| In this dissertation, femtosecond laser materials processing is studied. A time-of-flight mass spectrometer has been designed, constructed and utilized to measure the time-of-flight spectra of the ions ablated by femtosecond laser pulses. The velocities and kinetic energies of the ions are determined. Emission spectroscopy and imaging have been carried out to analyze the laser-induced plume. Craters ablated in vacuum and under ambient pressure are measured by a scanning electron microscope (SEM) and a light interferometric microscope. Numerical methods in microscale energy transfer are first reviewed. Subsequently, a numerical model based on the two-temperature concept is utilized to account for the non-equilibrium energy transfer processes dominating the ultrashort laser pulse excitation of materials.; Time-of-flight measurements reveal the presence of extremely energetic ions in femtosecond laser-induced plumes, with kinetic energies more than one order of magnitude higher than those of nanosecond laser-induced ions. Two different ablation regimes, exhibiting different laser fluence dependence of the total ion yields, and the corresponding percentage of energetic ions and the crater depths, are discovered for femtosecond laser ablation of titanium. Numerical modeling shows that the laser fluences associated with the second ablation regime can raise the lattice system to the range of the thermodynamic critical point and hence may cause explosive evaporation leading to the observed higher ablation rates. The first regime, however, occurs at lower laser fluences, when the laser energy is mainly deposited in the shallow region defined by the optical penetration depth. The more localized energy deposition is believed to lead to higher percentage of energetic ions, albeit to less total ablation volume.; Femtosecond laser-induced plumes have been found to be much smaller in spatial dimensions and weaker in intensity than those induced by nanosecond laser pulses. The ambient pressure is found to restrain the expansion of the plumes.; The non-equilibrium microscopic energy transfer between the electron and lattice subsystems needs to be considered to model femtosecond laser materials processing. The two-temperature approach is capable of at least semi-quantitatively modeling real engineering problems. Due to the large temperature range encountered in typical laser ablation applications, the temperature dependence of the materials thermal properties is found to be very important for accurate modeling. (Abstract shortened by UMI.)... |
