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Particle adsorption was explored in a model optical polishing system, consisting of silica colloids and like-charged silica surfaces. The adsorption was monitored in situ under various suspension conditions, in the absence of surfactants or organic modifiers, using a quartz crystal microbalance with dissipation monitoring (QCM-D). Changes in surface coverage with particle concentration, particle size, pH, ionic strength and ionic composition were quantified by QCM-D and further characterized ex situ by atomic force microscopy (AFM). A Monte Carlo model was used to describe the kinetics of particle deposition and provide insights on scaling with particle concentration. Transitions from near-zero adsorption to measurable adsorption were compared with equilibrium predictions made using the Deraguin-Verwey-Landau-Overbeek (DLVO) theory. In addition, the impact of silica surface roughness on the propensity for particle adsorption was studied on various spatial scale lengths by intentionally roughening the QCM sensor surface using polishing methods. It was found that a change in silica surface roughness at the AFM scale from 1.3 nm root-mean-square (rms) to 2.7 nm rms resulted in an increase in silica particle adsorption of 3-fold for 50-nm diameter particles and 1.3-fold for 100-nm diameter particles—far exceeding adsorption observed by altering suspension conditions alone, potentially because roughness at the proper scale reduces the total separation distance between particle and surface.

During optical glass polishing, a number of interactions between the workpiece (i.e., glass), polishing slurry, and pad can influence the resulting workpiece roughness at different spatial scale lengths. In our previous studies, the particle size distribution of the slurry, the pad topography, and the amount of material removed by a single particle on the workpiece were shown to strongly correlate with roughness at AFM scale lengths (nm-μm) and weakly at μ-roughness scale lengths (μm-mm). In this study, the polishing slurry pH and the generation of glass removal products are shown to influence the slurry particle spatial and height distribution at the polishing interface and the resulting μ-roughness of the glass workpiece. A series of fused silica and phosphate glass samples were polished with various ceria and colloidal silica slurries over a range of slurry pH, and the resulting AFM roughness and μ-roughness were measured. The AFM roughness was largely invariant with pH, suggesting that the removal function of a single particle is unchanged with pH. However, the μ-roughness changed significantly, increasing linearly with pH for phosphate glass and having a maximum at an intermediate pH for fused silica. In addition, the spatial and height distribution of slurry particles on the pad (as measured by laser confocal microscopy) was determined to be distinctly different at low and high pH during phosphate glass polishing. Also, the zeta potential as a function of pH was measured for the workpiece, slurry, and pad with and without surrogate glass products (K3PO4 for phosphate glass and Si(OH)4 for silica) to assess the role of interfacial charge during polishing. The addition of K3PO4 significantly raised the zeta potential, whereas addition of Si(OH)4 had little effect on the zeta potential. An electrostatic DLVO three-body force model, using the measured zeta potentials, was used to calculate the particle–particle, particle–workpiece, and particle–pad attractive and repulsive forces as a function of pH and the incorporation of glass products at the interface. The model predicted an increase in particle–pad attraction with an increase in pH and phosphate glass products consistent with the measured slurry distribution on the pads during phosphate glass polishing. Finally, a slurry “island” distribution gap (IDG) model has been formulated which utilizes the measured interface slurry distributions and a load balance to determine the interface gap, the contact area fraction, and the load on each slurry “island”. The IDG model was then used to simulate the workpiece surface topography and μ-roughness; the results show an increase in roughness with pH similar to that observed experimentally.

The chemical characteristics and the proposed formation mechanisms of the modified surface layer (called the Beilby layer) on polished fused silica glasses are described. Fused silica glass samples were polished using different slurries, polyurethane pads, and at different rotation rates. The concentration profiles of several key contaminants, such as Ce, K, and H, were measured in the near surface layer of the polished samples using Secondary Ion Mass Spectroscopy (SIMS). The penetration of K, originating from KOH used for pH control during polishing, decreased with increase in polishing material removal rate. In contrast, penetration of the Ce and H increased with increase in polishing removal rate. In addition, Ce penetration was largely independent of the other polishing parameters (e.g., particle size distribution and the properties of the polishing pad). The resulting K concentration depth profiles are described using a two-step diffusion process: (1) steady-state moving boundary diffusion (due to material removal during polishing) followed by (2) simple diffusion during ambient postpolishing storage. Using known alkali metal diffusion coefficients in fused silica glass, this diffusion model predicts concentration profiles that are consistent with the measured data at various polishing material removal rates. On the other hand, the observed Ce profiles are inconsistent with diffusion based transport. Rather we propose that Ce penetration is governed by the ratio of Ce–O–Si and Si–O–Si hydrolysis rates; where this ratio increases with interface temperature (which increases with polishing material removal rate) resulting in greater Ce penetration into the Beilby layer. Calculated Ce surface concentrations using this mechanism are in good agreement to the observed change in measured Ce surface concentrations with polishing material removal rate. These new insights into the chemistry of the Beilby layer, combined together with details of the single particle removal function during polishing, are used to develop a more detailed and quantitative picture of the polishing process and the formation of the Beilby layer.

The effects of temperature and material deposit on workpiece material removal spatial uniformity during optical pad polishing are described. Round and square-fused silica workpieces (25–265 mm in size) were polished on a polyurethane pad using ceria slurry under various conditions. Using a nonrotated workpiece on a rotating lap, elevated temperatures (as measured by IR imaging), due to frictional heating at the workpiece–lap interface, were observed having a largely radial symmetric profile (relative to the lap center) on both the workpiece and lap with a peak temperature corresponding to the workpiece center. A 3D steady-state thermal model of the polishing process, which accounts for the frictional heating and effective heat transfer from various surfaces, quantitatively describes the observed thermal profiles. The temperature spatial uniformity, which affects the material removal spatial uniformity, can be significantly improved using a rotated workpiece and a specially designed compensating septum during polishing. Next, using a rotating workpiece and lap, the workpiece surface develops two types of mid-range structure: (1) fine ripples (sub-mm scale length) that run circumferentially with respect to the lap, which have been attributed to microscopic islands of slurry on the lap leading to radial material removal nonuniformities; and (2) a center depression (cm scale length) which has been attributed to nonlinear slurry & glass products buildup at a specific radial lap location. A polishing simulator model (called Surface Figure or SurF), which accounts for workpiece wear, pad wear, and now deposition on the pad, correctly simulates the preferential material deposit on the pad and the center depression structure developed on the workpiece. Strategies, such as time averaging through kinematics and diamond conditioning, for preventing both these nonuniformities are demonstrated.

A method for chemically stabilizing metal oxide polishing slurries to prevent their agglomeration while maintaining their surface activity is demonstrated experimentally. Negatively charged ceria, zirconia, and alumina particles are reversibly size-stabilized under low ionic strength conditions at and above their isoelectric points using anionic surfactants. Stability is imparted only at surfactant concentrations above the critical micelle concentration and when the particle and the micelle have like-signed charges. Zeta potential measurements demonstrate that little adsorption of anionic surfactant occurs under conditions where the particles are negatively charged. Changes to pH, hydrophobicity, and ionic strength disrupt the surfactant's ability to size-stabilize the slurries. These results suggest that the charged micelles electrosterically hinder the agglomeration of oxide particles. Because the stabilization method does not rely on adsorption, the particle surface remains accessible for chemical reactions, such as those involved in polishing. Metal oxide slurries stabilized by this method remove material at a rate comparable to that of unstabilized slurry. In addition, stabilized slurry is easier to filter, which improves the quality of the polished surface. Stabilizing colloids by this method may prove valuable for systems where particle surface functionality is important, such as those used in ceramics processing, optical polishing, and chemical-mechanical planarization.

Loose abrasive grinding was performed on a wide range of optical workpiece materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2-Al2O3-P2O5-Li2O glass-ceramic (Zerodur); and glasses of SiO2 : TiO2 (ULE), SiO2 (fused silica), and P2O5-Al2O3-K2O-BaO (phosphate)]. Using the magneto rheological finishing (MRF) taper wedge technique (where a wedge was polished on each of the ground workpieces and the resulting samples were appropriately chemically etched), the subsurface mechanical damage (SSD) characteristics were measured. The SSD depth for most of the workpiece materials was found to scale as E11/2 / H1, where E1 is the elastic modulus and H1 is the hardness of the workpiece. This material scaling is the same as that for the growth of lateral cracks, suggesting that lateral cracks are a dominant source for SSD rather than radial/median cracks, as previously proposed. Utilizing the SSD depth data from both this study and others, semiempirical relationships have been formulated, which allows for estimating the SSD depth as a function of workpiece material and important grinding parameters (such as abrasive size and applied pressure).

A series of controlled grinding experiments, utilizing loose or fixed abrasives of either alumina or diamond at various particle sizes, were performed on a wide range of optical workpiece materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2−Al2O3−P2O5−Li2O glass ceramic (Zerodur); and glasses of SiO2:TiO2 (ULE), SiO2 (fused silica), and P2O5−Al2O3−K2O−BaO (phosphate)]. The material removal rate, surface roughness, and morphology of surface fractures were measured. Separately, Vickers indentation was performed on the workpieces, and the depths of various crack types as a function of applied load was measured. Single pass grinding experiments showed distinct differences in the spatial pattern of surface fracturing between the loose alumina abrasive (isolated indent-type lateral cracking) and the loose or fixed diamond abrasive (scratch-type elongated lateral cracking). Each of the grinding methods had a removal rate and roughness that scaled with the lateral crack slope, s l (i.e., the rate of increase in lateral crack depth with the applied load) of the workpiece material. A grinding model (based on the volumetric removal of lateral cracks accounting for neighboring lateral crack removal efficiency and the fraction of abrasive particles leading to fracture initiation) and a roughness model (based on the depth of lateral cracks or the interface gap between the workpiece and lap) are shown to quantitatively describe the material removal rate and roughness as a function of workpiece material, abrasive size, applied pressure, and relative velocity. This broad, multiprocess variable grinding model can serve as a predictive tool for estimating grinding rates and surface roughness for various grinding processes on different workpiece materials.

Various ceria and colloidal silica polishing slurries were used to polish fused silica glass workpieces on a polyurethane pad. Characterization of the slurries’ particle size distribution (PSD) (using both ensemble light scattering and single particle counting techniques) and of the polished workpiece surface (using atomic force microscopy) was performed. The results show the final workpiece surface roughness is quantitatively correlated with the logarithmic slope of the distribution function for the largest particles at the exponential tail end of the PSD. Using the measured PSD, fraction of pad area making contact, and mechanical properties of the workpiece, slurry, and pad as input parameters, an Ensemble Hertzian Gap (EHG) polishing model was formulated to estimate each particle’s penetration, load, and contact zone. The model is based on multiple Hertzian contact of slurry particles at the workpiece–pad interface in which the effective interface gap is determined through an elastic load balance. Separately, ceria particle static contact and single pass sliding experiments were performed showing ~1-nm depth removal per pass (i.e., a plastic type removal). Also, nanoindentation measurements on fused silica were made to estimate the critical load at which plastic type removal starts to occur (Pcrit~5 3 10 � 5 N). Next the EHG model was extended to create simulated polished surfaces using the Monte Carlo method where each particle (with the calculated characteristics described above) slides and removes material from the silica surface in random directions. The polishing simulation utilized a constant depth removal mechanism (i.e., not scaling with particle size) of the elastic deformation zone cross section between the particle and silica surface, which was either 0.04 nm (for chemical removal) at low loads ( Pcrit). The simulated surfaces quantitatively compare well with the measured rms roughness, power spectra, surface texture, absolute thickness material removal rate, and load dependence of removal rate.

The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct processes were identified, including: a) the onset of optical absorption in the material arising from the buildup of an electronic excitation, b) the expansion of the hot modified region (plasma) along the surface and inside the bulk, c) the formation of radial and circumferential cracks, d) the swelling of the affected region on the surface and, e) the onset of ejection of material clusters at about 30 ns delay and its progression to a well-defined jet by about 75 ns delay. Limited theoretical modeling is used to aid the interpretation of the data.

A new method (a variant of tumble finishing) for polishing and achieving local planarization on precision spherical, plastic capsules is described. Such capsules have niche applications, such as ablators used in high-peak-power laser targets for fusion energy research. The as-manufactured ablators contain many shallow domes (many 100's of nm high and a few 10's of μm wide) on the outer surface which are undesirable due to contributions to instabilities during implosion. These capsules were polished (i.e., tumble finished) by rotating a cylindrical vial containing the capsule, many borosilicate glass or zirconia media, and an aqueous-based colloidal silica polishing slurry. During tumble finishing, the relative media/capsule motions cause multiple, random sliding spherical–spherical Hertzian contacts, resulting in material removal, and possibly plastic deformation, on the capsule. As a result, the domes were observed to locally planarize (i.e., converge to lower heights). Utilizing the correct kinematics (i.e., the characteristics of the media/capsule motions), as controlled by the vial rotation rate and the fill fraction of media and slurry, the high velocity downward circumferential media motions were avoided, preventing fracturing of the fragile capsules. Also, the resulting post-polished surface roughness on the capsule was found to scale with the initial media surface roughness. Hence, pre-polishing the media greatly reduced the roughness of the media and thus the roughness of the polished capsule. A material removal model is described based on the Preston model and spherical–spherical Hertzian contacts which shows reasonable agreement with measured average removal rates of 35 ± 15 nm/day and which serves as a valuable tool to scale the polishing behavior with changes in process variables. Narrow domes were observed to planarize more rapidly than wider domes. A local planarization convergence model is also described, based on the concept of workpiece–lap mismatch where the local pressure, and hence removal, varies with the gap at the interface contact. The calculated rate and shape evolution of various size isolated domes compares well with the experimental data.

A new method of optical polishing termed “Convergent Polishing” is demonstrated where a workpiece, regardless of its initial surface figure, will converge to the lap shape in a single iteration. This method of polishing is accomplished by identifying the phenomena that contribute to non-uniform spatial material removal, and mitigating the non-uniformity for each phenomenon (except for the workpiece-lap mismatch due to the workpiece surface shape). The surface mismatch at the interface between the workpiece and lap causes a spatial and time varying pressure differential which decreases with removal, thus allowing the workpiece to converge to the shape of the lap. In this study, fused (amorphous) silica workpieces are polished using ceria slurry on various polyurethane pads. Polishing parameters were systematically controlled to prevent various sources of non-uniform material removal which include: (i) moment force, (ii) viscoelastic lap relaxation, (iii) kinematics, (iv) pad wear, and (v) workpiece bending. The last two are described herein. With these mitigations, removal uniformity has been demonstrated to within 1.0 μm over the surface after 83 μm of material removal corresponding to a within workpiece non-uniformity (WIWNU) of

The purpose of this paper is to compare simple and efficient pair-wise force fields for silica glass and assess their applicability for use in large scale molecular dynamic (MD) simulations of laser damage mitigation. A number of pair-wise force fields have been shown to give the random tetrahedral network of silica glass. Further, potentials obtained by fitting quantum mechanical results exhibit many of the properties of silica such as the low thermal expansion and densification. However with these potentials densification is observed at temperatures much higher than experiment. We also show that the thermodynamic melting point of β-crystobalite similarly occurs at temperatures much higher than observed experimentally. Softer empirical potentials can be constructed that do give liquid properties at experimental temperatures. However in all cases the activation energies for diffusion are lower than the experimental activation energies for viscosity.

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.

We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd:...

The effects of HF/NH4F, wet chemical etching on the morphology of individual surface fractures (indentations, scratches) and of an ensemble of surface fractures (ground surfaces) on fused silica glass has been characterized. For the individual surface fractures, a series of static or dynamic (sliding) Vickers and Brinnell indenters were used to create radial, lateral, Hertzian cone and trailing indentation fractures on a set of polished fused silica substrates which were subsequently etched. After short etch times, the visibility of both surface and subsurface cracks is significantly enhanced when observed by optical microscopy. This is attributed to the increased width of the cracks following etching, allowing for greater optical scatter at the fracture interface. The removal of material during etching was found to be isotropic except in areas where the etchant has difficulty penetrating or in areas that exhibit significant plastic deformation/densification. Isolated fractures continue to etch, but will never be completely removed since the bottom and top of the crack both etch at the same rate. The etching behavior of ensembles of closely spaced cracks, such as those produced during grinding, has also been characterized. This was done using a second set of fused silica samples that were ground using either fixed or loose abrasives. The resulting samples were etched and both the etch rate and the morphology of the surfaces were monitored as a function of time. Etching results in the formation of a series of open cracks or cusps, each corresponding to the individual fractures originally on the surface of the substrate. During extended etching, the individual cusps coalesce with one another, providing a means of reducing the depth of subsurface damage and the peak-to-valley roughness. In addition, the material removal rate of the ground surfaces was found to scale with the surface area of the cracks as a function of etch time. The initial removal rate for the ground surface was typically 3.5× the bulk etch rate. The evolving morphology of ground surfaces during etching was simulated using an isotropic finite difference model. This model illustrates the importance that the initial distributions of fracture sizes and spatial locations have on the evolution of roughness and the rate at which material is removed during the etching process. The etching of ground surfaces can be used during optical fabrication to convert subsurface damage into surface roughness thereby reducing the time required to produce polished surfaces that are free of subsurface damage.

The distribution and characteristics of surface cracks (i.e., sub-surface damage or scratching) on fused silica formed during grinding/polishing resulting from the addition of rogue particles in the base slurry has been investigated. Fused silica samples (10 cm diameter × 1 cm thick) were: (1) ground by loose abrasive grinding (alumina particles 9–30 μm) on a glass lap with the addition of larger alumina particles at various concentrations with mean sizes ranging from 15 to 30 μm, or (2) polished (using 0.5 μm cerium oxide slurry) on various laps (polyurethane pads or pitch) with the addition of larger rogue particles (diamond (4–45 μm), pitch, dust, or dried Ceria slurry agglomerates) at various concentrations. For the resulting ground samples, the crack distributions of the as-prepared surfaces were determined using a polished taper technique. The crack depth was observed to: (1) increase at small concentrations (>10 −4 fraction) of rogue particles; and (2) increase with rogue particle concentration to crack depths consistent with that observed when grinding with particles the size of the rogue particles alone. For the polished samples, which were subsequently etched in HF:NH 4 F to expose the surface damage, the resulting scratch properties (type, number density, width, and length) were characterized. The number density of scratches increased exponentially with the size of the rogue diamond at a fixed rogue diamond concentration suggesting that larger particles are more likely to lead to scratching. The length of the scratch was found to increase with rogue particle size, increase with lap viscosity, and decrease with applied load. At high diamond concentrations, the type of scratch transitioned from brittle to ductile and the length of the scratches dramatically increased and extended to the edge of the optic. The observed trends can be explained semi-quantitatively in terms of the time needed for a rogue particle to penetrate into a viscoelastic lap. The results of this study provide useful insights and ‘rules-of-thumb’ relating scratch characteristics observed on surfaces during optical glass fabrication to the characteristics of the rogue particles causing them and their possible source.

Laser induced damage (breakdown) initiated on the exit surface of transparent dielectric materials using nanosecond pulses creates a volume of superheated material reaching localized temperatures on the order of 1 eV and pressures on the order of 10 GPa or larger. This leads to material ejection and the formation of a crater. The volume of this superheated material depends largely on the laser parameters such as fluence and pulse duration. To elucidate the material behaviors involved, we examined the morphologies of the ejected superheated material particles and found distinctive morphologies. We hypothesize that these morphologies arise from the difference in the structure and physical properties (such as the dynamic viscosity and presence of instabilities) of the superheated material at the time of ejection of each individual particle. Some of the ejected particles are on the order of 1 µm in diameter and appear as “droplets”. Another subgroup appears to have stretched, foam-like structure that can be described as material globules interconnected via smaller in diameter columns. Such particles often contain nanometer size fibers attached on their surface. In other cases, only the globules have been preserved suggesting that they may be associated with a collapsed foam structure under the dynamic pressure as it traverses in air. These distinct features originate in the structure of the superheated material during volume boiling just prior to the ejection of the particles.

The distribution and characteristics of surface cracking (i.e., sub-surface damage or SSD) formed during standard grinding processes have been measured on fused silica glass using a surface taper polishing technique. The measured SSD depth distributions are described by a single exponential followed by an asymptotic cutoff in depth. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The length of the trailing indent is strongly correlated with a given grinding process. It is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and most likely it is the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth increased with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related to the SSD length distribution to gain insight into ‘effective’ size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth. These relationships can serve as useful rules-of-thumb for non-destructively estimating SSD depth and for identifying the process that caused the SSD.

Free electron lasers operating in the 0.1–1.5 nm wavelength range have been proposed for the Stanford Linear Accelerator Center (USA) and DESY (Germany). The unprecedented brightness and associated fluence (up to 30 J cm−2) predicted for pulses < 300 fs pose new challenges for optical components. A criterion for optical component design is required, implying an understanding of X-ray—material interactions at these extreme conditions. In our experimental effort, the extreme conditions are simulated by the currently available sources ranging from optical lasers, through X-ray lasers (XRLs) at 14.7 nm down to K-alpha sources (∼0.15 nm). In this paper, we present an overview of our research project on X-ray—matter interaction, including both computer modeling and preliminary results from optical laser experiments, the COMET tabletop high brightness ps XRL and a K-alpha experimental campaign carried out at the JanUSP laser facility at the Lawrence Livermore National Laboratory.© 2003 Elsevier Science B.V. All rights reserved.PACS: 41.50; 42.70; 41.60. Cr; 42.55. Vc; 07.85. F

Pressure waves generated by the surface ablation of water with ultrashort laser pulses in the 140 fs–10 ps duration range were studied with a Mach–Zehnder interferometry. Formation and propagation of spherical and, in ablation with longer pulses, combination of spherical and cylindrical pressure waves were observed. Measurements of the amplitude and dynamics of the pressure waves show that the conversion efficiency of laser energy to mechanical energy of the pressure waves is less than 1%, thus justifying the claim that ultrashort laser pulse ablation induces low-collateral damage and has a large potential in biomedical applications.

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~λ/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will review important optical damage precursors in silica up to UV fluences as high as 45J/cm2 (3ns) along with studies of the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily responsible for optic damage below about 10J/cm2; they can be mitigated by an optimized oxide etch processes. At fluences above about 10J/cm2, precipitates of trace impurities are responsible for damage; they can be mitigated by eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating fluences these optics see.

A model simulating transient optical properties during laser damage in the bulk of KDP/DKDP crystals is presented. The model was developed and tested using as a benchmark its ability to reproduce the well-documented damage initiation behaviors but most importantly, the salient behavior of the wavelength dependence of the damage threshold. The model involves two phases. During phase I, the model assumes a moderate localized initial absorption that is strongly enhanced during the laser pulse via excited state absorption and thermally driven generation of additional point defects in the surrounding material. The model suggests that during a fraction of the pulse duration, the host material around the defect cluster is transformed into a strong absorber that leads to significant increase of the local temperature. During phase II, the model suggests that the excitation pathway consists mainly of one photon absorption events within a quasicontinuum of short-lived vibronic defect states spanning the band gap that was generated after the initial localized heating of the material due to thermal quenching of the excited state lifetimes. The width of the transition (steps) between different number of photons is governed by the instantaneous temperature, which was estimated using the experimental data. The model also suggests that the critical physical parameter prior to initiation of breakdown is the conduction band electron density. This model, employing very few free parameters, for the first time is able to quantitatively reproduce the wavelength dependence of the damage initiation threshold, and thus provides important insight into the physical processes involved.

Progress on understanding phenomena influencing surface figure and roughness during pad polishing are reviewed, followed by a description of a new Convergent Polishing process allowing single iteration, process variation-free finishing of optical flats & spheres.

We present the description of powerful beam self-focusing in underdense plasma. The importance of electron cavitation, i.e. total electron evacuation under the effect of ponderomotive forces, is emphasized. Cavitation results in suppression of filamentation and the possibility to channel power well above the nominal critical power of self-focusing for a distance of many Rayleigh lengths.

Understanding physical processes accompanying ablation is necessary for optimal use of ultrashort laser pulse (USLP) material processing. We describe the implementation of self-consistent electromagnetic propagation -energy absorption in our numerical models and estimate effect on material removal of energy, pulselength and prepulses.

The high intensities present in and the non-thermal nature of ultrashort-pulse laser ablation provide a nearly ideal source for thin-film deposition. The high kinetic energies and high ion content in the ablation plume suggest that it would be useful for the creation of diamond thin films. We used a 120 fs, 3 W, 1 kHz laser to ablate a graphite target and characterized the resulting films. We were able to grow amorphous films of up to 18 μm thick and free from graphite particulates with no annealing necessary and at rates up to 25 μm/hr. The films had 40–50% sp3 bonds as measured by using EELS and had properties typical of PLD-generated diamond-like carbon films.

Although hole drilling using ultra-short laser pulses has been shown to produce holes of excellent quality, we have observed that during the evolution of the hole, the morphology of the hole bottom has a dependence on fluence, number of shots, and polarization. We describe the nature of this structure within the hole and under what conditions it is observed.

There is a strong deviation from the usual τ1/2 scaling of laser damage fluence for pulses below 10 ps in dielectric materials. This behavior is a result of the transition from a thermally dominated damage mechanism to one dominated by plasma formation on a time scale too short for significant energy transfer to the lattice. This new mechanism of damage (material removal) is accompanied by a qualitative change in the morphology of the interaction site and essentially no collateral damage. High precision machining of all dielectrics (oxides, fluorides, explosives, teeth, glasses, ceramics, SiC, etc.) with no thermal shock or distortion of the remaining material by this mechanism is described.

An improved cavitation model shows that stable beam channeling and electron cavitation occur for relativistic laser intensities even at powers hundreds of times larger than the critical power for self-focusing.

Ultrashort laser pulse ablation removes material with low-energy fluence required and minimal collateral damage. The ultimate usefulness of this technology for biomedical application depends, in part, on characterization of the physical conditions attained, and determination of the zone of shockwave and heat-affected material in particular tissues. Detailed numerical modeling of the relevant physics (deposition, plasma formation, shockwave generation and propagation, thermal conduction) are providing this information. A wide range of time scales is involved, ranging from picosecond for energy deposition and peak pressure and temperature, to nanosecond for development of shockwave, to microsecond for macroscopic thermophysical response.

We have extended the physics of the one-dimensional radiation hydrodynamics simulation code HYADES to include processes important for studying laser–matter interaction with dielectrics and metals in the eV and sub-eV electron temperature range. Ultra-short laser pulses (USLP) are advantageous in many material processing situations where most of the absorbed laser energy appears in mass motion, and little appears as thermal energy. Major additions to the code include (1) the transport, reflection and absorption of laser light, (2) thermal energy transport and improved shockwave physics, and (3) improved models for the equation of state (EOS), strength of materials, and fracture. Examples from material processing and plasma generation were chosen for simulation. Our numerical simulations include the effects of radiation transport, hydrodynamic expansion and shockwave phenomena.

An optical feedback system for controlled precise tissue ablation is discussed. Our setup includes an ultrashort pulse laser (USPL), and a diagnostic system utilizing analysis of either tissue fluorescence or plasma emission luminescence. Current research is focused on discriminating hard and soft tissues such as bone and spinal cord during spinal surgery using either technique. Our experimental observations exhibit considerable spectroscopic contrast between hard and soft tissue, and both techniques offer promise for a practical diagnostic system.

We discuss, with illustrations drawn from the simple example of a dielectric grating under total internal reflection illumination, the use of electric field, energy density and Poynting vector as tools for understanding phenomena associated with dielectric gratings. The electric field has greatest direct observational interest and exhibits patterns of nodes and antinodes that are both expected and intuitive. The energy density, although not directly linked with photoelectric response, has readily understood global patterns. The Poynting vector has more elaborate structure, involving patterns of curls, but the patterns are sensitive to small changes in illumination angle or groove depth. Plots of Poynting vectors may not be as useful for dielectric structures as they are for metals.

Interpretation of spatial and time resolved images of rear surface ns laser damage in dielectrics requires understanding of the dynamic interaction of the incoming laser beam with the confined expanding plasma in the material. The detailed kinetics of the plasma, involving both expansion and retraction, depends on details of reflection and absorption in the hot material. The growth of the hot region is treated using a model previously developed to understand laser peening. The pressure is found to scale as the square root of laser intensity and drops off slowly after energy deposition is complete. For the conditions of our experimental observations in fused silica, our model predicts a pressure of about 9 GPa and a surface expansion velocity of about 1.5 km/sec, in good agreement with experimental observation.

Crater formation that accompanies laser-induced damage is the result of material ejection following the rapid, localized heating to temperatures on the order of 1 eV. The objective of this work is to compare the material ejection behavior in fused silica and KDP crystals as captured using time-resolved shadowgraphy. These two materials are of fundamental importance in ICF class laser systems but they also represent materials with significantly different physical properties. We hypothesize that these different properties can affect the material ejection process.

To date, lasers have not succeeded in replacing mechanical tools in many hard tissue applications. Slow material removal rates and unacceptable collateral damage has prevented such a successful transition. Ultrashort pulses (

Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.

We report on the first experimental demonstration of a scalable fiber laser approach based on phase-locking multiple gain cores in an antiguided structure. A novel fabrication technology is used with soft glass components to construct the multiple core fiber used in our experiments. The waveguide region is rectangular in shape and comprised of a periodic sequence of gain and no-gain segments having nearly uniform refractive index. The rectangular waveguide is itself embedded in a lower refractive index cladding region. Experimental results confirm that our five-core Nd-doped glass prototype structure runs predominately in two spatial antiguided modes as predicted by our modeling.

Growing laser damage sites on multilayer high-reflector coatings can limit mirror performance. One of the strategies to improve laser damage resistance is to replace the growing damage sites with predesigned benign mitigation structures. By mitigating the weakest site on the optic, the large-aperture mirror will have a laser resistance comparable to the intrinsic value of the multilayer coating. To determine the optimal mitigation geometry, the finite-difference time-domain method was used to quantify the electric-field intensification within the multilayer, at the presence of different conical pits. We find that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Therefore, the optimal mitigation conical pit geometry is application specific. Furthermore, our simulation also illustrates an alternative means to achieve an optimal mitigation structure by matching the cone angle of the structure with the AOI of the incoming wave, except for the p-polarized wave at a range of incident angles between 30° and 45°.

The difference between front-surface and rear-surface energy deposition of a 35 ps laser pulse (λ=1064 nm) in fused silica was investigated using shadowgraphic and laser-deflection techniques. Shock waves were observed in the glass and in air. Shock waves generated in air at the front surface are stronger than at the rear surface. Less than 35% of the energy incident on the surface drives the air shock waves at the rear surface. Up to 90% of the incident energy drives the air shock waves at the front surface. Laser-plasma interaction is responsible for this difference and for limiting the amount of energy deposited inside the sample during front-surface ablation. Energy deposition at the rear surface is mostly limited by self-focusing inside the material.

We have constructed an optical parametric oscillator to generate 75-ps near-transform-limited pulses with wavelengths tunable about 1.053 µm for use in pump-probe studies of self-focusing. The singly resonant oscillator uses a Brewster-cut LiB(3)O(5) crystal that is oriented for type-II phase matching and synchronously pumped by the amplified and frequency-tripled pulse trains from a mode-locked and Q-switched Nd:YLP laser. An intracavity Pockels cell is used to switch out single 0.5-MW pulses at rates of 1 to 10 Hz. The design, construction, and performance of the oscillator are discussed. Measured performance is compared with design predictions and with detailed numerical simulations.

Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To date, the mechanism by which substrate surface defects affect the performance of coating layers under high power laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations to delineate the correlation between laser damage resistance of coating layers and the physical properties of the substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference time-domain (FDTD) method show that field intensification exists mostly near the top surface region of the coating near convex focusing structures. The light intensification could be responsible for the reduced damage threshold. Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our preliminary results show that damage occurs on the region of the coating where substrate scratches reside and etching of the substrate before coating does not seem to improve the laser damage resistance.