Femtosecond laser ablation: Visualization of the aerosol formation process by light scattering and shadowgraphic imaging

The shockwave propagation and aerosol formation during femtosecond laser ablation (fs-LA) of dielectric materials (Li2B4O7, Y:ZrO2) in ambient air were monitored using shadowgraphy and light scattering. Three independent shockwave fronts were observed originating from (i) the instantaneous compression of ambient gas during the initial stage of fs-LA, (ii) a secondary compression caused by material ejection, and (iii) an air breakdown well above the target surface. In addition, particle size distributions were found to be multimodal implying the co-existence of condensational growth and supplementary particle production pathways such as phase explosion or critical point phase separation (CPPS). As a consequence, fs-LA of Li2B4O7 resulted in the formation of primary aggregates reaching diameters of > 10 μm. In contrast, aggregates formed during fs-LA of Y:ZrO2 covered a size range < 1 μm. Our data, furthermore, indicate the existence of a breakdown channel in the ambient atmosphere being capable to carry plasmatic, i.e. non-condensed matter beyond the primary shockwave barrier which may occasionally causes a spatial separation of material released. Assuming the Taylor-Sedov model of explosion to be valid the over-all energy dissipated in acoustic transients was found to exceed values of 50%.