (wavelength 532 nm). Digital holograms were recorded for different time delays using collimated laser light passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps. The Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. Intensity maps have been calculated from the recorded digital holograms and used to calculate the attenuation of the probing laser beam by the ablated plume. Qualitative and quantitative information have been extracted from both the phase map and the intensity map to help describing the laser ablation process. Also 3D information about the induced plume has been obtained by numerical reconstruction of the digital holograms at different planes along the plume. The amount of released energy due to laser impact on a PCBM target has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and the target has been calculated and it seems to be constant around 80 %. The 3D refractive index fields have been used to calculate the shock wave front density and the electron number density distribution within the induced plasma. The electron number densities are found to be in the order of 1018 cm-3 and decay at a rate of 3x1015 electrons/cm3ns. The effect of the laser spot diameter on the shock wave generated in the ablation process of a Zn target has been studied. The induced shock wave has an ellipsoidal shape that approaches a sphere for decreasing spot diameter. A model was developed that approaches the density distribution that facilitates the derivation of the particle velocity field. The method provides quantitative results that are discussed; in particular a comparison with the point explosion theory. The effect of the physical properties of the target on the laser ablation process has been studied. The comparison of the laser ablation of Zn and Ti shows that different laser ablation mechanisms are observed for the same laser settings and surrounding gas. At a laser fluence of 5 J/cm2, phase explosion appears to be the ablation mechanism in case of Zn, while for Ti normal vaporisation seems to be the dominant mechanism. The results show that pulsed digital holographic interferometry is a promising technique to give a physical picture and increase the understanding of the laser ablation process in a time resolved manner.

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