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Femtosecond pulse damage and predamage behavior of dielectric thin films

The damage behavior of five different oxide dielectric thin films (Ta2O5, TiO2, Al2O3, HfO2, and SiO2) has been investigated with ultrashort laser pulses with durations from 25 fs to 1 ps. At all pulse durations the damage threshold is well defined and scales with the bandgap energy of the material. The damage behavior can be described with a phenomenological model taking into account multi-photon excitation, impact ionization, and electron relaxation. The temporal evolution of the dielectric constant of the film following the excitation with pulses below the damage threshold has been measured with time-resolved pump-probe spectroscopy. The complex dielectric constant was retrieved from transient reflection and transmission data.