Wavelength dependence of femtosecond laser-induced damage threshold of optical materials
Résumé
An experimental and numerical study of the laser-induced damage of the surface of optical material
in the femtosecond regime is presented. The objective of this work is to investigate the different
processes involved as a function of the ratio of photon to bandgap energies and compare the results
to models based on nonlinear ionization processes. Experimentally, the laser-induced damage
threshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to
310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system.
Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1
to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage
threshold with the bandgap and photon energy. A model based on the Keldysh photo-ionization
theory and the description of impact ionization by a multiple-rate-equation system is used to
explain the dependence of laser-breakdown with the photon energy. The calculated damage fluence
threshold is found to be consistent with experimental results. From these results, the relative
importance of the ionization processes can be derived depending on material properties and irradiation
conditions. Moreover, the observed damage morphologies can be described within the framework
of the model by taking into account the dynamics of energy deposition with one dimensional
propagation simulations in the excited material and thermodynamical considerations.