Double-pulse Femtosecond Laser-induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS) has been demonstrated as a convenient and powerful spectroscopic tool for elemental analysis of various types of samples. LIBS has attracted much attention due to its unique features such as simplicity, no sample preparation, speed of analysis. In the past decades, the advance of femtoseocond(fs) laser make it a unique laser source for LIBS due to its ultrashort pulse duration, ultrahigh peak power, and minimized thermal effects. Generally, fs-LIBS can improve the spatial resolution, reduce the amount of material removal, and realize remote detection in air. Nevertheless, compared to the ns-LIBS, the signal intensity from fs-LIBS is relatively low. It is therefore urgent to find solutions for signal enhancement for fs-LIBS. It was reported that fs-LIBS using a collinear double-pulse(DP) configuration may effectively improve the signal intensity compared to single pulse fs-LIBS. Still, the mechanism of signal enhancement has not been fully understood. Based on this, with the aim to discuss the mechanisms of signal enhancement, this thesis will study the DP fs-LIBS of metals, semiconductors, organic dielectrics, and combustion flame. The innovations of this thesis can be concluded as follows:(1) The study of DP fs-LIBS on copper reveals that the signal enhancement is caused by the heating effect of the second pulse on the first pulse generated plasma plume. This further ionizes the plume and increase its electron density and temperature. Moreover, it is found the enhancement factor at a lower pulse fluence is larger than that using a higher fluence. Additionally, the variation of the laser pulse duration in 1 ps has no effects on the DP fs-LIBS.(2) By comparing the trajectories of signal enhancement with the surface morphology and crater depth in term of DP delay, it is revealed that the signal enhancement of silicon is the increased amount of material removal by double pulse over that by single pulse.(3) DP fs-LIBS was firstly applied to the transparent organic material of polymethyl methacrylate(PMMA). Compared to the single pulse(SP) fs-LIBS, the emission enhancement of DP fs-LIBS is strongly related to the type of emission particles and the DP delay. The intensity enhancement of emission lines increased as molecules < neutral atoms < ions. The characterization of the ablated craters by SP and DP irradiations showed lesser contribution of ablation volume on the signal enhancement. The electron density and the temperature of the plasma exhibited similar variation trajectories in terms of DP delay and featured a distinct increase at an optimal DP delay of ~80 ps, indicating the reheating of preproduced plume was responsible for the emission enhancement.(4) The DP fs-LIBS was firstly applied to the equivalence ratio study of oxyacetylene flame. Due to the linearity between CN signal generated by fs-LIBS and the equivalence ratio of flame, the CN signal can be used as the indicator for the equivalence ratio study. The use of DP fs-LIBS on the flame can increase the signal intensity, and hence increase the detection sensitivity.