Laser ablation plume dynamics in nanoparticle synthesis

The dynamics of the plume ejected from the surface of solid targets (YSZ, Nd:YAG and graphite) by a CO₂ laser pulse with a duration of ~500 μs (at the 0.03 level), energy of 1.0 — 1.3 J and peak power of 6 — 7 kW have been studied using high-speed photography of the plume luminescence and shadow. The targets were used to produce nanopowders by laser evaporation. About 200 μs after termination of the pulse, shadowgraph images of the plumes above the YSZ and Nd:YAG targets showed dark straight tracks produced by large particles. The formation of large (~10 μm) particles is tentatively attributed to cracking of the solidified melt at the bottom of the ablation crater. This is supported by the fact that no large particles are ejected from graphite, which sublimes without melting. Further support to this hypothesis is provided by numerical 3D modelling of melt cooling in craters produced by laser pulses of different shapes.