Comparative study of laser-induced plasma emission from microjoule picosecond and nanosecond KrF-laser pulses

In this paper, the emission of laser produced silicon and aluminum plasmas is investigated in the energy range from 0.1 to 100 μJ (0.5–500 J/cm2) using 10 ns and 50 ps KrF laser pulses focused to a 5 μm diameter spot. For energies higher than 3 μJ, there is little difference between 50 ps and 10 ns pulses in the plasma emission both in terms of the intensity of the emission lines and in terms of lifetime of the emission. Differences become significant only at very low fluences approaching the plasma formation threshold, which is significantly lower for 50 ps pulses than for 10 ns pulses. Calculations using a plasma ablation model show that initial plasma conditions are significantly different for 50 ps and 10 ns pulses during irradiation by the laser pulses. However, the dominant process leading to plasma emission at later times is from expansion and cooling of the plasma plume in the form of a blast wave in the ambient air which is primarily dependent on the energy deposited in the plasma and not the pulse length. Calibrations have also been carried out in order to give the emission in absolute numbers of photons emitted and thus facilitate the comparison with modeling and future experiments.