## 3 Results and Discussion
### Suppressed Topography of Various FS Surfaces  
Figure 7 shows the surface morphology and corresponding root mean square (RMS) roughness in a 3 \(\times\) 3 \(\upmu\)m\({}^{2}\) scanning area of various FS surfaces. The roughness value of pristine FS is ~0.47 nm. With the increase in H\({}_{2}\)O\({}_{2}\) concentration, the roughness value of the FS surface gradually decreases, and it reduces to ~0.35 nm as the H\({}_{2}\)O\({}_{2}\) concentration increases to 20%, indicating a reduction in roughness of ~25%. Generally, the reduction in surface roughness is caused by the surface corrosion and reduction in surface defects [48; 49; 50]. Thus, based on the results from Figures 1 and 7, the treatment with the H\({}_{2}\)O\({}_{2}\) solution can not only affect the surface water adsorption and change the hydrophilicity of the FS surface, partial H\({}_{2}\)O\({}_{2}\) may penetrate into the subsurface and corrode the FS surface to some extent. As a result, the tiny structural defects (scratches, cracks, and pits, etc.) originated from the precision process can be removed, causing the decrease in surface roughness. Previously, it was reported that after traditionally grinding and polishing processes, a polishing redeposition layer with traceless photoactive impurities (such as ceria and iron) can exist on the top surface layer with a thickness of ~50 nm [51]. These surface defects can be migrated or removed by chemical etching [50], reactive ion etching [48], or a combined etching process [49]. The photoactive impurities at the near-surface region of FS can also be leached out of glass during the immersion in strong acid and/or H\({}_{2}\)O\({}_{2}\) solutions [50]. If the traceless photoactive impurities ions are leached out of the FS surface, the atomic sites that were originally taken by impurity ions would be occupied by the diffused water molecules, the FS surface would become more susceptible to damage by indentation and nanocxtaching, and the subsurface densification of FS would also become higher since the network on the top surface layer has more propensity to be compacted upon indentation and nanoscratching. Thus, the leaching of impurity ions on the top surface layer and the weak corrosion by H\({}_{2}\)O\({}_{2}\) solution can also explain the increased surface damage and subsurface densification by nanoindentation and in nanowear tests (Figures 3-6). These findings also imply the H\({}_{2}\)O\({}_{2}\) solution can be alternative method for removing the surface defects on FS optics. Previously, the chemical etching [50], reactive ion etching [48], or combined etching [49] methods have been used to remove the surface defects caused by traditional grinding and polishing processes, thus improving the LIDT and lifetime of FS optics. However, these methods are either hazardous for the environment  
Figure 7: (**a**) AFM surface topography and (**b**) RMS roughness value of various FS surfaces. and during handling operations, or difficult to operate or dependent on expensive equipment. In the present study, the surface roughness of FS was reduced using H\({}_{2}\)O\({}_{2}\) solution to some extent and weak surface corrosion of FS occurred (Figure 7). This suggests H\({}_{2}\)O\({}_{2}\) treatment could be a wet shallow etching method used to remove the surface defects and clean the surface of FS optics, which is environmentally friendly, easy to handle, and low cost. For example, the combination of reactive ion etching or annealing treatment with H\({}_{2}\)O\({}_{2}\) treatment of fused silica may be helpful to improving the LIDT of fused silica optics used in high-energy laser systems.