Investigation of mechanisms of ultrashort laser pulse ablation through experiments and simulations.

摘要

Ultrashort laser pulse (USLP) ablation has been investigated over a decade long. The machining quality, however, as well as ablation efficiency and understanding of the ablation mechanism, still need improvement.;First, a combined molecular dynamics (MD) and Monte Carlo (MC) method is used to investigate the particle transport and energy transport phenomena in USLP ablation. It is found that the ionization and ballistic electron motion greatly affect the surface material temperature. It is also found that the hydrodynamic motion and fast density change of the ablated material cannot be neglected and should be considered in the model.;Next, the MD-MC model is coupled with a particle-in-cell (PIC) method as well as a beam propagation method (BPM) to be an integrated atomistic model for the simulation of charged particle evolution and air breakdown. A series of shadowgraph measurements are performed to validate the simulation results in terms of early-stage plasma front locations. It is found that the location of the focal spot, whether slightly above the target surface or slightly below the target, has substantial effects on the early-stage plasma evolution.;To simulate later-stage plume plasma evolution, the output of this integrated atomistic model is used as the input to a hydrodynamic model. Various plasma properties, such as the plasma expansion length, temperature and electron number density, are obtained from this simulation model and validated against direct fluorescence photography and plasma emission spectroscopy measurements. The nonlinear relationship between ablation depth and laser fluence is found to directly relate to the relationship between plasma temperature and laser fluence. The nonlinear relationships are caused by the effect of early-stage plasma and air breakdown.;The effect of air breakdown on laser energy loss is further investigated with focal lenses of various focal lengths using the proposed simulation model. It is found that the laser energy loss increases as the focal length decreases.;A 10-picosecond (ps) laser is used for microhole drilling and microstructure machining on metals, alloys, and ceramics. It is demonstrated that ps laser has the capability of producing high-quality features comparable to femtosecond (fs) lasers and good stability as well as flexibility, which meet the requirements of precision applications in industry. The ablation depth per pulse is investigated via experiments and simulations. Two-temperature model (TTM) is used for single-pulse mode and further expanded for multi-burst mode laser ablation simulation. It is found that, at a fixed laser pulse energy, the ablation depth per pulse greatly depends on the pulse-to-pulse separation time in multi-burst mode but does not change much in single-pulse mode. The energy accumulation between two adjacent pulses in a burst enhances the ablation efficiency.