Abstract
In applications of high intensity lasers to materials processing, the formation of an ablation plume is of high importance. For wide bandgap insulators (e.g., oxides, halides, nitrides, carbides) irradiated with sub bandgap photon energies, the route to plume formation is not well understood. For example, contrary to metals and semiconductors, inverse bremsstrahlung (IB) is not possible for a wide range of laser intensities on these materials due to insufficient photon and electron densities. We present an alternative path to plume formation on nominally transparent materials. We first examine the interaction of photo-and thermally-emitted particles from exposure to pulsed laser irradiation of surfaces which include photoelectrons, energetic positive ions, and neutral metal atoms. We establish experimentally that there is overlap in space and time of significant portions of the distributions of these particles in the near surface region. We present a model for the collective motion of these particles and show that as laser fluence is increased we achieve sufficient densities, overlap, and kinetic energies to result in the onset of plume fluorescence and eventually ionization at fluences far below any IB or catastrophic breakdown process.
