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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]

[Display omitted] • There are amounts of point defects at the atomic scale in the residual stress zones. • The STE point defects at the atomic scale distributed on the fused silica surface are found to be closely related to the residual stress on the fused silica surface for the first time. • PL-detection method is proposed to characterize the residual stress on the material surface for the first time. • The relative amount of STE point defect (K = S / S 0) is first proposed to quantitatively characterize the residual stress. The residual stress would have a significant impact on the service performance and fatigue life of the fused silica (non-crystal material) components (e.g., Hemispherical Shell Resonator). However, there is still no effective method for characterizing the residual stress on their working surfaces. Therefore, it is essential to develop a novel method for characterizing the residual stress on fused silica surfaces. Herein, the residual stress is found to significantly affect the mechanical properties of fused silica surfaces. On the basis of the study on the photoluminescence (PL) properties of residual stress zones, atomic point defects are found to be closely related to the residual stress. Based on an investigation of the PL properties of different types of mechanically processed surfaces, four types of point defects (ODCII, STE, Si clusters and NBOHCI) have been determined. On the basis of the molecular dynamics (MD) simulation results and the PL experiments on different types of mechanically processed surfaces, STE point defects at the atomic scale are found to be closely related to the residual stress for the first time. Based on this, PL-detection method is proposed to characterize the residual stress on the material surface for the first time. Besides, the relative amount of STE point defect (K = S/S 0) is first proposed to quantitatively characterize the residual stress. In sum, this work determines the structure at the atomic scale which is closely related to the residual stress and opens a convenient door to quantitatively characterize the residual stress with PL-detection method. It could also provide a reference for the characterization and even the alleviation of the residual stress on the surfaces of other non-crystal materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A reciprocal relationship between the highest photoluminescence intensity (HPI) and the laser-induced damage threshold (LIDT) of the surface-defect zone is constructed for the first time. • The physical mechanism of the sharp decrease of the LIDT from plastic to brittle surface defects is revealed for the first time. • The slight increase of the HPI would lead to a sharp decrease of the LIDT from plastic to brittle surface defects. • E'-Center and STE point defects would increase sharply from plastic to brittle surface defects, causing a sharp increase of HPI. Micro-nano surface defects are inevitably introduced on mechanically processed fused silica surfaces and even almost all the optical glasses, severely decreasing their laser-induced damage thresholds (LIDTs). There are two types of defects: brittle surface defects and plastic surface defects, whose diverse structural characteristics could affect their influence on the LIDTs. Due to their tiny dimensions, brittle and plastic defects are difficult to be distinguished and identified clearly in industrial production. However, there is a sharp decrease (even to 10%) of LIDT from plastic to brittle surface defect, whose mechanisms have not been revealed. Herein, a reciprocal relationship between the highest photoluminescence intensity (HPI) and LIDT of the surface-defect zone is constructed for the first time. The increase of HPI is found to cause a sharp decrease of the LIDT when HPI is low, which is the first cause of the sharp decrease of LIDT. E'-Center and STE point defects would substantially increase from plastic to brittle surface defect, causing a sharp increase of HPI, which is the second cause. This work fundamentally reveals the mechanisms of the sharp decrease of the LIDT from plastic to brittle surface defect, which is beneficial to characterize and mitigate the hazardous brittle surface defects. [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]