Quantized behavior of the LIDT in the fs-range in dielectric layers

摘要

In comparison to studies at longer pulse regimes, investigations of laser induced damage threshold indicate a contrary behavior on the fs-scale for the dielectric coatings. In general, experiments reveal an electronic cause of the damage. The strong correlation of theoretical calculations with experimental data of laser induced damage thresholds for quartz verifies this assumption. Consequently, the characteristic function of the wavelength dependence of the damage threshold differs in this range from the classical behavior. The quantized structure of the electronic transition leads to a typical step function of the LIDT in dependence on the band gap energy of the materials. Hence, the step should be observed between energy levels from n to n+1 electron ionization. In detail, the probability for the transition of the electron from the valence band to the conduction band changes abruptly. In an international cooperation with the University of Vilnius the wavelength dependence of the LIDT was investigated for dielectric coatings of Ti_xSi_(1-x)O_2 as a function of the stoichiometry. The measurements were performed for a wavelength range from 600 to 800 nm and at a pulse duration of 130 fs by using an OPA laser system. The step from two photon to three photon ionization was measured. The assumption of the mentioned behavior of the fs-damage was proven. For different concentrations of silicon and titanium in the oxide, the electronic structure of the material changes. The experiments have shown an increasing gap energy and LIDT for a high content of silica.