| The breakdown of dielectric solids and transparent gases by high-intensity femtosecond laser pulses are experimentally investigated. The plasma emission from atmospheric-pressure gases is studied. The plasma emission strength is shown to exhibit much less shot-to-shot variance than the plasma generated by longer-pulse lasers; this observation is explained by the different breakdown mechanism active for very short pulses. The breakdown plasma defocuses the pulse which creates it; this process is studied experimentally and through a Finite-Difference Time-Domain (FDTD) simulation. The plasma also causes a spectral blueshift in the generating pulse; the spectrum of the transmitted pulse is investigated. Time-resolved measurements of the evolving plasma shape are presented. The defocus, blueshift, and plasma shape experiments are all conducted for both linear and circular polarization - and it is found that a circularly polarized pulse behaves exactly like a linearly polarized pulse of lower intensity. The intensity shift is determined to be 1.36 times, and is consistent among all three experiments, but disagrees with the 1.5 times shift seen in vacuum experiments. In spite of possessing a lower overall number of ions, the plasma generated with a circular pulse is shown to emit more brightly than one produced with a linear pulse. This is connected with theoretical descriptions of ionization which predict higher electron kinetic energies following a circularly polarized breakdown pulse, and shows that kinetic energy plays a major role in producing plasma emission. The role of impact excitation is reaffirmed in a mixed-gas experiment, in which it is shown that a lower-ionization-potential gas can increase emission from a higher-ionization-potential gas. In dielectric solids, it is found that circular pulses produce a narrower but deeper crater. The crater profiles are well fit by an atomic model which primarily considers lattice heating from electron collisions. In both solids and gases, the role of dephasing of the electron motion - induced either through collisions or continuous blueshifting of the pulse - is found to be the critical factor in moving energy from the pulse to the material. Finally, the effects of the shockwave generated when a solid is ablated in air are studied. |
