Fused Silica Surface Research Articles

Partially (resp. fully) reversible adsorption of monoterpenes (resp. alkanes and cycloalkanes) to fused silica.

This work compares the extent of reversibility and the thermodynamics of adsorption (Kads, ΔG°ads) of room-temperature vapors of common environmentally relevant monoterpenes (α-pinene, β-pinene, limonene, and 3-carene) and industrially relevant cyclic and acyclic non-terpene hydrocarbons (cyclohexane, hexane, octane, and cyclooctane) to fused silica surfaces. Vibrational sum frequency generation spectroscopy carried out in the C-H stretching region shows negligible surface coverage-dependent changes in the molecular orientation of all species surveyed except for cyclohexane. The group of monoterpenes studied here distinctly exhibits partially reversible adsorption to fused silica surfaces compared to the group of non-terpene hydrocarbons, demonstrating a link between molecular structure and adsorption thermodynamics. The standard Gibbs free energy of adsorption is nonlinearly correlated with the equilibrium vapor pressure of the compounds surveyed.

Toward little heat-affected area of fused silica materials using short pulse and high power CO2 laser

Abstract As a rapid and efficient processing technology, CO2 laser processing has been applied to remove fused silica materials. However, thermal effect caused by CO2 laser would lead to thermal modification layer, thermal deformation and residual stress in processed fused silica optics. To reduce the thermal effect, high power and short pulse CO2 laser is applied to remove the surface material of fused silica. In this work, a model coupling the physical processes of heat transfer and phase transition was established to simulate the complex interaction of CO2 laser with fused silica. Then, the model was applied to investigate the evolutions of temperature distribution and surface morphology involved in the laser ablating and melting processes. The results indicate that with the increase of laser power and decrease of pulse width, positive effect would take place that the radius-depth ratio of ablating area increases and the heat-affected area becomes smaller. For the application of CO2 laser with 0.2 μs pulse width and 800 kW peak power, fused silica material would quickly reach the ablation state. The ablation depth is less than 1 μm, and the thickness of heat-affected area is about 80 nm. It means that the high power and short pulse CO2 laser is equivalent to a hot grinding ball head, which can remove the material rapidly with small heat-affected area on fused silica surface. The simulation results of line scanning processing with CO2 laser were finally compared with the experiment ones, which verified the feasibility of the established model. The suggestions and prospective for improving the model were put forward as well.

Investigation of Non-Uniformity of Classically-Polished Fused Silica Surfaces via Laser-Induced Breakdown Spectroscopy

In this contribution, the surface uniformity of classically-manufactured fused silica windows was investigated via laser-induced breakdown spectroscopy. It is shown that for all investigated samples a comparatively high aluminium content was found at the edge of the surface with respect to its centre. This contamination can be attributed to residues from lapping and polishing agents and leads to a mentionable non-uniformity of the surface in terms of chemical composition and optical properties, respectively.

Optical transmission during mid-infrared femtosecond laser pulses ablation of fused silica

We report on the optical transmission of near-infrared (1.8 μm) femtosecond laser pulses during multiple pulses laser ablation of fused silica surface. In addition to the previously reported ciliary white light phenomenon (Phys. Rev. Lett. 110, 097601 (2013)), two new optical transmission phenomena were observed for relatively low energy laser pulses. We systemically examined the damage morphologies of the ablation site as a function of laser pulse energy and laser shot number. Laser induced periodic surface structures (LIPSS) with different periods and orientations were observed for different pulse energy regimes. Comparison of the damage morphologies and the optical transmission reveals that the two new optical transmission phenomena origin from the refraction of the incident near-infrared pulses on the LIPSS covered damage surface followed by nonlinear propagation of the beamlets and even filamentation if the laser energy was high enough. We found that the interference of the neighboring white light emissions gives rise to radially orientated optical fringes. Based on this observation, we proposed another interpretation for the ciliary white light phenomenon.

Investigation of global thermal effect of large fused silica surface figuring using inductively coupled plasma

Inductively coupled plasma (ICP) machining at atmospheric pressure has been used widely in fabrication of optical elements. However, the non-convergence caused by the global thermal effect is regarded as an inevitably issue for applying ICP to figure large fused silica optical surface. In this paper, according to the detailed investigation of the surface temperature distribution of the global thermal effect, a new method of round-trip repetitive tool path is proposed as a convenient way to balance the global thermal effect for making large fused silica surface error rapidly convergence. Experiment result shows that a work-piece with a size of 200 × 200 × 20 mm3 is successfully figured with this method. Result demonstrates that this method can realize rapid convergence for large fused silica optical surface. The presented study can provide convenient technical guidance of efficient fabrication for large fused silica optics by ICP surface figuring.

Effect of argon cluster ion beam on fused silica surface morphology

The gas cluster ion bombardment technique is promising for low-damage surface smoothing of optical materials. In this work, the morphology evolution of fused silica surfaces during argon cluster ion beam processing was investigated. To characterize the surface roughness, power spectral density functions (PSD) were obtained from a wide spatial frequency range of measurements. The power spectral density function dependencies on the depth of the removed material and cluster ion parameters are shown. Based on the experimental results, it is demonstrated that the use of gas clusters of different sizes and specific energy allows a fused silica surface to be smoothed in the middle spatial frequency (0.01–1 μm−1) and high spatial frequency (1–100 μm−1) ranges.

Characterisation of the fused silica surface quality with a [formula omitted]-source

A method to characterise the quality of a fused silica surface using a β-source is presented. Two fused silica bars (5×10×400mm3) were fabricated for the Cherenkov detector for proton Flux Measurement installed at vacuum chamber of the Super Proton Synchrotron at CERN. The resolution of such device is defined by the collection efficiency of the Cherenkov light, which is produced by relativistic charged particles in the fused silica. Thus, the surface quality of the radiator should be as good as possible to avoid light losses. The method is based on the scanning of the radiator surface with a 90Sr radioactive source and measurements of the Cherenkov light rate, detected by a PMT attached to the quartz bars. The data have been compared with a Monte-Carlo simulation, providing an estimation of the radiator’s probability of the total internal reflection and inefficient area at the edges of the bars.

Investigation of the Chemical Residuals on the Fused Silica during Chemical Mechanical Polishing

AbstractChemical residuals on fused silica after chemical mechanical polishing with silica and ceria have been analyzed. The surface of fused silica terminates with silanol after being polished with silica abrasives but ends up with Ce‐O−Si in a ceria‐based slurry under the CMP environment. The silanol firstly synthesizes on a fused silica surface polished with ceria, but then further reacts with hydroxyl cerium groups scattering in the slurry forming Ce‐O−Si. It also reveals that polishing fused silica with silica abrasives is an alternative way giving an ultra smooth surface.

Terrace morphology on fused silica surfaces by Ar+ ion bombardment with Mo co-deposition

The morphology evolution of self-organized nanopatterns induced during Ar+ ion bombardment (IB) with Mo co-deposition on fused silica (SiO2) surfaces at different incidence angles and fluences was investigated by using atomic force microscopy and transmission electron microscopy. For pure IB at incidence angles from 30° to 70°, SiO2 surfaces evolve from being flat, via ripples, to direction-transversed ripples. In contrast, at the same ion fluence and incidence angles, the simultaneous Mo co-deposition leads to significant terraced structures with significantly enhanced roughness and wavelength. Our observations show that the concurrent Mo co-deposition during IB can reduce the critical incidence angle and the fluence level of terrace formation. Owing to the guidance of the IB-induced morphology, at incidence angles where a well-ordered ripple-mode can be generated, well-ordered terrace morphology is more likely to be formed. Terraced structures are initiated and further grow until the appearance of the nonlinear phase, i.e., where the ripple amplitude is sufficiently high. The enhanced terrace morphology on smooth SiO2 results from the interplay between pure IB and Mo co-deposition. The phase separation is attributed to the formation of crystalline MoOx on the side facing the impurity.

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.

Nano-sized anion-exchangers as a stationary phase in capillary electrochromatography for separation and on-line concentration of carboxylic acids

Nano-sized anion-exchangers (NSAE) are promising materials in electrophoretic separation methods due to their high ion-exchange capacity, large surface-to-volume ratios, high adhesion to the quartz surface and pH-independent positive charge. In current research we describe a simple approach for NSAE synthesis, which includes two-step grinding of macroanionite followed by centrifugation. The synthesized stable aqueous suspension of NSAE particles was applied as physically adsorbed modifier of fused-silica capillary walls for CEC separation of carboxylic acids. We proposed fast and simple approach to formation of NSAE-based stationary phase on the internal fused-silica surface, which included 15 min rinsing of the capillary with diluted water suspension of NSAE. Formed physically-adsorbed coating turned out to be extremely stable in a wide range of pH (from 2 to 10). NSAE modified capillaries provided high separation efficiency (N = 148–732 *103 t.p./m) and selectivity (Rs = 1.2–5.7) of carboxylic acids. Simultaneous application of NSAE-modified capillaries with various on-line concentration techniques (such as field amplified sample stacking and field amplified sample injection) provided both low detection limits (up to 1–3 ng/mL) and high separation selectivity of carboxylic acids. It was useful for their quantitative determination in wines samples. Physically-adsorbed coatings based on NSAE exhibit higher selectivity and lower detection limits compared to commonly used dynamic modifier of fused-silica capillary walls - cetyltrimethylammonium bromide. NSAE-based coatings do not require equilibrium sustaining to maintain the surface coverage. It makes them appropriate for CE-MS application.

Adsorption characteristics of F and Cl atoms on fused silica surface defects

F and Cl atoms are always brought by the process of wet etching for fused silica devices, which dramatically influence the performance of the devices. The adsorption characteristics of F and Cl atoms with two main fused silica surface defects (E', NBOHC) were investigated on the basis of first-principle methods. Through the comparison of absorption capacity of different surface defects, we found that E' has larger capability to adsorb halogen atoms than NBOHC, and F atom is easier to be trapped by surface defects than Cl atom. Furthermore, the electronic structure and the bonding nature for surface defects are also studied and the calculations show that the introduction of F and Cl atoms will lead to the disappearing of defect states in band gap. And this adsorption process of F and Cl atoms on fused silica surface can be considered to chemical adsorption.

Ab initio molecular dynamics simulation of the effect of impurities on laser-induced damage of fused silica

The damages of fused silica surface after laser irradiation can be caused by the impurities introduced during manufacture processes. In this paper, Fe and Ce impurities are considered in fused silica. The first-principles method is employed to simulate the process of high-power ultraviolet laser irradiation to fused silica. The bond angle distribution, pair distribution function, the density of state (DOS) and other information of fused silica structures are calculated. There is a remarkable difference in bond length and bond angle distribution between pure and doped fused silica. New defect states are also observed in DOS with the narrower band gap, and new optical absorption peaks appear in the dielectric function for the fused silica with the dopants after laser irradiation. This means that fused silica with impurities has more significant absorption for laser energy than pure fused silica. FeO bonds are found more stable than CeO ones after irradiation. This indicates the damage of Fe doping is greater than that of Ce doping. So, doping of metal indeed leads to a decrease in damage resistance to laser for fused silica, and Fe is more harmful than Ce.

Multifunctional Hierarchical Surface Structures by Femtosecond Laser Processing.

Hierarchical surface structures were fabricated on fused silica by using a fs-laser with a pulse duration τ = 300 fs and a wavelength λ = 512 nm. The resulting surface structures were characterized by scanning electron microscopy, atomic force microscopy and white light interference microscopy. The optical properties were analyzed by transmittance measurements using an integrating sphere and the wettability was evaluated by measuring the water contact angle θ. The silanization of structured fused silica surfaces with trichloro(1H,1H,2H,2H-perfluorooctyl)silane allows to switch the wettability from superhydrophilic (θ = 0°) to superhydrophobic behavior with θ exceeding 150°. It was shown that the structured silica surfaces are a suitable master for negative replica casting and that the hierarchical structures can be transferred to polystyrene. The transmittance of structured fused silica surfaces decreases only slightly when compared to unstructured surfaces, which results in high transparency of the structured samples. Our findings facilitate the fabrication of transparent glass samples with tailored wettability. This might be of particular interest for applications in the fields of optics, microfluidics, and biomaterials.

Scan speed and fluence effects in femtosecond laser induced micro/nano-structures on the surface of fused silica

The influence of scan speed and fluence on the formation of micro/nano-structures were explored under 800 nm, 35 femtosecond (fs) laser irradiation. Surface morphology, roughness, and laser-induced defects of prepared samples were characterized. The antireflection property and wettability of laser treated samples were also tested and compared with that of original fused silica. Results showed that the laser irradiated structures included melt zone, nanofibers, laser-induced periodic surface structures (LIPSS), and fluffy structures. At a fix scan speed, high-spatial-frequency LIPSS (HFSL) were observed in the low fluence region. A transition from HSFL to LSFL (low-spatial-frequency LIPSS) occurs when a critical fluence threshold is exceeded. Otherwise, the scan speed significantly influences the surface roughness and should be controlled carefully during the preparation process. The material structural modification in laser-irradiated fused silica surface was verified by the Raman spectra. The concentration of oxygen deficient centers (ODCs), and non-bridging oxygen hole centers (NBOHCs) increased after femtosecond laser irradiation by analyzing the photoluminescence. The laser irradiated surface has a better antireflection property in comparison with that of original fused silica, which may be due to the scattering and absorption of micro/nano structures. After laser treatment, the micro/nano structured fused silica presented super-hydrophilic property. The contact angle of water droplet decreases can be explained by Wenzel's model.

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.

Plasma-Induced, Self-Masking, One-Step Approach to an Ultrabroadband Antireflective and Superhydrophilic Subwavelength Nanostructured Fused Silica Surface.

In this work, antireflective and superhydrophilic subwavelength nanostructured fused silica surfaces have been created by one-step, self-masking reactive ion etching (RIE). Bare fused silica substrates with no mask were placed in a RIE vacuum chamber, and then nanoscale fluorocarbon masks and subwavelength nanostructures (SWSs) automatically formed on these substrate after the appropriate RIE plasma process. The mechanism of plasma-induced self-masking SWS has been proposed in this paper. Plasma parameter effects on the morphology of SWS have been investigated to achieve perfect nanocone-like SWS for excellent antireflection, including process time, reactive gas, and pressure of the chamber. Optical properties, i.e., antireflection and optical scattering, were simulated by the finite difference time domain (FDTD) method. Calculated data agree well with the experiment results. The optimized SWS show ultrabroadband antireflective property (up to 99% from 500 to 1360 nm). An excellent improvement of transmission was achieved for the deep-ultraviolet (DUV) range. The proposed low-cost, highly efficient, and maskless method was applied to achieve ultrabroadband antireflective and superhydrophilic SWSs on a 100 mm optical window, which promises great potential for applications in the automotive industry, goggles, and optical devices.

Effect of CHF3/Ar Gas Flow Ratio on Self-masking Subwavelength Structures Prepared on Fused Silica Surface

Based on an advanced technology, randomly-aligned subwavelength structures (SWSs) were obtained by a metal-nanodot-induced one-step self-masking reactive-ion-etching process on a fused silica surface. Metal-fluoride (mainly ferrous-fluoride) nanodots induce and gather stable fluorocarbon polymer etching inhibitors in the reactive-ion-etching polymers as masks. Metal fluoride (mainly ferrous fluoride) is produced by the sputtering of argon plasma and the ion-enhanced chemical reaction of metal atoms. With an increase in CHF3/Ar gas flow ratio, the average height of the SWSs increases, the number of SWSs per specific area increases and then decreases, and the optical transmittance of visible light increases and then decreases. The optimum CHF3/Ar gas flow ratio for preparing SWSs is 1:5.

Large-area uniform periodic microstructures on fused silica induced by surface phonon polaritons and incident laser

A simple and convenient means to self-organize large-area uniform periodic microstructures on fused silica by using multiple raster scans of microsecond CO2 laser pulses with beam spot overlapping at normal incidence is presented, which is based on laser-induced periodic surface structures (LIPSS) attributed to the interference between surface phonon polaritons and incident CO2 laser. The evolution of fused silica surface morphologies with increasing raster scans indicates that the period of microstructures changed from 10.6 µm to 9 µm and the profiles of microstructures changed from a sinusoidal curve to a half-sinusoidal shape. Numerical simulation results suggest that the formation of the half-sinusoidal profile is due to the exponential relationship between evaporation rate and surface temperature inducing by the intensive interference between surface phonon polaritons and incident laser. The fabricated uniform periodic microstructures show excellent structural color effect in both forward-diffraction and back-diffraction.

Surface forces between hydrophilic silica surfaces in a moisture-sensitive oleophilic diacrylate monomer liquid

We measured the surface forces generated between fused silica surfaces in a low-viscosity oleophilic diacrylate monomer for reliably repeated ultraviolet (UV) nanoimprinting, and studied the influence of water in monomer liquids on the forces. Fused silica surfaces, with a static contact angle of 52.6 ± 1.7° for water, owing to the low degree of hydroxylation, hardly showed reproducible surface forces with repeated scan cycles, comprising approach and separation, even in an identical liquid monomer medium with both of low and high water content. The monomer liquid with a high water content of approximately 420 ppm showed a greater tendency to increase the surface forces at longer surface-surface distances compared with the monomer liquid with a low water content of approximately 60 ppm. On the other hand, silica surfaces with a water contact angle of < 5° after exposure to vacuum UV (VUV) light under a reduced air pressure showed reproducible profiles of surfaces forces using the monomer with a low water concentration of approximately 60 ppm for repeated surface forces scan cycles even in separately prepared silica surfaces, whilst they showed less reproducible profiles in the liquids with high water content of 430 ppm. These results suggested that water possibly adsorbed on the hydrophilic and hydrophobic silica surfaces in the monomer liquid of the high water concentration influenced the repeatability of the surface forces profiles.