A Fundamental Understanding of the Dependence of the Laser-Induced Breakdown Spectroscopy (LIBS) Signal Strength on the Complex Focusing Dynamics of Femtosecond Laser Pulses on Either Side of the Focus

We correlate the focusing dynamics of 50 femtosecond (fs) laser radiation as it interacts with a silicon sample to laser-induced breakdown spectroscopy (LIBS) signal strength. Presented are concentric ring-shaped variations in the electric field in the prefocus region due to lens aberrations and nonsymmetry between the prefocus and post-focus beam profile as a result of continuum generation, occurring around the focus. Experimental results show different signal trends for both atmospheric and vacuum conditions, attributed to the existence of a continuum for the former. Lens aberrations effects on the LIBS signal strength are investigated using a plano-convex spherical lens and an aspherized achromatic lens. High-resolution scanning electron micrographs of the silicon surface after ablation, along with theoretical simulations, reveal the electric field patterns near the focus. The research results contribute to fundamental understanding of the basic physics of ultrashort, femtosecond laser radiation interacting with materials.