A combination of several surface etching techniques and a superprecise polishing process was used to remove subsurface damage from fused silica surface. The effect of subsurface damage removal on laser-damage resistance was determined by measuring the laser-induced damage threshold (LIDT) at 1064 nm, 532 nm, 355 nm and 266 nm. Surface-damage resistance was enhanced as the irradiated laser wavelength shortens. At 266 nm, the greatest advantage was obtained from virtually subsurface removed surfaces. The surface LIDT improved 2.8-fold compared with that of conventionally polished fused silica surfaces.
The surface-damage resistance of fused silica was enhanced 2.8-fold by removing a subsurface damage. For the conventionally polished fused silica surface, µm-scale subsurface damage and a shallow (20 nm to 100 nm) structurally modified zone produced during grinding and polishing were formed on the top of surface. Several surface etching techniques and super-precise polishing process were used to remove subsurface damage from a fused silica surface. First the conventionally polished surfaces were chemically etched in a buffered HF solution to remove 300µm of surface material, and then super-precise polishing was performed to obtain an optical surface. After that, the polishing compound was removed by using ion-beam etching. The effect of subsurface damage on laser damage resistance was characterized by the measuring of the laser-induced damage threshold (LIDT) for the laser radiations of 1064 nm and 266 nm respectively. For the wavelength of 1064 nm, the effect of the removal of subsurface damage wasn't clearly seen, although the enhancement of surface-damage resistance by the ion-beam etching could be confirmed. However, in the case of 266 nm, enhanced LIDT of 28 J/cm 2 was obtained from the subsurface damage removed surface. The surface LIDT increased by 2.8 times compared to that of conventionally polished fused silica surfaces.
In recent years, it has become clear that 1 μm or less sized fine air bubbles have different properties from those of mm or cm sized air bubbles. This study examined the effect of fine bubbles on the removal of salt fixed in the gap of aluminum plate as a verification of cleaning action by fine bubbles. As a result, it was confirmed that the removal effect of fixed salt is higher in fine bubble water than in control water, and that the degree of effect depends on the concentration of fine bubbles. It was also confirmed that less than 1 μm sized ultrafine bubble water has a higher removal effect than the control water. As a mechanism of removing fixed salt, it is conceivable that peeling effect, like lifting up part of fixed salt, in which fine bubbles are dissolved by pressure change around the fixed salt to become supersaturated, causing further bubbling around there, and/or physical collision effect of microbubbles. In addition, it was confirmed that the removal effect of a system in which fine bubbles were regularly introduced was much higher than the one that does not introduce fine bubbles regularly. In addition, even when the gap between fixed salt and aluminum plate was narrowed, the superiority of fine bubble cleaning was confirmed.
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