| Abstract [eng] |
The development and output power of ultrashort pulse lasers is limited by the the quality of optical elements. Laser induced damage is usually the limiting quality factor. Resistance to electromagnetic radiation usually depends on many material and irradiation parameters. Characterization of optical elements and identification of damage causing physical processes leads to a creation of next generation optics. Laser induced damage threshold (LIDT) is now usually evaluated using standardized experimental methods. Computational LIDT evaluation would speed up and reduce the cost of this process, however, there are no realiable and accurate models for calculating LIDT in ultrashort pulse regime for optical coatings. Therefore, the main goal of this work was to fit experimental time-resolved digital-holography pump-probe data to evualuate physical properties of Ta2O5 coating and use those properties to calculate laser induced damage threshold dependence on pulse duration and laser wavelength in the femtosecond regime. The electromagnetic field in dielectric coatings was simulated using finite-difference time-domain method. Standard rate equation model was improved by adding multiple rate equations model for conduction band electrons. Laser induced damage threshold was computationally investigated in 490 nm Ta2O5 optical coating in 10 - 1000 fs pulse duration and 400 - 1200 nm wavelength regimes, using absorbed volumetric energy as damage criteria. It was shown that time-resolved digital-holography pump-probe experiment combined with the developed model is a sensitive technique that allows accurate description of physical parameters for self-trapped excitonic levels, because the determined energy of excitonic level (2,30 eV) is within 2% of that found in literature. It was also demonstrated that the proposed model allows using these material parameters to evaluate laser induced damage threshold by a single laser pulse with high accuracy, because the LIDT evaluated for 1030 nm, 300 fs pulse is within 15% of the experimentally measured value. The findings of this work will aid in further development of ultrafast laser damage analysis and computational evaluation of LIDT. |
