Surface and subsurface damage in optical element will greatly decrease the laser induced damage threshold (LIDT) in the intense laser optical system. Processing damage on the workpiece surface can be inevitably caused when the material is removed in brittle or plastic mode. As a non-contact polishing technology, hydrodynamic effect polishing (HEP) shows very good performance on generating an ultra-smooth surface without damage. The material is removed by chemisorption between nanoparticle and workpiece surface in the elastic mode in HEP. The subsurface damage and surface scratches can be effectively removed after the polishing process. Meanwhile ultra-smooth surface with atomic level surface roughness can be achieved. To investigate the improvement of LIDT of optical workpiece, polishing experiment was conducted on a magnetorheological finishing (MRF) silica glass sample. AFM measurement results show that all the MRF directional plastic marks have been removed clearly and the root-mean-square (rms) surface roughness has decreased from 0.673nm to 0.177nm after HEP process. Laser induced damage experiment was conducted with laser pulse of 1064nm wavelength and 10ns time width. Compared with the original state, the LEDT of the silica glass sample polished by HEP has increased from 29.78J/cm2 to 45.47J/cm2. It demonstrates that LIDT of optical element treated by HEP can be greatly improved for ultra low surface roughness and nearly defect-free surface/subsurface.
Subsurface damage, especially photoactive impurities embedded in the redeposition layer, degrades the performance of high-energy optics in UV or high-power laser systems. The features and distributions of the redeposition layer in classical and magnetorheological finishing polished fused silica were detected and evaluated by a variety of measurements, such as secondary ion mass spectroanalyzer, atomic force microscope, scanning electron microscope, and x-ray photoelectron spectroscopy. Then, a critical particle Reynolds number approach and chemical contribution were applied to interpret the deposition mechanism of impurities, on the basis of which a comprehensive redeposition model of polished optics was presented. Eventually, the relationship between distributions of redeposition materials in depth and freshly polished surface structure was investigated. Results show that the redeposition process of nanoparticles is dominated with the particle Reynolds number and the formation of a Ce─O─Si bond. The impurities in the redeposition layer are mixed with removed glass and present as a uniform dopant. Furthermore, there exists explicit correlation between redeposition layer and subsurface defected layer; so it is easy to achieve planarized surface in the polishing process.
Differing from the traditional pad polishing, hydrodynamic effect polishing (HEP) is non-contact polishing with the wheel floated on the workpiece. A hydrodynamic lubricated film is established between the wheel and the workpiece when the wheel rotates at a certain speed in HEP. Nanoparticles mixed with deionized water are employed as the polishing slurry, and with action of the dynamic pressure, nanoparticles with high chemisorption due to the high specific surface area can easily reacted with the surface atoms forming a linkage with workpiece surface. The surface atoms are dragged away when nanoparticles are transported to separate by the flow shear stress. The development of grand scale integration put extremely high requirements on the surface quality on the silicon wafer with surface roughness at subnanometer and extremely low surface damage. In our experiment a silicon sample was processed by HEP, and the surface topography before and after polishing was observed by the atomic force microscopy. Experiment results show that plastic pits and bumpy structures on the initial surface have been removed away clearly with the removal depth of 140nm by HEP process. The processed surface roughness has been improved from 0.737nm RMS to 0.175nm RMS(10μm×10μm) and the section profile shows peaks of the process surface are almost at the same height. However, the machining ripples on the wheel surface will duplicate on the silicon surface under the action of the hydrodynamic effect. Fluid dynamic simulation demonstrated that the coarse surface on the wheel has greatly influence on the distribution of shear stress and dynamic pressure on the workpiece surface.
A material removal mechanism of ceria particles with different sizes in a glass polishing process was investigated in detail. Contrast polishing experiments were carried out using ceria slurries with two kinds of particle sizes and different amounts of hydrogen peroxide (H 2 O 2 ) added in the slurries. The Ce 3+ ions on the surface of the ceria particles were gradually oxidized to Ce 4+ with increased H 2 O 2 concentration. It was found that the material removal rate (MRR) decreased sharply with an increasing concentration of H 2 O 2 . There was no material removal when the concentration reached 2.0% for nanoparticle slurry. Nevertheless, the application of microparticles made the MRR decrease to a constant value when excessive H 2 O 2 was added. By comparison, we conclude that the material is removed by chemical reaction for ceria nanoparticles, while chemical reaction and mechanical abrasion simultaneously take place for ceria particles with sizes at scale of micrometers in the glass polishing process. It is clearly demonstrated from the experimental results that Ce 3+ instead of Ce 4+ ions play an important role in chemically reacting with the glass surface. An ultrasmooth surface with root-square-mean roughness of 0.272 nm was obtained after being polished by ceria nanoparticles.
This article [Opt. Eng.. 52, (4 ), 043401 (2013)] was originally published on 15 April 2013 with an error on page 6, Sec. 4, paragraph 2. The first sentence “Figure 8 shows the AFM image of surface microstructure images of 10×10 μm on the silica glass surface…” has been corrected to read “Figure 8 shows the AFM image of surface microstructure images of 5×5 μm on the silica glass surface…”
Nanoparticle jet polishing (NJP) is presented as a posttreatment to remove magnetorheological finishing (MRF) marks. In the NJP process the material is removed by chemical impact reaction, and the material removal rate of convex part is larger than that of the concave part. Smoothing thus can progress automatically in the NJP process. In the experiment, a silica glass sample polished by MRF was polished by NJP. Experiment results showed the MRF marks were removed clearly. The uniform polishing process shows that the NJP process can remove the MRF marks without destroying the original surface figure. The surface root-mean-square roughness is improved from 0.72 to 0.41 nm. power spectral density analysis indicates the surface quality is improved, and the experimental result validates effective removal of MRF marks by NJP.