Fused Silica Surface Research Articles

Experimental and numerical investigation of CO2 laser ablation of fused silica with sub-microsecond pulses

The CO2 laser material interaction is commonly used for thermal treatments and processing of fused silica glasses. As the laser pulse duration decreases down to a few tens of microseconds, the heat-affected depth in the material decreases up to the point where it has the same magnitude as the laser radiation penetration depth, which is an interesting operating point for applications that require minimal heat-affected zone. In this work, we explore the effects of CO2 laser pulses in the range of 100 μs to a few milliseconds on the laser ablation of polished fused silica surfaces, based on experiments and numerical simulations. We particularly study the evolution of surface profile as a function of the number of applied pulses. The results suggest that the ablation depth can be accurately controlled from a few hundreds of nanometers to a few tens of micrometers by adjusting the combination of the number of applied pulses and pulse duration.

Study on Pitch Performance Deterioration in Chemical Mechanical Polishing of Fused Silica

Fused silica (FS) has been widely applied in astronomical telescopes, laser systems, and optical communication. The excellent fabrication of a fused silica surface requires high efficiency and high smoothness. Here, based on the ceria slurry and the pitch lap, the study of chemical mechanical polishing (CMP) of FS surface was carried out. The variations of the polishing removal rate (RR) and polished surface roughness Rq of FS with the polishing time were investigated. Meanwhile, the variations of the pitch performance during the CMP process were analyzed. Compared with the original pitch, the Rq of the used pitch presented an obvious decrement and the thickness of the used pitch was reduced. Scanning electronic microscopy was used to observe the morphologic change of the pitch. Attenuated total reflection-Fourier transform infrared spectroscopy was used to evaluate the chemical change of the pitch. The surface roughness Rq of the pitch was characterized. The viscoelastic characteristics of the pitch were investigated by dynamic shear rheometer tests. The relative mechanism of the pitch performance deterioration was discussed.

Molecular investigation on the desorption process of alkane contaminant from fused silica surface in nonionic surfactant solution

In this article, the molecular level investigation on the detachment process of the alkane contaminant from the fused silica surface in nonionic surfactant solution is performed by using molecular dynamics simulation. Three important cleaning parameters including surfactant molecule structure, temperature and surfactant concentration are discussed. We found that the hydrophilic tail of the nonionic surfactant molecule plays a leading role in the desorption process of the oil droplet. The activated chemical activity in surfactant solution with the increasing temperature of solution facilitates the desorption of alkane contaminant. The critical surfactant concentration is vital for cleaning in a real situation. High surfactant concentration can facilitate the desorption of oil droplet in a static solution. The efficiency of water flooding decrease, when the concentration of surfactant is high. Our study can help to understand the cleaning mechanism in surfactant solution and is important for its application in industrial cleaning.

A recycling strategy of ion beam removal and recoating of sol-gel film on fused silica surface

A recycling strategy is presented on ion beam removal and recoating of sol-gel film on fused silica surface, which is based on non-contact inert ion beam etching to remove sol-gel films followed by recoating on the surface of fused silica to realize the recycling of the anti-reflection films on optical components. Through the characterization and analysis of optical properties of the samples before and after etching, the influence of ion energy on the film removal is studied and further optimized. The results indicate that the ion beam removal method does not degrade the performance of the fused silica substrate. After recoating of sol-gel porous silica film, the surface contact angle, surface roughness, optical transmittance, and laser damage threshold are characterized and compared with that of the original coated samples. The results show that the performance is as good as or even better than that of the original coated fused silica, which verifies the effectiveness of the recycling method presented in this work.

Role of each step in the combined treatment of reactive ion etching and dynamic chemical etching for improving the laser-induced damage resistance of fused silica.

We investigate the role of each step in the combined treatment of reactive ion etching (RIE) and dynamic chemical etching (DCE) for improving the laser-induced damage resistance of fused silica optics. We employ various surface analytical methods to identify the possible damage precursors on fused silica surfaces treated with different processes (RIE, DCE, and their combination). The results show that RIE-induced defects, including F contamination, broken Si-O bonds, luminescence defects (i.e., NBOHCs and ODCs), and material densification, are potential factors that limit the improvement of laser-induced damage resistance of the optics. Although being capable of eliminating the above factors, the DCE treatment can achieve rough optical surface with masses of exposed scratches and pits which might serve as reservoirs of the deposits such as inorganic salts, thus limiting the further improvement in damage resistance of fused silica. The study guides us to a deep understanding of the laser-induced damage process in achieving fused silica optics with enhanced resistance to laser-induced damage by the combined treatment of RIE and DCE.

Quantification of Carbonic Contamination of Fused Silica Surfaces at Different Stages of Classical Optics Manufacturing.

The chemical composition of ground and polished fused silica glass surfaces plays a decisive role in different applications of optics. In particular, a high level of carbon impurities is often undesirable for further processing and especially for gluing or cementing where adhesion failure may be attributed to carbonic surface-adherent contaminants. In this study, the surface carbon content at different stages of classical optics manufacturing was thus investigated. Two different standard processes—grinding and lapping with two final polishing processes using both polyurethane and pitch pads—were considered. After each process step, the chemical composition and roughness of the surface were analysed using X-ray photoelectron spectroscopy and atomic force microscopy. An obvious correlation between surface roughness and effective surface area, respectively, and the proportion of carbon contamination was observed. The lowest carbon contamination was found in case of lapped and pitch polished surfaces.

Alteration of the chemical composition of fused silica surfaces via combined hydrogenous plasma treatment and UV laser irradiation

In this contribution, we report on the influence of nitrogen-hydrogen plasma treatment on the chemical composition of fused silica surfaces with and without additional argon fluoride laser irradiation. Therefore, the compositions of pristine and plasma treated fused silica samples were investigated by secondary ion mass spectrometry before and after laser irradiation. Transmission data were moreover obtained via UV-VIS spectroscopy. The plasma treatment most likely generates optically active defects such as non-bridging oxygen hole centers (NBOHCs), oxygen-deficiency-related defects (ODCs), and E' centers, which serve as precursor sites for hydrogen saturated defects. The resulting enhanced laser coupling leads to a loss of oxygen from the fused silica network and a further generation of defect centers. Possible explanatory approaches for the observed plasma- and laser-induced changes in carbon, hydrogen, and oxygen content are discussed. This includes the formation of silicon monoxide, hydrogen saturated defects, and nanocrystalline domains in the matrix containing silicon-carbon bonds.

Effect on nanoscale damage precursors of fused silica with wet etching in KOH solutions

We investigate the nanoscale damage precursors that will cause laser damage initiation on fused silica surface during KOH-based wet etching. Some nanoscale damage precursors, like impurity contamination and chemical structure defects on different etched surface with a KOH solution, are explored through a variety of testing methods at nanoscale spatial resolution. The etched surface roughness and photothermal absorption level are also studied. The results show that KOH-based etching can keep a good surface roughness, reduce impurity contamination significantly, and thus decrease surface photothermal absorption level. However, it can mitigate little chemical structure defect and has a risk of secondary pollution induced by residual deposition such as K2SiO3. The work can be a reference on using KOH-based wet etching technology to mitigate nanoscale damage precursors of fused silica ultraviolet optics.

Understanding the effect of HF-based wet shallow etching on optical performance of reactive-ion-etched fused silica optics.

The optical performance of fused silica optics used in high-power lasers is known to depend not only on their surface damage resistance, but also on their surface quality. Previous studies have shown that good fused silica damage performance and surface quality can be achieved by the use of reactive ion etching (RIE), followed by HF-based wet shallow etching (3 μm). In this study, two kinds of HF-based etchants (aqueous HF and HF/NH4F solutions) were employed to investigate the effect of HF-based etching on the optical performance of reactive-ion-etched fused silica surfaces at various HF-based shallow etching depths. The results showed that the addition of NH4F to HF solution makes it possible to produce a high-quality optical surface with a high laser-induced damage threshold, which is strongly associated with the surface roughness and fluorescence defect density. Additionally, changing the HF-based etching depth over the range from 1 μm to 3 μm can affect the surface damage resistance and absorption performance of RIE-treated fused silica. The light-scattering results indicate that the point defect density plays an important role in the determination of the HF-based etching depth. Understanding these trends can enable the advantages of the combined technique of RIE and HF-based etching during the fabrication of high-quality fused silica optics.

Light modulation performance control of the coating on the repaired damage sites in fused silica surface

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Influence of Adsorbed Metal Atoms on Light Absorption by a Fused Silica Surface

Optical properties of submonolayer metal (silver, gold, and indium) films obtained by laser ablation on a fused silica surface have been investigated. Extinction and absorption spectra of silica and substrates with deposited films and difference spectra are obtained. The minimum values of the effective metal-film thicknesses, at which islands begin to nucleate and localized plasmon resonances manifest themselves, are determined. It is shown that the absorption of the silica substrate–metal film complex exceeds the sum of absorptions by the silica surface and metal atoms. A relationship between this phenomenon and charge transfer from metal to defects on the substrate surface is revealed.

Adsorption properties of iodine on fused silica surfaces when evaporated from tellurium in various atmospheres

The evaporation of iodine containing species from tellurium has been investigated together with their adsorption behavior on a fused silica surface. In inert gas, the formation of two species was observed with adsorption enthalpies of around − 90 to − 100 and − 110 to − 120 kJ/mol, respectively. For reducing environments, a single species identified as monatomic iodine was observed with an adsorption enthalpy around − 95 kJ/mol. In oxidizing conditions, species with low adsorption enthalpies ranging from − 65 to − 80 kJ/mol were observed. Presumably, these are iodine oxides as well as oxo-acids of iodine (HIOx). The results of the thermochromatography experiments performed here prove the usefulness of the employed production method for carrier-free iodine isotopes and enhance the understanding of the evaporation and deposition behavior of iodine under various chemical conditions.

Rapid CO2 laser processing technique for fabrication of micro-optics and micro-structures on fused silica materials

Micro-optics and micro-structures play important roles in the field of optics. A multi-step processing strategy with the combination of various processing technologies has been applied to fabricate micro-optics and micro-structures. However, the multi-step processing method is complex and expensive. In this work, a rapid CO2 laser processing technique is proposed to fabricate micro-optics and micro-structures on fused silica materials. First, high-power and short-pulse CO2 laser is used to achieve rapid prototyping of micro-optics and micro-structures with pre-designed geometry on fused silica. In this step, a maximum material removal rate of 1.53 mm3/min could be achieved with surface roughness better than 100 nm. Then, using the same CO2 laser source with reduced laser power density, the initially processed fused silica surface could be smoothed to improve the surface quality. By simulating the CO2 laser interaction with fused silica material, the formation mechanism of smooth surface is revealed, and the processing parameters for achieving smooth silica surface are proposed. The surface roughness of finally processed silica surface could reach 10.8 nm. Finally, as an application example of the processed two-step CO2 laser processing method, a micro-structure with special hexagonal shape on fused silica optics is successfully processed. The proposed rapid CO2 laser processing technique for the fabrication of micro-optics and micro-structures on fused silica materials can be realized with only one equipment, which can not only ensure the processing accuracy and efficiency but also reduce the processing cost.

The impurity study of MRF processed fused silica surface

MRF (magnetorheological finishing) will leave impurities and affect the LIDT (laser induced damage threshold) performance of fused silica components. This manuscript first measured the impurity distribution after MRF. Then three-dimensional FDTD (finite-difference time-domain) simulation was employed to study the light field distribution of impurities under laser irradiation and the relationship between the dielectric constant, the size of the impurities and LIEF (light intensity enhancement factor). At last the HF etching was verified to remove the impurities. Studies have shown that carbonyl iron and cerium oxide impurities exist after MRF. FDTD simulation showed for impurities of the same size, the LIEF value first decreases and then increases with the dielectric constant; while for impurities of the same dielectric constant, the LIEF value increases as the size increases. Finally, HF etching can effectively remove impurities on MRF processed fused silica components.

Study of UV repetition laser-induced absorption on fused silica surface using a surface thermal lensing technique.

A surface thermal lensing technique is used to study the time evolution of the absorption of a fused silica surface when exposed to UV pulsed laser irradiation in the nanosecond regime. The time evolution of the absorption is characterized as a saturated exponential increase, and the disruptive change indicates that the macrodamage occurs. A time-dependent absorption defect model is used to fit the experimental data, and a single-photon process is proposed to explain the results. This method can be used to estimate the operation lifetime of optical components.

Laser-Induced Dissociation of the Monolayer of Adsorbed Methanol Molecules

Astronomical observations indicate a high abundance of methanol molecules in the gas phase of molecular-cloud dense cores, which cannot be explained by gas-phase chemical reactions only. A significant contribution to the methanol abundance should be provided by chemical reactions on the dust particle surface with subsequent desorption of the produced molecules into the gas phase. For the development and refinement of models involving these processes, laboratory studies of photo-induced processes occurring in the adsorbed material are necessary. In this paper, the experiment results of adsorbed methanol molecules are presented. A methanol molecule monolayer, physically adsorbed on fused silica surface cooled by liquid nitrogen (Т ∼ 100 K), was irradiated in high vacuum by nanosecond pulses of an excimer KrF laser with a fixed wavelength λ = 248 nm. The photodissociation products of three-photon laser excitation were recorded by a quadrupole mass spectrometer. Relative yields of photofragments H, OH, and CH3 were determined. Photolysis of partially deuterated CH3OH molecules has shown that hydrogen atoms can be ejected both from hydroxyl and methyl groups. In contrast to the isolated molecule photolysis in the gas phase and dissociation of the multilayer molecular coatings, photoexcitation of adsorbed methanol monolayer even in the energy region of 10 eV does not cause noticeable chemical transformations and does not lead to the formation of molecular components H2 and CH4. Due to existing astrochemical modeling problems, possible application methods of the obtained laboratory results are considered.

Anisotropic ion beam etching of fused silica to mitigate subsurface damage

The laser damage resistance of fused silica optics depends significantly on the surface quality. In this work, anisotropic etching with inert ion beams at various ion incident angles was performed to investigate the evolution of the fused silica surface. The results show that the surface is smoothed when the incident angle is below [Formula: see text]. However, the fused silica surface starts to become coarse owing to the formation of nanostructures on the surface when the incident angle exceeds [Formula: see text]. Further, ion beam etching at a large incident angle of [Formula: see text] removes subsurface defects and less induces nanostructures, resulting in reduction of the surface roughness. The concentrations of impurities and defects are both significantly reduced after ion beam etching. The surface quality, subsurface and surface defects, and surface impurities determine the variation in the laser damage threshold of fused silica with the ion incident angle. The results demonstrate successful application of ion beam etching to improve the laser damage resistant characteristics of fused silica optics. Ion beam etching is a very versatile tool that provides physical erosion to anisotropically mitigate surface damage of fused silica.

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.

Top-hat and Gaussian laser beam smoothing of ground fused silica surface

Laser smoothing is a promising technique for processing optical glass due non-contact characteristics and easy-adaptability to non-spherical surfaces. The smoothing effects of CO2 laser of top-hat and Gaussian intensity on ground fused silica surfaces were examined in the work. The results show that ground surface with surface micro-roughness of 500 nm (RMS) can be smoothed to <0.5 nm (RMS) with both intensity profiles. Best surface roughness rises with increasing power density of laser and therefore lower power density combined with slow scanning speed is favored to improve surface quality. Numerical model was established to investigate thermal and hydrodynamic transient process to elucidate the underlying mechanism. Simulations show that smoothing process takes place at the same time as the temperature rises due to ever-decreasing viscosity of fused silica. Beam shaping is able to reduce peak power density of high power processing laser and smoothing efficiency might be increased without the loss of the processed quality of glass.

Three dimensional numerical simulation of modulation by scratches on fused silica surface

The scratches generated during optical manufacturing influence the LIDT (laser induced damage threshold) performance of fused silica components. In order to get a better understanding of this influence, the three dimensional FDTD (finite-difference time-domain) simulation is employed to study the light field distribution of scratches under laser irradiation and the relationship between the location, geometry and distribution of scratches and LIEF (light intensity enhancement factor). The results show that scratches located on the exit surface are more likely to induce damage. The geometric size of a single scratch greatly affects its LIEF value. The LIEF of a scratch with the same geometry increases as the width increases. While if the width of the scratch is given, its LIEF increases firstly, then decreases, eventually it tends to be stable as the geometry varies. For Hertzian scratches, the LIEF of adjacent scratches is larger than that of a single scratch due to the superposition of field strength; when the distance between Hertzian scratches becomes farther, the superposition of field strength gradually weakens, and finally reaches the same level as that of a single scratch. These results can help controlling the scratches to improve the quality of optical components.