Temporal-spatial dynamics of electronic plasma in femtosecond laser induced damage

In this study, transient temporal-spatial evolutions of femtosecond (fs) laser pulse-induced filaments and electronic plasma when laser induced damage occurred in fused silica were investigated using fs time-resolved pump-probe shadowgraphy. The transient peak electron density increased and then decreased as delay time of probe beam increased. Its corresponding spatial positions moved from the sample surface to the inside of the sample, but remained at the nonlinear focus for a relatively long time. The maximum electron density increased as pump energies increased and then became saturated at 8 μJ, above which laser-induced material damage occurred. The material damage threshold electron density was approximately 1.27×10²⁰ cm^-3. The laser-induced material damage position corresponded to the position of the maximum electron density. Furthermore, the material damage was extended from the nonlinear focus to the deeper parts of the sample at pump energies above 8 μJ. This tendency agreed well with the spatial distribution of the maximum transient electron density at each propagation depth, implying that the fs time-resolved pump-probe shawdowgraphy is a meaningful tool for predicting the distribution of laser-induced microstructures in ultrafast laser micromachining.