The role of plasma shielding in collinear double-pulse femtosecond laser-induced breakdown spectroscopy

We report an experimental and theoretical study of the mechanism for the enhancement of the laser-induced breakdown signal produced by two collinear femtosecond pulses separated by a suitable delay. A bilayer sample consisting of a 500 nm thick film of Ag deposited on Al was used in the experiments, and a particle-in-cell (PIC) simulation was implemented in the theoretical part of the study. Experiments on the effect of laser polarization, performed at a 30° angle of incidence over a wide range of fluences, together with the PIC results, showed that the plasma produced by the first pulse was further excited by the second pulse. Experiments at normal incidence and a fluence of 200 J/cm2 showed that the second pulse did not penetrate the Ag layer. In addition, measurements of the effect of pulse delay on the signal supported the conclusion that double pulse enhancement is produced by plasma heating rather than by increased surface ablation.