International audienceAn experimental and numerical study of the laser-induced damage of the surface of optical materialin the femtosecond regime is presented. The objective of this work is to investigate the differentprocesses involved as a function of the ratio of photon to bandgap energies and compare the resultsto models based on nonlinear ionization processes. Experimentally, the laser-induced damagethreshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to310 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 1to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damagethreshold with the bandgap and photon energy. A model based on the Keldysh photo-ionizationtheory and the description of impact ionization by a multiple-rate-equation system is used toexplain the dependence of laser-breakdown with the photon energy. The calculated damage fluencethreshold is found to be consistent with experimental results. From these results, the relativeimportance of the ionization processes can be derived depending on material properties and irradiationconditions. Moreover, the observed damage morphologies can be described within the frameworkof the model by taking into account the dynamics of energy deposition with one dimensionalpropagation simulations in the excited material and thermodynamical considerations

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In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica (FS) substrates which are widely used in laser applications and directly influence performances of optical elements. Two procedures were developed: 1 -acid etching and 2 -superpolishing. Additionally, surface roughness and total integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD.Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO 2 layer coated FS samples increased ~3 times, while of the ones that were not coated -4 times. It has been revealed that for nonetched surface a single nano-to micro-scale absorbing defect ensemble most likely associated with polishing particles within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects observed.More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with conventional CeO 2 abrasive polishing.

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