| Laser induced dielectric breakdown and related physical mechanisms were investigated in the sub-picosecond pulse regime using oxide films (TiO 2, Ta2O5, HfO2, Al2O 3, and SiO2).; The experimentally observed scaling of the single-pulse breakdown threshold fluence of our films with respect to pulse duration was explained by an interplay of multiphoton ionization, impact ionization, and sub-picosecond electron decay out of the conduction band. Within the framework of this model, the observed linear scaling of the breakdown fluence with band gap energy was attributed to the band gap dependence of the multiphoton absorption coefficient from Keldysh theory.; The dependence of the multiple-pulse breakdown threshold on pulse number and repetition rate was attributed to the accumulation of photoinduced defects with a large binding energy. This model can also explain the experimentally observed exponential increase of the temperature and the quadratic scaling of the luminescence power with excitation laser fluence.; Pump-probe measurements were performed on TiO2, Ta2O 5, and HfO2 films to monitor the ultrafast temporal evolution of the dielectric constant at excitation densities close to the breakdown threshold. The dielectric constant was determined from the reflection and transmission data using a retrieval algorithm that takes into account the nonuniform excitation of the film material by the standing-wave pump field and the resulting complications due to Fabry-Perot effects for the probe pulse. The necessary and sufficient conditions for a unique retrieval, and the criteria for choosing a set of angles of incidence for the pump and probe pulses were identified. The observed sign change of the real part of the induced change of the dielectric constant after a few hundred femtoseconds was attributed to the formation of deep defects.; To explain our pump-probe data, we extended a theoretical model, which describes the interaction of electrons, phonons, photoinduced defects, and a femtosecond laser pulse using the Boltzmann transport equation. The contribution of the excited electron and phonon gas to the complex dielectric constant was determined from the linearized Boltzmann equation. The simulation results suggest that in addition to the deformation potential interaction, electron-phonon scattering mechanisms with higher interaction strengths are also needed in the model to explain the experimental data. |
