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Reaction rate calculations via transmission coefficients

Description: The transmission coefficient of a wavepacket traversing a potential barrier can be determined by steady state calculations carried out in imaginary time instead of by real time dynamical calculations. The general argument is verified for the Eckart barrier potential by a comparison of transmission coefficients calculated from real and imaginary time solutions of the Schroedinger equation. The correspondence demonstrated here allows a formulation for the reaction rate that avoids difficulties due to both rare events and explicitly time dependent calculations. 5 refs., 2 figs.
Date: July 11, 1985
Creator: Feit, M.D. & Alder, B.J.
Partner: UNT Libraries Government Documents Department

Effect of Random Clustering on Surface Damage Density Estimates

Description: Identification and spatial registration of laser-induced damage relative to incident fluence profiles is often required to characterize the damage properties of laser optics near damage threshold. Of particular interest in inertial confinement laser systems are large aperture beam damage tests (>1cm{sup 2}) where the number of initiated damage sites for {phi}>14J/cm{sup 2} can approach 10{sup 5}-10{sup 6}, requiring automatic microscopy counting to locate and register individual damage sites. However, as was shown for the case of bacteria counting in biology decades ago, random overlapping or 'clumping' prevents accurate counting of Poisson-distributed objects at high densities, and must be accounted for if the underlying statistics are to be understood. In this work we analyze the effect of random clumping on damage initiation density estimates at fluences above damage threshold. The parameter {psi} = a{rho} = {rho}/{rho}{sub 0}, where a = 1/{rho}{sub 0} is the mean damage site area and {rho} is the mean number density, is used to characterize the onset of clumping, and approximations based on a simple model are used to derive an expression for clumped damage density vs. fluence and damage site size. The influence of the uncorrected {rho} vs. {phi} curve on damage initiation probability predictions is also discussed.
Date: October 29, 2007
Creator: Matthews, M J & Feit, M D
Partner: UNT Libraries Government Documents Department

Modeling laser-induced surface cracks in silica at 355 nm

Description: Starting from the absorption of laser energy at a subsurface nanoparticle in fused silica, we simulate the consequent buildup of stresses and resulting mechanical material damage . The simulation indicates the formation of micropits with size comparable to a wavelength, similar to experimental observation. Possible mechanisms for enhanced local light absorbtion are discussed.
Date: December 1, 1997
Creator: Feit, M.D., LLNL
Partner: UNT Libraries Government Documents Department

Laser intensity modulation by nonabsorbing defects

Description: Nonabsorbing bulk defects can initiate laser damage in transparent materials. Defects such as voids, microcracks and localized stress concentrations can serve as positive or negative lenses for the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka){sup 2}{Delta}{epsilon}, where the wavenumber K is 2{pi}/{lambda} linear size of the defect and AF, is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. Three dimensional (eg. voids) as well as linear and planar (eg. cracks) microlenses can all have strong effects. The present paper evaluates the intensification due to spherical voids and high refractive index inclusions. We wish to particularly draw attention to the very large intensification that can occur at inclusions.
Date: January 1, 1997
Creator: Feit, M.D. & Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Laser intensity modulation by nonabsorbing defects

Description: Nonabsorbing defects can lead to laser damage. Defects such as voids, microcracks, and localized stressed concentrations, even if they differ from the surrounding medium only by refractive index, can serve as positive or negative lenses for the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka){sup 2}{Delta}{epsilon}, where the wavenumber K is 2{pi}/{lambda}, 2a is the linear size of the defect, and {Delta}{epsilon} is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. Three dimensional (e.g. voids) as well as linear and planar (e.g. cracks) microlenses can all have strong effects. This paper evaluates intensification due to spherical voids and high refractive index inclusions.
Date: November 20, 1996
Creator: Feit, M.D., Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Simple model of laser damage initiation and conditioning in frequency conversion crystals

Description: Laser conditioning, i.e. pre-exposure to less than damaging laser fluence, has been shown to improve the damage resistance of KDP/DKDP frequency conversion crystals. We have extended our damage model, small absorbing precursors with a distribution of sizes, to describe various damage related properties such as damage density and effects of laser conditioning in crystals. The model assumes the rate limiting process for both initiation and conditioning depends on temperature and that separate threshold temperatures exist for either conditioning or damage initiation to occur. This is reasonable in KDP/DKDP since the melting temperature is far below the temperatures associated with plasma formation and damage events. This model is capable of accounting for some recently observed damage-conditioning behaviors.
Date: October 28, 2005
Creator: Feit, M D; Rubenchik, A M & Trenholme, J B
Partner: UNT Libraries Government Documents Department

Scratch Forensics

Description: Scratches on optical components which are formed during fabrication, cleaning, handling and end-use, are widespread and almost always detrimental. The impact of scratches on the end-use of the optic includes increased optical scatter, reduced system performance, and reduced strength. In the case of optics used in high intensity laser applications, prevention of scratches is paramount because they are closely associated with laser damage. Evaluation of the characteristics (dimensions, location on optic, shape, and orientation) of a scratch can serve a powerful tool to identify the cause of the scratch and lead to mitigations to prevent their reoccurrence. It is likely that opticians have used such techniques for hundreds of years. In recent years, by applying techniques of fracture mechanics and tribology, several new semi-quantitative rules-of-thumb have been developed allowing one to estimate the size and shape of the scratch inducing asperity or rogue particle, the load on the particle, the depth of the fractures in the scratch, and properties of material housing the rogue particle. The following discussion reviews some these techniques, which as a whole, we refer to as 'Scratch Forsenics'.
Date: July 9, 2008
Creator: Suratwala, T I; Miller, P E; Feit, M D & Menapace, J A
Partner: UNT Libraries Government Documents Department

Size-selection initiation model extended to include shape and random factors

Description: The Feit-Rubenchik size-selection damage model has been extended in a number of ways. More realistic thermal deposition profiles have been added. Non-spherical shapes (rods and plates) have been considered, with allowance for their orientation dependence. Random variations have been taken into account. An explicit form for the change of absorptivity with precursor size has been added. A simulation tool called GIDGET has been built to allow adjustment of the many possible parameters in order to fit experimental data of initiation density as a function of fluence and pulse duration. The result is a set of constraints on the possible properties of initiation precursors.
Date: November 2, 2005
Creator: Trenholme, J B; Feit, M D & Rubenchik, A M
Partner: UNT Libraries Government Documents Department

Contributions of kinematics and viscoelastic lap deformation on the suface figure during full aperture polishing of fused silica

Description: A typical optical fabrication process involves a series of basic process steps including: (1) shaping, (2) grinding, (3) polishing, and sometimes (4) sub-aperture tool finishing. With significant innovation and development over the years in both the front end (shaping using CNC machines) and the back end (sup-aperture tool polishing), these processes have become much more deterministic. However, the intermediate stages (full aperture grinding/polishing) in the process, which can be very time consuming, still have much reliance on the optician's insight to get to the desired surface figure. Such processes are not presently very deterministic (i.e. require multiple iterations to get desired figure). The ability to deterministically finish an optical surface using a full aperture grinding/polishing will aid optical glass fabricators to achieve desired figure in a more repeatable, less iterative, and more economical manner. Developing a scientific understanding of the material removal rate is a critical step in accomplishing this. In the present study, the surface figure and material removal rate of a fused silica workpiece is measured as a function of polishing time using Ceria based slurry on a polyurethane pad or pitch lap under a variety of kinematic conditions (motion of the workpiece and lap) and loading configurations. The measured results have been applied to expand the Preston model of material removal (utilizing chemical, mechanical and tribological effects). The results show that under uniform loading, the surface figure is dominated by kinematics which can be predicted by calculating the relative velocity (between the workpiece and the lap) with time and position on the workpiece. However, in the case where the kinematics predict a time-averaged removal function over the workpiece that is uniform, we find experimentally that the surface deviates significantly from uniform removal. We show that this non-uniform removal is caused by the non-uniform stress distribution resulting from ...
Date: October 9, 2007
Creator: Suratwala, T I; Steele, R A & Feit, M D
Partner: UNT Libraries Government Documents Department

Material removal and surface figure during pad polishing of fused silica

Description: The material removal and surface figure after ceria pad polishing of fused silica glass have been measured and analyzed as a function of kinematics, loading conditions, and polishing time. Also, the friction at the workpiece/lap interface, the slope of the workpiece relative to the lap plane, and lap viscoelastic properties have been measured and correlated to material removal. The results show that the relative velocity between the workpiece & lap (determined by the kinematics) and the pressure distribution determine the spatial and temporal material removal and hence the final surface figure of the workpiece. In the case where the applied loading and relative velocity distribution over the workpiece are spatially uniform, a significant non-uniform spatial material removal from the workpiece surface is observed. This is due to a non-uniform pressure distribution resulting from: (1) a moment caused by a pivot point and interface friction forces; (2) viscoelastic relaxation of the polyurethane lap; and (3) a physical workpiece/lap interface mismatch. Both the kinematics and these contributions to the pressure distribution are quantitatively described, and then combined to form a spatial and temporal Preston model & code for material removal (called Surface Figure or SurF{copyright}). The surface figure simulations are consistent with the experiment for a wide variety of polishing conditions. This study is an important step towards deterministic full-aperture polishing, which would allow optical glass fabrication to be performed in a more repeatable, less iterative, and hence more economical manner.
Date: May 4, 2009
Creator: Suratwala, T I; Feit, M D & Steele, W A
Partner: UNT Libraries Government Documents Department

Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation

Description: Characterizing laser-induced damage in optical materials is important for laser design and operation. Previous methods of evaluating optical materials damage resistance to high-power laser irradiation have typically suffered from shot to shot uncertainties in laser energy output and/or have insufficient sensitivity. More importantly such methods do not address the aspects of laser-induced damage important to laser beam propagation, namely the amount of light scattered by the damage. We present a method for the quantitative correlation of material modification on the surface or in the bulk of optical materials to laser parameters, which deconvolutes the effects of laser output instability. Image analysis, whereby two images, one a fluence spatial profile and the other a visible light scatter image of the damage, are directly compared to extract scatter as a function of fluence. An automated microscope is used to record the location and number of bulk damage sites and determine a calibration factor between the scatter signal observed and damage density pinpoints (ppt)/mm{sup 3}. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distributions in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of +/- 0.3 pp/mm{sup 3} in the range 1-100 ppts/mm{sup 3} with our minimum detectable density being 0.01 ppts/mm{sup 3}. We also determined the ppt size distribution for 351-nm, 3-ns damage with the average size being 5.5 +/2.5 mm (1/e{sup 2}) diameter.
Date: March 22, 2005
Creator: Carr, C W; Feit, M D; Nostrand, M C & Adams, J J
Partner: UNT Libraries Government Documents Department

Densification and residual stress induced by CO2 laser-based mitigation of SiO2 surfaces

Description: Knowing the ultimate surface morphology resulting from CO{sub 2} laser mitigation of induced laser damage is important both for determining adequate treatment protocols, and for preventing deleterious intensification upon subsequent illumination of downstream optics. Physical effects such as evaporation, viscous flow and densification can strongly affect the final morphology of the treated site. Evaporation is a strong function of temperature and will play a leading role in determining pit shapes when the evaporation rate is large, both because of material loss and redeposition. Viscous motion of the hot molten material during heating and cooling can redistribute material due to surface tension gradients (Marangoni effect) and vapor recoil pressure effects. Less well known, perhaps, is that silica can densify as a result of structural relaxation, to a degree depending on the local thermal history. The specific volume shrinkage due to structural relaxation can be mistaken for material loss due to evaporation. Unlike evaporation, however, local density change can be reversed by post annealing. All of these effects must be taken into account to adequately describe the final morphology and optical properties of single and multiple-pass mitigation protocols. We have investigated, experimentally and theoretically, the significance of such densification on residual stress and under what circumstances it can compete with evaporation in determining the ultimate post treatment surface shape. In general, understanding final surface configurations requires taking all these factors including local structural relaxation densification, and therefore the thermal history, into account. We find that surface depressions due to densification can dominate surface morphology in the non-evaporative regime when peak temperatures are below 2100K.
Date: October 21, 2010
Creator: Feit, M D; Matthews, M J; Soules, T F & Stolken, J S
Partner: UNT Libraries Government Documents Department

Surface contamination initiated laser damage

Description: We are engaged in a comprehensive effort to understand and model the initiation and growth of laser damage initiated by surface contaminants. This includes, for example, the initial absorption by the contaminant, heating and plasma generation, pressure and thermal loading of the transparent substrate, and subsequent shockwave propagation, ``splashing`` of molten material and possible spallation, optical propagation and scattering, and treatment of material fracture. The integration use of large radiation hydrodynamics codes, optical propagation codes and material strength codes enables a comprehensive view of the damage process The following picture of surface contaminant initiated laser damage is emerging from our simulations. On the entrance optical surface, small particles can ablate nearly completely. In this case, only relatively weak shockwaves are launched into the substrate, but some particulate material may be left on the surface to act as a diffraction mask and cause further absorption. Diffraction by wavelength scale scattering centers can lead to significant intensity modulation. Larger particles will not be completely vaporized. The shockwave generated in this case 1642is larger and can lead to spallation of contaminant material which then may be deposited in the substrate. A gaseous atmosphere can lead to radiation trapping with concomitant increases in temperature and pressure near the surface. In addition, supersonic ionization waves in air may be generated which greatly extend the plasma plume spatially and temporally. Contaminants on the exit optical surface behave differently. They tend to heat and pop off completely in which case significant damage may not occur. Since plasma formed at the interface of the optic and absorbing particle is confined, much stronger pressures are generated in this case. Imaging of contaminants resulting in ``writing`` a diffraction pattern on the exit surface due to contamination on the entrance surface has been observed experimentally and predicted theoretically. Such imprinted damage ...
Date: January 24, 1997
Creator: Feit, M.D.; Rubenchick, A.M. & Faux, D.R.
Partner: UNT Libraries Government Documents Department

Poynting vectors and electric field distributions in simple dielectric gratings

Description: The authors 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, though 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.
Date: February 7, 1996
Creator: Shore, B. W.; Feit, M. D. & Li, L.
Partner: UNT Libraries Government Documents Department

Fiber amplifiers and lasers in Yb:silica

Description: We have measured gain and saturation in sing;e mode Yb:silica fiber, and developed fiber lasers and amplifiers at 1053 nm. The lasers are tunable over 10`s of nanometers, with amplifier gain flattened by fiber gratings or dielectric filters.
Date: November 15, 1996
Creator: Wilcox, R.B.; Browning, D.F.; Feit, M.D. & Nyman, B.
Partner: UNT Libraries Government Documents Department

Contamination effects on optical damage

Description: The development and construction of high fluence lasers for inertial confinement fusion such as the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) or the Laser Megajoule in France continues to generate strong interest in the behavior of optical components under intense laser irradiation. the design of such lasers has created significant technological challenges in the area of laser glass, KDP crystal growth, surface finishing, and fused silica damage in the ultraviolet (UV). The damage of fused silica lenses at 355 nm is of particular concern since the optical design requires peak fluences that are very close to the damage threshold of the most resistant fused silica lenses available today.
Date: March 2, 1998
Creator: Genin, F.Y.; Kozlowski, M.R. & Feit, M.D.
Partner: UNT Libraries Government Documents Department

Unique aspects of laser energy deposition in the fs pulse regime

Description: Ultrashort laser pulse tissue ablation has demonstrated advantages of greatly reduced required energy and collateral damage. These advantages stem directly from the fact that laser energy is absorbed nonlinearly in a time too hsort for significant thermal dn hydrodynamic response. The high peak power and short pulse duration both have implications for practical fiber delivery systems.
Date: February 2, 1996
Creator: Feit, M.D.; Rubenchik, A.M. & Shore, B.W.
Partner: UNT Libraries Government Documents Department

Increased damage thresholds due to laser pulse modulation

Description: Nonlinear self-focusing in laser glass imposes limits on the energy fluence that can be safely transmitted without risking damage. For this reason, it is desirable to strictly limit the peak to average spatial variations of fluence by smoothing schemes such as Smoothing by Spectral Dispersion (SSD). While spatial variations are problematic, the same is not necessarily true of temporal variations since normal group velocity dispersion tends to smooth out temporal peaks caused by spatial self-focusing. Earlier work indicated that increased bandwidth can delay the onset of self focusing. The present work re-examines the question of self focusing threshold increases due to high bandwidth by investigating another source of such increase in three dimensional beam breakup--the bending instability. For simplicity, the authors consider the behavior of a single space-time speckle. Normal dispersion can lead to splitting of the pulse and delay of self focusing for short enough pulses as noted above. In addition to the self focusing instability, the laser beam is also subject to the so-called bending (sausage like) instability which can spatially disperse the field maxima over time. Because the bending instability breaks an initial axial symmetry, a full three dimensional numerical simulation is required to study it accurately. Such calculations are possible, but costly. The authors have used a modified 2D nonlinear Schroedinger equation with a high power nonlinearity since this mimics the 3D behavior of the competition between self focusing and bending. This study is relevant for inertial confinement conditions.
Date: May 30, 1995
Creator: Feit, M.D.; Musher, S.L.; Shapiro, E.G. & Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Measurement of the Bespalov-Talanov gain spectrum in a dispersive medium with large n{sub 2}

Description: Conditions which seed the self focussing of high-power broadband laser beams are determined by examining growth rates for plane-wave perturbations on a strong pump field as a function of frequency and angle. Measurements verifying predictions of growth based on the linearized stability analysis of Bespalov and Talanov extended to broadband fields are reported.
Date: June 15, 1995
Creator: Wegner, P. J.; Feit, M. D.; Fleck, A. J., Jr. & Eimerl, D.
Partner: UNT Libraries Government Documents Department

Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses. II. Theory

Description: The authors have reported extensive measurements of damage thresholds for fused silica and several fluorides (LiF, CaF, MgF, and BaF) at 1053 and 526 nm for pulse durations, {tau}, ranging from 275 fs to 1 ns. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in good agreement with experimental results.
Date: December 1, 1994
Creator: Feit, M.D.; Rubenchik, A.M. & Shore, B.W.
Partner: UNT Libraries Government Documents Department

Pump and probe damage testing for investigation of transient material modifications associated with laser damage in optical materials

Description: Laser-induced breakdown in the bulk of transparent dielectric materials is associated with the generation of extreme localized conditions of temperatures and pressures. In this work, we perform pump and probe damage testing experiments to investigate the evolution of transient absorption by the host material arising from modifications following confined laser energy deposition in fused silica and DKDP materials. Specifically, we measure the size of the damage sites observed in the region of spatial overlap between the pump and probe pulses versus probe time delay and energy. Results of this proof-of-principle experimental work confirm that material modifications under extreme conditions created during a damage event include transient optical absorption. In addition, we found that the relaxation times of the induced absorption are very distinct for DKDP and SiO{sub 2} even under identical excitation conditions, on the order of 100 ns and 100 {micro}s, respectively.
Date: October 18, 2007
Creator: Negres, R A; Feit, M D; DeMange, P; Bude, J D & Demos, S G
Partner: UNT Libraries Government Documents Department

Investigation of laser-induced damage in DKDP under multi-color irradiation

Description: Laser-induced initiation of bulk damage sites in DKDP crystals is investigated under simultaneous exposure to 532- and 355-nm nanosecond laser pulses in order to simulate the operational conditions during harmonic conversion as well as probe the damage mechanisms. The results demonstrate synergetic damage effects under the dual-wavelength excitation. Furthermore, the damage performance is directly related to and can be predicted from the damage performance at each wavelength separately. The measured relative effective absorption coefficients at the two wavelengths are found to depend on the laser fluence. Laser-induced damage sites initiated within the bulk of optical components is a key limiting factor in the development of high power laser systems. Potassium dihydrogen phosphate (KH{sub 2}PO{sub 4} or KDP) and its deuterated analog (KD{sub 2-x}H{sub x}PO{sub 4} or DKDP) have been widely used for over three decades as Pockels cells and frequency converters and are still the only nonlinear materials suitable for large-aperture laser systems [1,2]. Damage thresholds in these materials have increased over time, primarily due to purer raw materials and improvement in growth processes, though localized damage sites still arise from laser intensities far below that necessary for intrinsic dielectric breakdown [3]. The damage precursors and their absorption mechanism leading to damage initiation are still unknown despite more than four decades of research [4,5]. In the case of KDP and DKDP crystals, recent work has highlighted the importance of synergetic effects between the second and third harmonics in Nd:glass lasers present during harmonic conversion to the observed damage density [6]. In this work, we quantitatively assess the damage performance of KDP/DKDP crystals under simultaneous exposure to the second and third harmonics of a nanosecond Nd:YAG laser system in order to (a) probe the underlying mechanism of damage initiation and (b) simulate the conditions taking place during harmonic conversion towards developing ...
Date: August 21, 2006
Creator: DeMange, P; Negres, R A; Rubenchik, A M; Radousky, H B; Feit, M D & Demos, S G
Partner: UNT Libraries Government Documents Department