Investigation of surface damage precursor evolutions and laser-induced damage threshold improvement mechanism during Ion beam etching of fused silica

Study on the Absorption Characteristics and Laser Damage Properties of Fused Silica Optics under Flexible Polishing and Shallow DCE Process

The enhancement of laser damage resistance of fused silica optics was a hotspot in scientific research. At present, a variety of modern processes have been produced to improve the laser induced damage threshold (LIDT) of fused silica optics. They included pre-treatment processes represented by flexible computer controlled optical surfacing (CCOS), magnetorheological finishing (MRF), ion beam finishing (IBF), and post-treatment processes represented by dynamic chemical etching (DCE). These have achieved remarkable results. However, there are still some problems that need to be solved urgently, such as excessive material removal, surface accuracy fluctuation in the DCE process, and the pollution in MRF process, etc. In view of above problems, an MRF, CCOS, IBF and shallow DCE combined technique was used to process fused silica optics. The surface morphology could be greatly controlled and chemical etching depth was reduced, while the LIDT increased steadily. After processing by this combined technique, the LIDT increased to 12.1 J/cm2 and the laser damage resistance properties of fused silica were significantly enhanced. In general, the MRF, IBF, CCOS and shallow DCE combined technique brought much help to the enhancement of laser damage resistance of fused silica, and could be used as a process route in the manufacturing process of fused silica.

Rapid and Non-Destructive Repair of Fused Silica with Cluster Damage by Magnetorheological Removing Method

The damage repair of fused silica based on the CO₂ laser repair technique has been successfully applied in high-power laser systems in the controllable nuclear fusion field. However, this kind of repairing technique mainly focuses on large-scale laser damage with sizes larger than 200 μm, but ignores the influence of cluster small-scale damage with sizes smaller than 50 μm. In order to inhibit the growth of small-scale damage and further improve the effect of fused silica damage repair, this paper carried out a study on the repair of fused silica damage using the magnetorheological (MR) removing method. The feasibility of fused silica damage repairing was verified, and the evolution law of the number, morphology, and the surface roughness of small-scale damage were all analyzed. The results showed that the MR removing method was non-destructive compared to traditional repairing technologies. It not only effectively improved the whole damage repairing rate to more than 90%, but it also restored the optical properties and surface roughness of the damaged components in the repairing process. Based on the study of the MR removing repair law, a combined repairing process of 4 μm MR removal and 700 nm computer controlled optical surfacing (CCOS) removal is proposed. A typical fused silica element was experimentally repaired to verify the process parameters. The repairing rate of small-scale damage was up to 90.4%, and the surface roughness was restored to the level before repairing. The experimental results validate the effectiveness and feasibility of the combined repairing process. This work provides an effective method for the small-scale damage repairing of fused silica components.

Ground fused silica processed by combined chemical etching and CO2 laser polishing with super-smooth surface and high damage resistance

Laser damage in fused silica, particularly ultraviolet laser damage, is still a key problem limiting the development of high-power laser systems. In this Letter, a combined process of chemical etching and CO₂ laser polishing was applied to ground fused silica. A super-smooth surface with a root-mean-square roughness of 0.25 nm was achieved through this combined process. Furthermore, the combined process can reduce the introduction of photoactive metal impurity elements, destructive defects, and chemical-structure defects, resulting in a 0% probability damage threshold nearly 33% higher than a conventional chemical mechanical polished sample for a 7.6 ns pulse at a wavelength of 355 nm.

Enhancement of the Load Capacity of High-Energy Laser Monocrystalline Silicon Reflector Based on the Selection of Surface Lattice Defects

Various defects during the manufacture of a high-energy laser monocrystalline silicon reflector will increase the energy absorption rate of the substrate and worsen the optical properties. Micron-scale or larger manufacturing defects have been inhibited by mechanism study and improvement in technology, but the substrate performance still fails to satisfy the application demand. We focus on the changes in the optical properties affected by nanoscale and Angstrom lattice defects on the surface of monocrystalline silicon and acquire the expected high reflectivity and low absorptivity through deterministic control of its defect state. Based on the first principles, the band structures and optical properties of two typical defect models of monocrystalline silicon-namely, atomic vacancy and lattice dislocation-were analyzed by molecular dynamics simulations. The results showed that the reflectivity of the vacancy defect was higher than that of the dislocation defect, and elevating the proportion of the vacancy defect could improve the performance of the monocrystalline silicon in infrared (IR) band. To verify the results of simulations, the combined Ion Beam Figuring (IBF) and Chemical Mechanical Polishing (CMP) technologies were applied to introduce the vacancy defect and reduce the thickness of defect layer. After the process, the reflectivity of the monocrystalline silicon element increased by 5% in the visible light band and by 12% in the IR band. Finally, in the photothermal absorption test at 1064 nm, the photothermal absorption of the element was reduced by 80.5%. Intense laser usability on the monocrystalline silicon surface was achieved, and the effectiveness and feasibility of deterministic regulation of optical properties were verified. This concept will be widely applied in future high-energy laser system and X-ray reflectors.

Effects of Ion Beam Etching on the Nanoscale Damage Precursor Evolution of Fused Silica

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.

Understanding the role of fluorine-containing plasma on optical properties of fused silica optics during the combined process of RIE and DCE

Reactive ion etching (RIE) is crucial for fabricating high-quality fused silica optics since this technique can be used as a first step before dynamic chemical etching (DCE) for tracelessly removing the fractured defects in subsurface layer. The final quality of the optics is dramatically influenced by the plasma etching condition but still lacks sufficient information for practical application. In this work, combination of RIE and DCE was investigated deeply on polished fused silica surface by changing the gas type and flow rate. We show that the proper choice of fluorine-containing plasma condition during the RIE process allows the simultaneous occurrence of high surface quality and a low concentration of etching-introduced defects on fused silica. This leads to an ultrahigh laser-induced damage threshold at 355 nm while substantially keeping the surface roughness unchanged. This study paves the way for designing and developing a next-generation surface modification ability of high-quality fused silica with the great potential for high-power laser application.

Detailed near-surface nanoscale damage precursor measurement and characterization of fused silica optics assisted by ion beam etching

Near-surface nanoscale damage precursor generated from the fabrication process has great influence on laser-induced damage threshold improvement of fused silica. In this work, high-resolution transmission electron microscopy (HRTEM) is used to characterize the arrangement of material particles near surface. The nanoscale defects in the Beilby layer could be clearly distinguished. And we find ion beam etching (IBE) has little effect on the arrangement of material particles. This microscopic phenomenon makes IBE a promising technique for the detection of nanoscale near-surface damage precursors. To further investigate the nanoscale near-surface damage after chemical mechanical polishing, a trench is generated by ion sputtering to contain the nature and characteristics of nanoscale precursors in different depths. The evolutions of chemical structure defects and nanoparticles are measured and their laser-induced absorption performance are tested. The results show that there is a nanoscale defect layer (~360nm) beneath the Beilby layer. A model for nanoscale defect layer of fused silica after CMP is offered. In the model, the quantitative density of nanoparticles falls exponentially with increasing the depth and the contents of ODC and NBOHC decreases linearly, respectively. Research results can be a reference on characterizing nanoscale defects near surface and conducting post-processing technologies to improve the laser damage resistance property of fused silica.

Research on the Surface Evolution of Single Crystal Silicon Mirror Contaminated by Metallic Elements during Elastic Jet Polishing Techniques

Metallic elements can contaminate single crystal silicon mirror during ion beam etching (IBE) and other postprocessing methods, which can affect the performance of components in an infrared laser system. In this work, scanning electron microscope (SEM) and atomic force microscope (AFM) were used to characterize the distribution of contaminant represented by aluminum (Al). After characterizing contaminated area, elastic jet polishing (EJP), EJP, and static alkaline etching (SAE) combined technique were used to process the mirror. The morphology and laser-induced absorption were measured. Results show that metallic elements can mix with silicon and generate bulges due to the sputtering effect. In addition, SAE and EJP combined technique can remove metallic contaminant and stabilize the surface quality. Research results can be a reference on conducting postprocessing technologies to improve laser damage resistance property of single crystal silicon mirror in infrared laser system.

Effects of combined process of reactive ion etching and dynamic chemical etching on UV laser damage resistance and surface quality of fused silica optics

We investigate the interest of combined process of reactive ion etching (RIE) and dynamic chemical etching (DCE) as a final step after polishing to improve the laser damage resistance of fused silica optics at the wavelength of 355 nm. The investigation is carried out on the polished fused silica optics by changing the RIE depth while keeping the DCE depth fixed. We evidence that the combined etching process can effectively remove the damage precursors on the fused silica surface and thus improve its laser-induced damage threshold exceeding the level of the deep HF-etched surface. The effects of the combined etching depth on the surface roughness and surface error are also studied systematically. We show that the combined shallow etching can achieve better overall surface quality. Deeper etching will cause surface quality degradation of the fused silica optics, which is believed to be associated with the chemical etching during the combined process. Given that HF acid processing will degrade the surface quality of fused silica optics, the combined shallow etching appears as a pertinent alternative to HF-based deep etching.

Ultraviolet Laser Damage Dependence on Contamination Concentration in Fused Silica Optics during Reactive Ion Etching Process

The reactive ion etching (RIE) process of fused silica is often accompanied by surface contamination, which seriously degrades the ultraviolet laser damage performance of the optics. In this study, we find that the contamination behavior on the fused silica surface is very sensitive to the RIE process which can be significantly optimized by changing the plasma generating conditions such as discharge mode, etchant gas and electrode material. Additionally, an optimized RIE process is proposed to thoroughly remove polishing-introduced contamination and efficiently prevent the introduction of other contamination during the etching process. The research demonstrates the feasibility of improving the damage performance of fused silica optics by using the RIE technique.

Investigation of surface damage precursor evolutions and laser-induced damage threshold improvement mechanism during Ion beam etching of fused silica

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.