The dielectric constant of several oxide dielectric thin-films (TiO2, Ta2O5 and HfO2) excited close to the laser-induced damage threshold is retrieved from
reflection and transmission measurements with a 40-fs time resolution. The experiments were compared with the results of a
numerical solution of the coupled Boltzmann equations for conduction band electrons and phonons, including nonlinear carrier excitation and relaxation processes as well as defect formation. The observed fast sub-100-femtosecond decay is shown to be caused by the interaction of non-equilibrium electrons with phonons and is in qualitative agreement with the results of the computer simulation. The observed sign reversal of the real part of the dielectric function from negative to positive after several hundred femtoseconds is attributed to the formation of self-trapped excitons (STE's) in the forbidden bandgap. Both real and imaginary part of the dielectric function are successfully modeled with the Boltzmann equation when defect formation is included. The simulations show that STE formation leads to efficient, non-thermal excitation of phonon modes on a sub-picosecond time scale.
Pulse duration and band-gap scaling of the laser breakdown threshold fluence of oxide dielectrics were measured using various (TiO2, Ta2O5, HfO2, Al2O3, and SiO2) single layer thin films. The observed scaling with pulse duration was explained by an empirical model including multi-photon and avalanche ionization, and conduction band electron decay. The results suggest the formation of self-trapped excitons on a sub-ps time-scale, which can cause significant energy transfer to the lattice. At constant pulse duration, the band-gap scaling was found to be approximately linear. This linear scaling can be explained by the Keldysh photo-ionization theory and avalanche ionization in the flux-doubling approximation.
Dielectric oxide and fluoride films used for optical coatings are
studied with femtosecond laser pulses with respect to their breakdown and pre-breakdown behavior. A phenomenological model with only three figures of merit is used to explain the measured breakdown thresholds for pulse durations from 25 fs to 1 ps. The temporal evolution of the dielectric constant in the pre-breakdown
regime is obtained from transient reflection and transmission measurements after taking into account standing wave effects of pump and probe. In addition to electron-electron and electron-phonon scattering processes, the creation of a new sample state after a few hundred fs is observed. The experimental data are explained with a computer simulation based on the Boltzmann equation.
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