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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] • 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]