For the development of standard measurement procedures in optics characterization, comparative measurement campaigns (Round-robin experiments) are indispensable. Within the framework of the CHOCLAB project in the mid-90s, several international Round-robins were
successfully performed qualifying procedures for e. g. 1 on 1-LIDT, laser-calorimetry and total scattering. During the recent years, the demand for single pulse damage investigations has been overtaken by the more practically relevant S on 1-LIDT. In contrast to the
industrial needs, the comparability of the multiple-pulse LIDT has not been proven by Round-robin experiments up to now. As a consequence of the current research activities on the interaction of ultra-short pulses with matter as well as industrial applications, numerous fs-laser systems become available in universities and research institutes. Furthermore, special problems for damage testing may be expected because of the intrinsic effects connected with the interaction of ultrashort pulses with optical materials. Therefore, a Round-robin experiment on S on 1-damage testing
utilizing fs-pulses was conducted within the framework of the EUREKA-project CHOCLAB II. For this experiment, seven parties investigated different types of mirrors and windows. Most of the partners were guided by the International Standard ISO 11254-2, but one partner employed his own damage testing technique. In this presentation, the results of this comparative experiment are compiled demonstrating the problems induced by special effects of damage testing in the ultra-short pulse regime.
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.
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.
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.