Mechanisms of the sharp decrease of the LIDT from the plastic surface defect to the brittle surface defect on optical surface

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.