Ultrashort laser pulse-matter interaction: Fundamentals and early stage plasma dynamics

摘要

Despite extensive studies for many years, the detailed mechanisms of ultrashort laser pulse (USLP)-matter interaction are still not fully understood and further fundamental investigation is required. This study seeks to provide an improved understanding of the USLP-material interaction by both theoretical and experimental investigations and to find ways to enhance laser energy coupling with different materials.;A two-dimensional comprehensive hydrodynamic model for USLP ablation of metals and semiconductors is developed in this study. The model comprises a two-temperature model and a hydrodynamic model supplemented with a quotidian equation of state model, considering the relevant multiphysical phenomena during the laser-matter interaction. The models are capable of simulating the ablation process and the resultant plasma evolution in a wide range of laser intensity, and are valid both in air and in vacuum.;The developed model is applied to investigate the ablation of metals in various laser intensity ranges. The dependence of ablation rates on laser intensity in air and in vacuum is studied by the model and validated against the experimental data in literature. It is revealed that there appears to be a sudden increase of the ablation rate in the high intensity range in vacuum, due to switching of the dominant absorption mechanisms. On the other hand, much lower ablation efficiency at high laser intensity in air is caused by the strong early plasma absorption of incident laser beam energy. The evolutions of both early plasma and plume plasma are measured by a shadowgraphic technique and a direct fluorescence method, respectively, and are analyzed by the numerical simulation. It is found that the electron emission process greatly affects the surface electron temperature.;The femtosecond laser ablation of silicon in air is also investigated by the integrated model. The numerical analysis results are validated and supplemented by the experimental measurements for the ablation rate and early plasma dynamics over a wide laser intensity range. It is found that ablation efficiency first increases with laser intensity, and then begins to drop in the high laser intensity range, because of the early plasma absorption of the laser beam energy. By investigating the ion expansion speed, electric field distribution, and velocity distribution of different ions, the occurrence of Coulomb explosion (CE) is demonstrated during the ablation of silicon at high laser intensity, which leads to a fast ion ejection from the target surface, thereby increasing the material removal rate at the early stage.;Next, double-pulse (DP) ablation of silicon is investigated by an integrated atomistic model, combined by molecular dynamics (MD), Monte Carlo (MC), particle-in-cell (PIC), and beam propagation (BP) methods. The plasma emission spectrum is measured by a spectrometer to calculate the plasma temperature and electron number density. It is observed that the double-pulse ablation could significantly increase the ablation rate of silicon, which is totally different from the case of metals. Electronic excitation and metallic transition of melted silicon are revealed to be responsible reasons of ablation enhancement at ultrashort (below 1 ps) and long (1 ps to 10 ps) pulse delays, respectively. At even longer pulse delay (over 20 ps), the plasma temperature and electron number density can be effectively increased, accompanied by the ablation rate suppression. The spatial analysis of plasma temperature shows that the second pulse energy is mainly absorbed by the front portion of the plasma, where the temperature is increased the most. The plasma reheating leads to a faster expansion of the plasma.