We report on time-resolved measurements of the plasma evolution during metal ablation with ultrashort laser pulses in the range from 200 fs to 3.3 ps. The plasma transmission exhibits two distinctive minima. Almost total attenuation is observed a few nanoseconds after the ablation pulse, while a second decrease of the transmission to approximately 50% is observed after about 150 ns. Images taken with a gated ICCD-camera confirm these data and allow determining the expansion velocity of the plasma plume. The attenuation in the first nanoseconds can be attributed to electrons and sublimated mass emitted from the target surface, while attenuation after several 10 ns is due to particles and droplets after a thermal boiling process. The possibility of a normal or an explosive boiling process, also called phase explosion, is discussed. Despite of the physical insight into the ablation process, these data provide valuable information for scaling the speed of ultrashort pulse laser materials processing in a fluence regime of several J/cm2 since they allow estimating the maximum usable pulse repetition rate.
Fundamental results of ablation processes of metals with ultrashort laser pulses in the far threshold fluence regime are shown and discussed. Time-resolved measurements of the plasma transmission exhibit two distinctive minima. The minima occurring within the first nanoseconds can be attributed to electrons and sublimated material emitted from the target surface, whereas the subsequent minimum after several 10 ns is due to particles and droplets after a thermal boiling process.Industrial applications of ultrashort pulsed laser micro machining in the Bosch Group are also shown with the production of exhaust gas sensors and common rail diesel systems. Since 2007, ultrashort laser pulses are used at the BOSCH plant in Bamberg for producing lambda-probes, which are made of a special ceramic layer system and can measure the exhaust gas properties faster and more accurately. This enables further reduction of emissions by optimized combustion control. Since 2009, BOSCH uses ultrashort pulsed lasers for micro-structuring the injector of common rail diesel systems. A drainage groove allows a tight system even at increased pressures up to 2000 bar. Diesel injection is thus even more reliable, powerful and environment-friendly.
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