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A two-step physical-chemical etching method, namely large incident angle ion beam etching followed by HF etching, is proposed to remove the damage precursors in fused silica surface to achieve good surface quality and significantly improve the laser damage resistance of fused silica. Ion beam etching with large incident angle is utilized to remove the metal impurities and most subsurface defects in fused silica surface, which can reduce the removal amount of subsequent HF etching of fused silica, thus suppressing the replication and expansion of the subsurface defects, deposition of reaction products and deterioration of surface shape during the HF etching process. After that, HF etching is used to remove the atomic-scale damaged layer and reduce the P–V value of the reflection surface shape induced by ion beam etching. Therefore, a better laser damage performance of fused silica can be obtained by the two-step etching method compared with single ion beam etching and single HF etching. The results show that the two-step physical-chemical etching can be developed to fabricate high-quality and high damage-resistant fused silica optical components with small etching amount and good surface quality for high-power laser application.

The basic concept of effective stress describes the characteristics of fluid and solid interaction in porous media. In this paper, based on the theory of fractal geometry, a fractal model was built to analyze the relationship between the microstructure and the effective stress of porous media. From the microscopic point of view, the influence of effective stress on pore structure of porous media was demonstrated. Theoretical analysis and experimental results show that: (i) the fractal model of effective stress can be used to describe the relationship between effective stress and the microstructure of porous media; (ii) a linear increase in the effective stress leads to exponential increases in fractal dimension, porosity and pore number of the porous media, and causes a decreasing trend in the average pore radius. Keywords: Porous media, Effective stress, Porosity, Fractal dimension, Size distribution