Nanoscale laser damage precursors generated from fabrication have emerged as a new bottleneck that limits the laser damage resistance improvement of fused silica optics. In this paper, ion beam etching (IBE) technology is performed to investigate the evolutions of some nanoscale damage precursors (such as contamination and chemical structural defects) in different ion beam etched depths. Surface material structure analyses and laser damage resistance measurements are conducted. The results reveal that IBE has an evident cleaning effect on surfaces. Impurity contamination beneath the polishing redeposition layer can be mitigated through IBE. Chemical structural defects can be significantly reduced, and surface densification is weakened after IBE without damaging the precision of the fused silica surface. The photothermal absorption on the fused silica surface can be decreased by 41.2%, and the laser-induced damage threshold can be raised by 15.2% after IBE at 250 nm. This work serves as an important reference for characterizing nanoscale damage precursors and using IBE technology to increase the laser damage resistance of fused silica optics.
Surface damage precursor evolution has great influence on laser-induced damage threshold improvement of fused silica surface during Ion beam etching. In this work, a series of ion sputtering experiment are carried out to obtain the evolutions of damage precursors (dot-form microstructures, Polishing-Induced Contamination, Hertz scratches, and roughness). Based on ion sputtering theory, surface damage precursor evolutions are analyzed. The results show that the dot-form microstructures will appear during ion beam etching. But as the ion beam etching depth goes up, the dot-form microstructures can be mitigated. And ion-beam etching can broaden and passivate the Hertz scratches without increasing roughness value. A super-smooth surface (0.238nm RMS) can be obtained finally. The relative content of Fe and Ce impurities both significantly reduce after ion beam etching. The laser-induced damage threshold of fused silica is improved by 34% after ion beam etching for 800nm. Research results can be a reference on using ion beam etching process technology to improve laser-induced damage threshold of fused silica optics.
In high-energy laser systems, the energy absorption coefficient of silicon optical elements is one of the most critical performance indicators. The absorption coefficient of substrate limits the absorption of the overall elements. Since mono-crystalline silicon is transparent in working wavelength range, the subsurface absorption precursors also influence the entire absorption dramatically. In this paper, the subsurface of a super-polished silicon substrate is exposed by ion beam etching (IBE) as deep as 4.6 μm. In different depth layers, morphology and energy absorption are measured with an atom force microscope and photothermal instrument, respectively. In the 100 nm layer, microstructures are found, and their heights decrease while widths increase with IBE. Finally, structures are diminished below the 1.12 μm layer. Absorption increases with the structures' appearance. When the structures are fully exposed, absorption reaches the peak value, 327.5% of the unremoved surface. Once structures are removed, the absorption value falls down to the lowest point, 67.5%, which verifies that structures influence the absorption significantly. According to the structure depth and energy dispersive spectrometer results, the structures are most likely the densificated micro zones, generated by fabrication processes. In practical fabrication, a subsurface layer of 1.12 μm thick needs to be removed by stress-less processes, to obtain a low-absorption element.
To realize low-damage ultra-precision grinding on fused silica, the surface quality and subsurface damage (SSD) distribution with fine-grained grinding wheel under different depth-of-cut and cutting speed are experimentally studied. The material removal mechanism under different grinding parameters is revealed by calculating undeformed chip thickness and observed with the help of transmission electron microscopy. The results show that brittleductile surfaces and ductile-like surfaces are generated during grinding. With the decrease of depth-of-cut and the increase of wheel cutting speed, the ultra-precision grinding changes to ductile-regime grinding with plastic flow removal. Besides, the surface roughness (SR) and SSD depth are reduced. The fracture defects such as fractured pits and grinding streaks on brittle-ductile surface gradually decrease. Instead, a ductile-like surface covered with grinding streaks is found. On brittle-ductile surfaces, the nonlinear relationship SSD∝SR 4/3 is no longer proper under the influence of plastic flow. Using surface roughness Ra to predict SSD depth is more accurate. When depth-of-cut is 1 μm, cutting speed is 23.4 m/s and the material removal mode is dominated by plastic flow removal, the surface Ra is improved to 2.0 nm and there is no crack but only a 3.4 nm deep plastic flow layer in subsurface after grinding.
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