Relationship between the photoluminescence envelope area of surface defects and the laser-induced damage thresholds of mechanically machined fused silica optical surfaces

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 = N0). 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 = N0) 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.