Fused Silica Surface Research Articles

Localized CO2 laser treatment and post-heating process to reduce the growth coef f icient of fused silica surface damage

The lifetime of optical components in high-fluence ultraviolet (UV) laser applications is typically limited by laser-initiated damage and its subsequent growth. Using 10.6-μm CO2 laser pulses, we successfully mitigate 355-nm laser induced damage sites on fused silica surface with dimensions less than 200 μm. The damage threshold increases and the damage growth mitigates. However, the growth coefficients of new damage on the CO2 laser processed area are higher than those of the original sample. The damage grows with crack propagation for residual stress after CO2 laser irradiation. Furthermore, post-heating is beneficial to the release of residual stress and slows down the damage growth.

熔石英表面加工引入金属微粒的三倍频激光损伤机制

熔石英表面加工引入金属微粒的三倍频激光损伤机制

Ultra-fast photoluminescence in fused silica surface flaws susceptible to laser damage

ABSTRACTUsing high-sensitivity confocal time-resolved photoluminescence (PL) techniques, we found an ultrafast PL (40 ps-5 ns) from impurity-free surface flaws on fused silica. This PL is excited by the single-photon absorption of sub-band gap light. Regions which exhibit this PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5 eV nanosecond-scale laser pulses. Very high defect densities are needed to explain the damage thresholds observed. For such high defect densities, significant interactions between defects may strongly affect the temporal characteristics of the emission of electronic excitations. We propose that the distribution in lifetimes observed is not simply due to a large variety of defect states, but due to a variety of energy transfer interactions between defect states.

Surface integrity of precision ground fused silica for high power laser applications

Surface integrity of optics used in high power laser systems is vital to their successful application. Reduction of subsurface damage is demanded during their processing. Rapid grinding of large and complex shape optics can be performed using the BoX® ultra precision grinding machine. Importantly, the depth of subsurface damage induced, even at high material removal rates, has been shown to be very small in comparison to previous studies. In this paper, grinding experiments have been conducted to verify the amount of defects beneath ground fused silica surfaces produced under selected processing conditions. The subsurface damage levels were revealed using a sub-aperture polishing process in combination with an etching technique. Observed subsurface damage can be separated in two distinct depth zones characterised as ‘process’ and ‘machine dynamics’ related.

Impact of two CO_2 laser heatings for damage repairing on fused silica surface

CO(2) laser is an interesting tool to repair defects on silica optics. We studied UV nanosecond laser-induced damage in fused silica after CO(2) laser heating. The localization of damage sites and the laser damage threshold are closely related to stress area in silica induced by heating. By applying a suitable second laser heating, we managed to eliminate the debris issued from redeposited silica and to modify the stress area. As a consequence, a significant increase of laser resistance has been observed. This process offers the possibility to improve damage repairing sufficiently to extend the lifetime of the silica components.

Single‐molecule imaging of protein adsorption mechanisms to surfaces

Protein-surface interactions cause the desirable effect of controlled protein adsorption onto biodevices as well as the undesirable effect of protein fouling. The key to controlling protein-surface adsorptions is to identify and quantify the main adsorption mechanisms: adsorptions that occur (1) while depositing a protein solution onto dry surfaces and (2) after the deposition onto wet surfaces. Bulk measurements cannot reveal the dynamic protein adsorption pathways and thus cannot differentiate between the two adsorption mechanisms. We imaged the interactions of single streptavidin molecules with hydrophobic fused-silica surfaces in real-time. We observed both adsorbed proteins on surfaces and diffusing proteins near surfaces and analyzed their adsorption kinetics. Our analysis shows that the protein solution deposition process is the primary mechanism of streptavidin adsorption onto surfaces at the subnanomolar to nanomolar protein concentrations. Furthermore, we found that hydrophilic fused-silica surfaces can prevent the adsorption of streptavidin molecules.

HF‐Based Etching Processes for Improving Laser Damage Resistance of Fused Silica Optical Surfaces

The effect of various HF‐based etching processes on the laser damage resistance of scratched fused silica surfaces has been investigated. Conventionally polished and subsequently scratched fused silica plates were treated by submerging in various HF‐based etchants (HF or NH4F:HF at various ratios and concentrations) under different process conditions (e.g., agitation frequencies, etch times, rinse conditions, and environmental cleanliness). Subsequently, the laser damage resistance (at 351 or 355 nm) of the treated surface was measured. The laser damage resistance was found to be strongly process dependent and scaled inversely with scratch width. The etching process was optimized to remove or prevent the presence of identified precursors (chemical impurities, fracture surfaces, and silica‐based redeposit) known to lead to laser damage initiation. The redeposit precursor was reduced (and hence the damage threshold was increased) by: (1) increasing the SiF62− solubility through reduction in the NH4F concentration and impurity cation impurities, and (2) improving the mass transport of reaction product (SiF62−) (using high‐frequency ultrasonic agitation and excessive spray rinsing) away from the etched surface. A 2D finite element crack‐etching and rinsing mass transport model (incorporating diffusion and advection) was used to predict reaction product concentration. The predictions are consistent with the experimentally observed process trends. The laser damage thresholds also increased with etched amount (up to ∼30 μm), which has been attributed to: (1) etching through lateral cracks where there is poor acid penetration, and (2) increasing the crack opening resulting in increased mass transport rates. With the optimized etch process, laser damage resistance increased dramatically; the average threshold fluence for damage initiation for 30 μm wide scratches increased from 7 to 41 J/cm2, and the statistical probability of damage initiation at 12 J/cm2 of an ensemble of scratches decreased from ∼100 mm−1 of scratch length to ∼0.001 mm−1.

Attenuated total reflectance spectroscopy of coumarin organosilane molecules adsorbed on a fused silica surface

Attenuated total reflectance (ATR) spectroscopy was used to investigate the adsorption of coumarin organosilane molecules onto a fused silica surface. The difference between the absorption spectra of the molecules on the surface and in solution was explained by the interaction of the adsorbed coumarin organosilane molecules with the hydroxyl groups on the fused silica surface. This interaction produces a perturbation of the π electron distribution and the electronic transitions of the coumarin chromophore of the organosilane molecules adsorbed on the surface. From the kinetics adsorption curves, the calculated enthalpy values of 74.8 ± 5.2 kJ mol −1 and free energy of −38.22 ± 0.70 kJ mol −1 at 23 °C indicates a chemisorption process. The high sensitivity of ATR spectroscopy allows the detection of a monolayer formed by a 10 nM concentration of coumarin organosilane molecules, which covers more than half of the maximum surface coverage at 60 °C.

Investigation of the behavior of serum and plasma in a microfluidics system

There are common problems with adsorption of analytes on the surfaces of microfluidic systems with physiological samples such as blood serum, plasma, and urine. The authors’ investigation involves the interaction of serum components with fused-silica surfaces under various flow regimes in microcapillaries. Their focus will include the individual components of serum as well as fresh whole serum. The authors studied the whole serum components in our microfluidic system to uncover the responses of proteins in capillary and microchannel surfaces when influenced by the highly variable serum constituents. They have observed the whole serum with a total protein assay by using the bicinchoninic acid assay in combination with a characterization method, such as SDS polyacrylamide gel electrophoresis, and repeated observations for any change of flow rate in fused-silica capillaries (50 μm inside diameter) under continuous flow. The authors’ preliminary results contradict anecdotal evidence that proteins and other components of serum clog or prevent flow at steady low flow rates.

Plasmon-assisted photoluminescence enhancement of single-walled carbon nanotubes on metal surfaces

We demonstrated photoluminescence (PL) enhancement in single carbon nanotubes using localized surface plasmons. Single nanotube spectroscopy revealed triple the PL intensity enhancement for carbon nanotubes on rough Au surfaces as on fused silica surfaces. The PL enhancement depends on the excitation wavelength and distance between the carbon nanotubes and the Au surface. The degree of PL enhancement is determined by the electric field enhancement from the localized surface plasmon and the energy transfer from the carbon nanotube to the metal surface.

Fracture-induced subbandgap absorption as a precursor to optical damage on fused silica surfaces

The optical damage threshold of indentation-induced flaws on fused silica surfaces was explored. Mechanical flaws were characterized by laser damage testing, as well as by optical, secondary electron, and photoluminescence microscopy. Localized polishing, chemical leaching, and the control of indentation morphology were used to isolate the structural features that limit optical damage. A thin defect layer on fracture surfaces, including those smaller than the wavelength of visible light, was found to be the dominant source of laser damage initiation during illumination with 355 nm, 3 ns laser pulses. Little evidence was found that either displaced or densified material or fluence intensification plays a significant role in optical damage at fluences >35 J/cm(2). Elimination of the defect layer was shown to increase the overall damage performance of fused silica optics.

Repairing Damage on Ground Fused Silica by CO<sub>2</sub> Laser Irradiation

Fused silica is an important material for optics of ultraviolet laser transmission. It is difficult to be ground in ductile mode because of a high brittleness. In this paper, ductile mode grinding was achieved by a taper grinding method when the initial surface was a polished surface. However, when the initial surface was an unpolished surface, ductile mode grinding could not be realized because of the existence of surface and subsurface damages. The damages on the ground fused silica surface were repaired by CO2 laser irradiation with optimal laser power and scanning velocity. The optimal parameters were determined by studying the viscosity of fused silica and etching depth of laser irradiated groove. Besides laser processing parameters, surface roughness of laser irradiated surface is also greatly influenced by the initial surface roughness.

Interactions of Al(III), La(III), Gd(III), and Lu(III) with the Fused Silica/Water Interface Studied by Second Harmonic Generation

The interactions of the trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) with the silica/water interface were studied using the nonlinear optical technique of second harmonic generation (SHG). Specifically, the Eisenthal chi(3) technique was used to quantify the thermodynamics of trivalent ion adsorption to the bare fused silica surface. SHG adsorption isotherms were measured and fit with the triple layer surface complexation model to obtain adsorption free energies, binding constants, and interfacial charge densities. The adsorption free energy for Al(III) was found to be -37.2(5) kJ/mol, while the adsorption free energies for the three trivalent lanthanide cations ranged from -29.9(9) to -32.2(7) kJ/mol. Despite identical ionic charges, the metals under investigation displayed different affinities for the fused silica/water interface, and this finding is analyzed and interpreted in the context of size-dependent metal cation properties and metal ion speciation. The thermodynamic results from this work are valuable benchmarks for computer simulations of trivalent metal transport in the environment.

Monitoring annealing via CO_2 laser heating of defect populations on fused silica surfaces using photoluminescence microscopy

Photoluminescence (PL) microscopy and spectroscopy under 266 nm and 355 nm laser excitation are explored as a means of monitoring defect populations in laser-modified sites on the surface of fused silica and their subsequent response to heating to different temperatures via exposure to a CO(2) laser beam. Laser-induced temperature changes were estimated using an analytic solution to the heat flow equation and compared to changes in the PL emission intensity. The results indicate that the defect concentrations decrease significantly with increasing CO(2) laser exposure and are nearly eliminated when the peak surface temperature exceeds the softening point of fused silica (approximately 1900K), suggesting that this method might be suitable for in situ monitoring of repair of defective sites in fused silica optical components.

Growth Behavior of an Adsorbed Monolayer from a Benzophenone-Containing Trimethoxysilane Derivative on a Fused Silica Surface for Nanoimprint Molds by Chemical Vapor Surface Modification

The formation of a monolayer from a benzophenone-containing trimethoxysilane derivative was studied to understand the growth behavior of an antisticking layer from a trimethoxysilane derivative with an alkyl long tail often used as an antisticking layer in nanoimprint lithography. An adsorbed monolayer was formed from 4-{[(3-trimethoxysilyl)propyl]oxy}benzophenone on a fused silica surface by chemical vapor surface modification (CVSM). The growth behavior of the adsorbed monolayer was monitored by UV–visible spectroscopy, and the results were compared with those obtained by contact angle measurement for water and atomic force microscopy. The monolayer formation was confirmed with absorption spectra showing a characteristic absorption band derived from a benzophenone moiety. Changes in contact angle and absorbance with an increase in CVSM period suggested that the monolayer formation was completed in 2 h. The period of 2 h suitable for CVSM was also supported by atomic force microscopy topographic images. These results suggested that the monolayer growth comprises the following three steps: the surface adsorption, condensation and packing, and excess adsorption steps.

Study on the loss of nucleoside mono-, di- and triphosphates and phosphorylated peptides to a metal-free LC–MS hardware

In our earlier LC–MS experiments on the analysis of phosphocompounds like nucleotides (mono-, di- and triphosphate) and phosphopeptides and in literature, low sensitivity and severe losses of analyte to the instrumental setup were observed. Since we noticed that the stainless steel parts of the setup (e.g., the electrospray needle) adsorbed important quantities of the phosphorylated analytes, we made a LC–ESI setup without metallic surfaces. The first results were disappointing since also the fused silica surface of the LC-ESI coupler adsorbed part of the nucleotides and phosphopeptides injected in flow analysis experiments. We present experiments documenting the contribution of the different components of the setup. A number of potential solutions to the adsorption problem are proposed and tested. Only dimethyldichlorosilane deactivation of fused silica capillaries gave satisfactory results as adsorption of nucleotides and phosphopeptide was minimised.

Comparing the use of mid-infrared versus far-infrared lasers for mitigating damage growth on fused silica

Laser-induced growth of optical damage can limit component lifetime and, therefore, increase operating costs of large-aperture fusion-class laser systems. While far-infrared (IR) lasers have been used previously to treat laser damage on fused silica optics and render it benign, little is known about the effectiveness of less-absorbing mid-IR lasers for this purpose. In this study, we quantitatively compare the effectiveness and efficiency of mid-IR (4.6 μm) versus far-IR (10.6 μm) lasers in mitigating damage growth on fused silica surfaces. The nonlinear volumetric heating due to mid-IR laser absorption is analyzed by solving the heat equation numerically, taking into account the temperature-dependent absorption coefficient α(T) at λ=4.6 μm, while far-IR laser heating is well described by a linear analytic approximation to the laser-driven temperature rise. In both cases, the predicted results agree well with surface temperature measurements based on IR radiometry, as well as subsurface fictive temperature measurements based on confocal Raman microscopy. Damage mitigation efficiency is assessed using a figure of merit (FOM) relating the crack healing depth to laser power required, under minimally ablative conditions. Based on our FOM, we show that, for cracks up to at least 500 μm in depth, mitigation with a 4.6 μm mid-IR laser is more efficient than mitigation with a 10.6 μm far-IR laser. This conclusion is corroborated by direct application of each laser system to the mitigation of pulsed laser-induced damage possessing fractures up to 225 μm in depth.

Simulation of laser-induced backside wet etching of fused silica with hydrocarbon liquids

The mechanism of laser-induced backside wet etching (LIBWE) is important for the optimization of application processes but is still ambiguous. Extremely high surface absorption coefficients of more than 40×104 cm−1 at λ=248 nm that decay exponentially within less than 25 nm were measured for LIBWE-etched fused silica surface. Therefore, the resulting laser-induced temperatures quickly exceed the boiling point and result in surface erosion of the modified material. Numerical calculations of the temperature considering the measured absorption and phase transitions have been used to evaluate the etching depth of fused silica with a pyrene/toluene solution that agrees well with the measured rates well. A model of LIBWE is proposed that bases on the laser ablation of the high-absorbing modified fused silica as the dominating erosion process.

Laser smoothing of sub-micron grooves in hydroxyl-rich fused silica

Nano- to micrometer-sized surface defects on UV-grade fused silica surfaces are known to be effectively smoothed through the use of high-temperature localized CO 2 laser heating, thereby enhancing optical properties. However, the details of the mass transport and the effect of hydroxyl content on the laser smoothing of defective silica at sub-micron length scales are still not completely understood. In this study, we examine the morphological evolution of sub-micron, dry-etched periodic surface structures on type II and type III SiO 2 substrates under 10.6 μm CO 2 laser irradiation using atomic force microscopy (AFM). In situ thermal imaging was used to map the transient temperature field across the heated region, allowing assessment of the T-dependent mass transport mechanisms under different laser-heating conditions. Computational fluid dynamics simulations correlated well with experimental results, and showed that for large effective capillary numbers ( N c > 2), surface diffusion is negligible and smoothing is dictated by capillary action, despite the relatively small spatial scales studied here. Extracted viscosity values over 1700–2000 K were higher than the predicted bulk values, but were consistent with the surface depletion of OH groups, which was confirmed using confocal Raman microscopy.

Effect of thermal stresses on fused silica surface on the laser induced damage

To study the effect of the thermal stresses on the optics, experiment is performed to measure the damage threshold of the two fused silica samples with one annealed to eliminate thermal stresses while the other not. The measurement includes the effects of the thermal stresses on the initial damage threshold of the fused silica, the damage growth threshold, and the damage growth laws. The results show that the damage threshold decreases as the thermal stresses increase, and the damage induced by the laser is accelerated by the thermal stresses. For the same laser intensity, the damage growth factor is higher for the area with higher thermal stresses, but the damage growth still obeys the exponential increase rule and no obvious effect of the thermal stresses is found on the damage growth threshold. The study here will determine to some extent whether the technology of the CO2 laser pretreatment can be applied to the large-aperture optics or not.