Femtosecond double pulse laser-induced breakdown spectroscopy: Investigation of the intensity enhancement

Abstract

Femtosecond laser-induced breakdown spectroscopy (fs LIBS) experiences a growing number of applications. We used a fs double pulse (DP) laser system to enhance the line intensities and we quantified the major mechanisms responsible for intensity enhancement in fs DP LIBS. The laser-induced plasma was excited by a 400 nm pulse and a delayed 800 nm pulse focused on metallic and oxide materials. The 400 nm pulses were produced by frequency doubling of the fundamental laser pulses at 800 nm. The laser radiation not converted by second harmonic generation was used as the 800 nm pulses. The LIBS spectra and the time-resolved plasma parameters were measured as function of pulse energies and correlated to the volume of ablation craters and the amount of nanoparticles formed in the process. Our results showed that the fs LIBS line intensities measured with double pulse excitation IDP were much higher than with single pulse excitation ISP. The intensity enhancement on metallic Cu was IDP/ISP = 2–30 depending on the energy of the first fs pulse. The dominating mechanisms of DP intensity enhancement were the increased number of atoms in plasma N and the higher plasma temperature T. The increase of N was due to the larger volume of DP ablation craters. The atomization of nanoparticles by laser-plasma-particle interaction contributed much less. The intensity enhancement was highest at lowest energy of the first pulse. Fs DP LIBS may thus enable elemental imaging with improved lateral resolution, higher sensitivity and increased elemental contrast.