Determination of plasma ignition threshold fluence during femtosecond single-shot laser ablation on metallic samples detected by optical emission spectroscopy

Abstract

The minimum laser fluence needed for plasma ignition on a sample during single-shot femtosecond laser ablation has been determined for a set of metals and alloys. Threshold fluence values ranged between 4 and 9 J cm⁻² for ablation craters with a diameter of 40 μm. The craters exhibited similar size and shape regardless of the sample type, energy range (for energies per pulse <400 μJ) and accumulated energy dosage (<500 laser shots on the same sample position). The results obtained for the metals (Pb, Ag, Zn, Sn, Cr, Cu, W, Ni and Fe) show a clear trend of the fluence threshold with the work function, evidencing the importance of the surface electron excitation in femtosecond laser–matter interactions. Experiments with the alloyed samples reveal that fluence thresholds for femtosecond plasma ignition are matrix independent.