Plasma diagnostics at early stage of laser ablation

Plasma diagnostics by optical and electric signal detection at the early stage of laser ablation have been investigated. An ultrafast phototube was applied to capture the optical signal. There are two peaks in the optical signal, with the first peak attributed to laser scattering and the second one to plasma generation. As the laser fluence increases, the second peak appears earlier to overlap with the first one. The dependence of the peak distributions on the laser fluence was analyzed. The time interval between the plasma starting and the end of the laser pulse is proposed as a quantitative parameter for characterizing the laser-plasma interaction. A tiny metal probe was used to record the electrical signal, with two negative peaks detected. The first peak has a duration of about 50 ns and its maximum amplitude position does not change with probe distance. It is attributed to a plasma-induced electric field. The field results from an electric dipole due to charge separation in the early stages of laser ablation. Variation of the first peak profile with probe distance and substrate bias was also studied.