Electric signal diagnostics of plasma dynamics at early stage of laser ablation

Abstract

Plasma dynamics at early stage of laser ablation is investigated by a tiny metal probe. There are two negative peaks with different distributions in an electric signal. The first peak has a duration about 50 ns and delays 30 ns with respect to laser irradiation. The waveform and peak maximum position do not change with probe distance. It is attributed to plasma-induced electric field at the early stage. The second peak appears about 250 ns later with a profile duration higher than 1 μs. The peak maximum position moves forward as probe distance reduces due to earlier charge particle arrival of the probe. Experimental results and theoretical modeling show that electric signal of plasma-induced electric field is resulted from an electric dipole with negative charge in front. It is constructed by electrons and positive ions emitted at early stage of laser ablation. Dependence of electric signal profile on probe distance, laser fluence and pulse number is studied. The electric signal detection can be used to monitor laser removal of metallic oxide layer in real time. Influence of substrate bias on the signal waveform is also analyzed. It may be applied to modify plasma dynamics and laser ablation.