Sub-micro scale driven LIBS signal enhancement on femtosecond laser microstructured copper

Overview & Synopsis

This paper investigates the laser induced breakdown spectroscopy (LIBS) signal enhancement mechanism provided by femtosecond laser microstructuring of copper samples. The surface morphology of such microstructured samples was characterized using confocal microscopy, scanning electron microscopy, and atomic force microscopy. Although little change was observed using confocal microscopy on the microstructures pre and post-LIBS laser shot, structures and particles in the sub-micrometer range were shown to partially melt on scanning electron microscopy images. This change was correlated with a sharp drop of the LIBS signal enhancement provided by a microstructured sample compared to a polished copper sample for successive LIBS shots on the same spot. Additionally, time resolved spectra revealed that the signal enhancement appeared after a delay of 100–800 ns. A physical model allowed demonstrating that such values of delay are compatible with the vaporization and ionization time of sub-micron spherical particles in the plasma, hence providing a delayed source of seed electrons responsible for the observed LIBS signal enhancement.