## 3 Results and Discussion
### Removal of Subsurface Impurity Defects by HF Acid Etching
#### 3.3.3 Laser Damage Resistance of Etched Fused Silica Surface  
Now that the contents of impurity defects are effectively controlled, another key index, laser-induced damage threshold (LIDT) of etched fused silica optics should be checked as well. The LIDTs of fused silica etched for various times using etchant of 5% HF and 10% NH\({}_{4}\)F are measured with the testing protocol and setup described in Section 2.3. A 10-min step of etching period is chosen in the HF-etching procedure and the reason as follows. Firstly, it can be derived from Figure 10 that an 8-min etching period and a 200 nm etching depth are required to ensure a steady etching rate for the polished fused silica optics with SSD layers. It means that the impurity defects mainly distributed among the uppermost re-deposition layer could be effectively removed within 10 min. Besides, the general trend of the tested Ce impurity content shown in Figure 13c would be clearly recognized with an etching period of 10 min. The tested LIDTs for various etching times are shown in Figure 14a. It can be seen that the LIDT rises firstly (etching for less than 10 min) and then gradually decreases (etching for more than 10 min) as the etching proceeds. It is indicated by the results in Figure 10 that the re-deposition layers containing most of the impurity defects and hydrolysis products are initially removed during the first 8-min etching times. Combined with the results shown in Figure 14a, one can conclude that not only the impurity defects are effectively removed, but the LIDT of etched fused silica optics could be enhanced from 24.4 J/cm\({}^{2}\) to 27.1 J/cm\({}^{2}\) (355 nm, 10 ns). However, when the re-deposition layers are fully removed, the LIDT presents gradual decrease as the etching proceeds due to the appearance of subsurface scratches and cracks. Based on this, in the practical etching process of fused silica optics, the etching time should be accurately and strictly controlled to prevent the appearing and deepening of subsurface cracks and scratches. The morphological comparisons of laser damage spots on un-etched and etched fused silica optics are exhibited in Figure 14b,c. It can be seen that the damage spot on an un-etched silica surface is very large, consisting of many local small and concentrated damage sites. Besides, the damaged silica materials are all peeled off. As for the damage spots on etched silica surfaces, they are very small and discretely distributed right on the surface scratches. The difference in laser damage morphology on etched and un-etched silica surfaces indicates that the un-etched surfaces possess more impurity defects, which are the main initiators for inducing serious optical damage. When the polished fused silica surfaces are etched by HF acid, the amounts of impurity defects are greatly reduced, and correspondingly the laser damage resistance of silica optics could be effectively improved. This is consistent with the tested results of impurity defect contents on etched and un-etched fused silica surfaces. The laser damage experiment directly validates the negative effect of impurity defects on the laser damage resistance of fused silica. Since the CeO\({}_{2}\), Al and Fe impurity defects are very difficult to be separated from each other and a silica sample with a single impurity defect is difficult to prepare in actual experiments, the respective effects of single impurity defects on laser damage resistance of fused silica optics are hard to be directly and quantitatively validated. Furthermore, the transient temperature caused by absorptive impurity defects is very high and it only lasts for such a short time that experimentally acquiring the transient high temperature is quite a challenge. Thus, it is very difficult to detect the accurate temperature evolution on the laser-irradiated silica surface to directly justify the simulation results in this work. However, the experimental results of the laser damage test show that the impurity defects are probably the absorbing initiators that incur the laser damage event.