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Micro surface defects (SDs) would be unavoidably induced on most fused silica surfaces (FSSs), substantially reducing their laser-induced damage thresholds (LIDTs). There is still no clear understanding of the relation between SD structural feature and their LIDTs, which poses a challenge to the recognition and mitigation of the dangerous SDs with low LIDTs in industrial production. In this work, the brittle-plasticity and stress states of the SDs are found to dominate the atomic point-defect densities on FSSs and resultantly dominate their LIDTs for the first time. Based on this, the SDs have been divided into four categories: pressure-induced brittle defects (PBDs), pressure-induced plastic defects (PPDs), tension-induced brittle defects (TBDs) and tension-induced plastic defects (TPDs). Based on the tested LIDTs, laser damage morphologies and photoluminescence (PL) characteristics, the laser damage resistance of different kinds of SDs are compared, whose reasons are revealed on the basis of the molecular dynamics (MD) simulations. The sharp increase of the atomic point-defect density from the plasticity to the brittleness of the material, the significant disparity in tensile and compressive strength of fused silica, the distinctly different material response behaviours under tensile and compressive forces, as well as the opposite effects of the tensile and compressive forces on the material compactness significantly affect the atomic point-defect densities on FSSs and resultantly contribute to the different laser damage resistance of different kinds of SDs. This work possesses great significance in solving the laser damage on FSSs. It could also provide a new understanding of the impact of the tensile and compressive forces on the LIDTs of various optical elements. [ABSTRACT FROM AUTHOR]

The defect-induced laser damage on mechanically machined fused silica optical surfaces under intense laser irradiation severely threatens the laser damage resistance of optical components. The lack of a quantitative relationship between the feature information of surface defects and the laser-induced damage thresholds (LIDTs) of optical surfaces poses a challenge to effectively evaluate the influence of the surface defects on the LIDTs of optical surfaces. Herein, the photoluminescence (PL) properties of the areas where laser damage tends to occur in surface-defect zones are explored. The PL induced by the point defects at the atomic scale in these areas is found to be obviously higher than that of other positions. The PL envelope area (S) at the position with the highest PL intensity among the whole surface defect is found to be inversely proportional to the LIDT (Φ) of the optical surface for the first time (SΦ = N 0). It is beneficial to quantitatively characterize the influence of the surface defect and provide new insights in elucidating the defect-induced laser damage mechanisms. Besides, we found that there is a minimum LIDT for the mechanically machined optical surfaces with various surface defects. On basis of this, the maximum influence of the surface defects with different species on the LIDTs of optical surfaces could be determined. Based on the constructed relationship (SΦ = N 0) and the developed MD model, two causes of the minimum LIDT are determined. The PL envelope area of the surface defect would slightly affect the LIDT when the PL envelope area is large enough, which is the first cause. The mechanical force could compact the optical surface, increasing the densities of Si and O atoms. However, the densities of Si and O atoms become stable when the mechanical force is large enough, causing the highest densities of point defects, which is the second cause. [ABSTRACT FROM AUTHOR]

• A PIC model is developed to investigate the plasma behaviors in laser damage processes under intense laser irradiation. • The evolution laws of plasma characteristics under intense laser irradiation are proposed to study from various aspects. • The particle ejection and the formation of craters are reproduced and the internal physical mechanisms are obtained. The laser-induced damage on KDP crystal optical surfaces under intense laser irradiation significantly limits the energy outputs of the high-power laser systems in many regions. The energy deposition in the non-heat stage of the laser-induced damage process is regarded as the main energy source, directly determining the laser-induced damage thresholds. It can cause severe damage to the optical surfaces under intense laser irradiation and significantly affect the final damage morphology on the optical surface. Nevertheless, there is still no available model that can well reproduce the behaviors of the plasma in the non-heat stage to reveal the internal physical mechanisms of the non-heat stage currently. Hence, a particle-in-cell (PIC) model is proposed to investigate the behaviors of the plasma in the non-heat stage. Afterward, the feature information of the plasma in the non-heat stage on the KDP crystal optical surface without surface defects under intense laser irradiation has been obtained from three aspects: the electron density, the electron phase-space distribution on the subatomic scale, and the electric field distribution at different times. Then two types of typical damage behaviors of particle ejection and the formation of the damaged craters on optical surfaces under intense laser irradiation are studied based on the developed model. Finally, the plasma behaviors in the processes of particle ejection and the formation of the damaged craters are well reproduced and the internal physical mechanisms of these two types of damage behaviors are obtained. To sum up, this work proposes a PIC model to investigate the plasma behaviors in the non-heat stage. Based on this, this work reveals the energy deposition mechanism of the non-heat stage in the process of the laser-induced damage on KDP crystal optical surfaces under intense laser irradiation and obtains the interaction laws between the intense laser and the plasma. Moreover, this work opens a door for the study on the non-heat stage of the laser-induced damage process on the optical surface from the subatomic scale, which is beneficial to the revelation of the internal physical mechanisms in the non-heat stage and the solution of the laser-induced damage issues in high-power laser systems. [ABSTRACT FROM AUTHOR]