Near threshold femtosecond laser interactions with materials: Ablation thresholds, morphologies, and dynamics

This dissertation reports the results of femtosecond (fs, 10-15 seconds) laser ablation studies performed on single crystal silicon with oxide thin films and the single crystal Ni-based superalloy, CMSX-4. Emphasis is placed on near threshold ablation (or material removal) phenomena where fs pulsed lasers show significant promise for industrial machining, characterization, materials processing, fabrication of structures, and other applications. Three specific topics are addressed: fs laser ablation thresholds, ablation morphologies, and ablation dynamics. These investigations demonstrate both fundamental aspects of the interaction of fs laser pulses with materials while also introducing novel, previously unobserved phenomena.; The fs laser ablation threshold of single crystal silicon was observed to depend on its naturally occurring oxide, the presence of which increased the ablation threshold of silicon by ∼39% at grazing laser incidence relative to atomically clean silicon. Extension of these studies to ablation threshold measurements on silicon with thermally grown oxide films of varying thickness revealed that despite the high intensities and short timescales of such interactions, the linear optical properties of a surface significantly influence near threshold fs laser ablation.; Femtosecond laser induced blistering or buckling of thin oxide films (20--1200 nm) from silicon substrates was observed. Thin film buckling mechanics were used to study the blister features, revealing that the fs laser induced ablation at the oxide film interface participated in the buckling by adding energy to the delaminated oxide film. Furthermore, isolated blisters could be connected together to create linear fluidic channels. A simple device for performing electrophoresis was fabricated with these channels, the characteristic of this device were studied.; The dynamics of the ablation event were studied in-situ using the technique of pump-probe microscopy with sub-picosecond resolution. Previously developed models of near threshold fs laser ablation were verified and extended by comparing the dynamics captured from two different viewing angles. Furthermore, the dynamics within the first 10 ns of ablation were found to correlate with the final ablation morphology. Finally, an orthogonal, dual-pulse laser induced breakdown spectroscopy method was used to reduce the surface damage associated with this versatile spectroscopy technique.