Generation of long-living charged nanoparticles at ablation in air and their role in pulsed microdrilling

The ablation of steel in air by short laser pulses was shown to form a long-living cloud of electrically charged submicron particles. These particles, being resident in the atmosphere of deep laser-produced channels within tens of seconds and carrying an electric charge during portions of a second, are able to initiate low-threshold gas breakdown resulting in the significant screening of the following pulses with a duration of ～100 ps. The clouds contained mostly positively charged particles shaped as ideal spheres. The statistics of their diameters nearly followed the Poisson law with the peak at 400 nm. The total volume of the charged particles was nearly equal to the volume of the ablatively removed material. A new approach was proposed to eliminate the screening, which implied the use of an external electric field. This enabled the enhancement of drilling rates by up to 50 times. The electric charge, mobility, and discharge rates of the particles were measured using a conductivity technique.