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Femtosecond breakdown and pre-breakdown behavior in thin dielectric films

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.