Diagnostics of silicon plasmas produced by visible nanosecond laser ablation

The second harmonic of a pulsed Nd:YAG laser (532 nm) has been used for the ablation of silicon samples in air at atmospheric pressure. In order to study the interaction for silicon targets, the laser-induced plasma characteristics were examined in detail with the use of a space- and time-resolved technique. Electron temperatures, ionic temperatures and electron number densities were determined. A discussion of thermodynamic equilibrium status of the silicon-microplasma is presented. Electron number densities are deduced from the Stark broadening of the line profiles of atomic silicon. Plasma ionization and excitation temperatures were determined from the Boltzmann plot and the Saha–Boltzmann equation, respectively. A limited number of suitable silicon lines for the studies of temperatures were found and the effect of these lines on the temperature measurements is discussed. Electron temperatures in the range of 6000–9000 K and ionic temperatures of 12 000–17 000 K with electron number densities of the order of 1018 cm−3 were observed. The breakdown threshold fluence has been also measured. Silicon plasmas were also characterized in terms of their morphology (shape and size) as a function of laser energy and delay time.