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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

Laser supported solid state absorption fronts in silica

Description: We develop a model based on simulation and experiment that explains the behavior of solid-state laser-supported absorption fronts generated in fused silica during high intensity (up to 5GW/cm{sup 2}) laser exposure. We find that the absorption front velocity is constant in time and is nearly linear in laser intensity. Further, this model can explain the dependence of laser damage site size on these parameters. This behavior is driven principally by the temperature-activated deep sub band-gap optical absorptivity, free electron transport and thermal diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. The regime of parameter space critical to this problem spans and extends that measured by other means. It serves as a platform for understanding general laser-matter interactions in dielectrics under a variety of conditions.
Date: February 9, 2010
Creator: Carr, C W & Bude, J D
Partner: UNT Libraries Government Documents Department

Temperature activated absorption during laser-induced damage: The evolution of laser-supported solid-state absorption fronts

Description: Previously we have shown that the size of laser induced damage sites in both KDP and SiO{sub 2} is largely governed by the duration of the laser pulse which creates them. Here we present a model based on experiment and simulation that accounts for this behavior. Specifically, we show that solid-state laser-supported absorption fronts are generated during a damage event and that these fronts propagate at constant velocities for laser intensities up to 4 GW/cm{sup 2}. It is the constant absorption front velocity that leads to the dependence of laser damage site size on pulse duration. We show that these absorption fronts are driven principally by the temperature-activated deep sub band-gap optical absorptivity, free electron transport, and thermal diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. In addition to the practical application of selecting an optimal laser for pre-initiation of large aperture optics, this work serves as a platform for understanding general laser-matter interactions in dielectrics under a variety of conditions.
Date: October 26, 2010
Creator: Carr, C W; Bude, J D; Shen, N & Demange, P
Partner: UNT Libraries Government Documents Department

Radiation in an Emitting and Absorbing Medium: A Gridless Approach

Description: A gridless technique for the solution of the integral form of the radiative heat flux equation for emitting and absorbing media is presented. Treatment of non-uniform absorptivity and gray boundaries is included. As part of this work, the authors have developed fast multipole techniques for extracting radiative heat flux quantities from the temperature fields of one-dimensional and three-dimensional geometries. Example calculations include those for one-dimensional radiative heat transfer through multiple flame sheets, a three-dimensional enclosure with black walls, and an axisymmetric enclosure with black walls.
Date: July 27, 2000
Partner: UNT Libraries Government Documents Department

The neutron physics of concrete reflectors

Description: It has long been known that concrete reflection can be an important factor in determining the critical state of any fissile system, single unit or storage array. Since there can be a large variation in the chemical makeup of concrete, mass-limit reduction factors are necessarily conservative, and may lead to a very uneconomical storage arrangement. This study was undertaken to clarify the importance of the various concrete constituents and to determine some general guidance as to the magnitude of the reactivity effects for the more likely fissile material storage conditions.
Date: September 1, 1995
Creator: Monahan, S.P.
Partner: UNT Libraries Government Documents Department

Transient radiation-induced absorption in materials for the DOI laser

Description: This is the final report on a series of experiments concerned with transient radiation-induced absorption in materials for a Cr,Nd:GSGG laser. Both the Sandia National Laboratories SPR III pulsed reactor and the Hermes III pulsed X-ray machine are used as radiation sources. The time dependence and the magnitude of the induced absorption in filter glasses and in doped and undoped LiNbO{sub 3} Q-switch materials have been measured. Gain has been observed in Cr,Nd:GSGG, the laser medium, when it is irradiated by X-rays.
Date: January 1, 1995
Creator: Brannon, P. J.
Partner: UNT Libraries Government Documents Department

Comparison Between Keyhole Weld Model and Laser Welding Experiments

Description: A series of laser welds were performed using a high-power diode-pumped continuous-wave Nd:YAG laser welder. In a previous study, the experimental results of those welds were examined, and the effects that changes in incident power and various welding parameters had on weld geometry were investigated. In this report, the fusion zones of the laser welds are compared with those predicted from a laser keyhole weld simulation model for stainless steels (304L and 21-6-9), vanadium, and tantalum. The calculated keyhole depths for the vanadium and 304L stainless steel samples fit the experimental data to within acceptable error, demonstrating the predictive power of numerical simulation for welds in these two materials. Calculations for the tantalum and 21-6-9 stainless steel were a poorer match to the experimental values. Accuracy in materials properties proved extremely important in predicting weld behavior, as minor changes in certain properties had a significant effect on calculated keyhole depth. For each of the materials tested, the correlation between simulated and experimental keyhole depths deviated as the laser power was increased. Using the model as a simulation tool, we conclude that the optical absorptivity of the material is the most influential factor in determining the keyhole depth. Future work will be performed to further investigate these effects and to develop a better match between the model and the experimental results for 21-6-9 stainless steel and tantalum.
Date: September 23, 2002
Creator: Wood, B C; Palmer, T A & Elmer, J W
Partner: UNT Libraries Government Documents Department


Description: We report the development of the composite superconducting transition edge bolometer. The temperature sensitive element is an aluminum strip evaporated onto the sapphire substrate. A bismuth film evaporated on the reverse side of the substrate is used to absorb the submillimeter radiation. The noise limitation of the bolometer is calculated. The fabrication and measured performance are described. The best electrical NEP (noise-equivalent-power) obtained is (1.7 {+-} 0.1) x 10{sup -15} WHz{sup -1/2} at 2 Hz at an operating temperature of 1.27 K. This NEP is within a factor of 2 of the thermal noise limit. The effective absorptivity of the bismuth film is measured to be 0.47 {+-} 0.05, and the corresponding detectivity D* is calculated to be (1.1 {+-} 0.1) x 10{sup 14} cm w{sup -1}Hz{sup 1/2}. Suggestions are made for further improvements in sensitivity. The current-dependent noise in thin metal films at the superconducting transition has been further investigated. The measured noise power spectrum of the tin film on sapphire substrate at the superconducting transition is compared with the cosine transforms of the decay curves obtained from step-function and {delta}-function thermal perturbations. The nature of the noise driving term is found to be a random current flowing inside the sample, in agreement with the uncorrelated thermal fluctuation model. This result is consistent with the case of a freely suspended tin fiber at the superconducting transition, but in contrast to the room temperature measurement where the cosine transform of the step-function response gives the noise power spectrum, in agreement with the correlated fluctuation model.
Date: June 1, 1978
Creator: Yeh, Nan-Hsiung
Partner: UNT Libraries Government Documents Department

Synthesis, optical properties, and microstructure of semiconductor nanocrystals formed by ion implantation

Description: High-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. The authors have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdS{sub x}Se{sub 1{minus}x}, CdTe, ZnS, ZnSe) in fused silica, Al{sub 2}O{sub 3} and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.
Date: December 1, 1996
Creator: Budai, J.D.; White, C.W.; Withrow, S.P.; Zuhr, R.A. & Zhu, J.G.
Partner: UNT Libraries Government Documents Department

High reflector absorptance measurements by the surface thermal lensing technique

Description: Surface thermal lensing is an alternate configuration of a photothermal deflection system that was used to measure low levels of optical absorption. The thermal lensing configuration facilitated the alignment of the pump and probe laser beams by using a larger diameter probe beam. This technique was applied to high performance optical coatings, specifically high reflectors at 511 nm, zero degrees angle of incidence. The absorptance of these coatings was previously measured using a high power copper vapor laser system. A high power copper laser beam is focused onto a -2 mm diameter spot. A thermal camera senses the temperature rise with respect to the rest of the coating. The temperature change, power density and beam diameter were used with an empirical formula that yields optical absorption. The surface thermal lensing technique was able to resolve absorption levels lower than that achieved with the copper laser method.
Date: November 1, 1996
Creator: Chow, R.; Taylor, J.R.; Wu, Z.L.; Krupka, R. & Yang, T.
Partner: UNT Libraries Government Documents Department

Pulsed laser surface hardening of ferrous alloys.

Description: A high power pulsed Nd:YAG laser and special optics were used to produce surface hardening on 1045 steel and gray cast iron by varying the process parameters. Unlike CO{sub 2} lasers, where absorptive coatings are required, the higher absorptivity of ferrous alloys at the Nd:YAG laser wavelength eliminates the necessity of applying a coating before processing. Metallurgical analysis of the treated tracks showed that very fine and hard martensitic microstructure (1045 steel) or inhomogeneous martensite (gray cast iron) were obtained without surface melting, giving maximum hardness of HRC 61 and HRC 40 for 1045 steel and gray cast iron respectively. The corresponding maximum case depths for both alloys at the above hardness are 0.6 mm. Gray cast iron was more difficult to harden without surface melting because of its lower melting temperature and a significantly longer time-at-temperature required to diffuse carbon atoms from the graphite flakes into the austenite matrix during laser heating. The thermal distortion was characterized in term of flatness changes after surface hardening.
Date: September 30, 1999
Creator: Xu, Z.; Reed, C. B.; Leong, K. H. & Hunter, B. V.
Partner: UNT Libraries Government Documents Department

Optical and Electronic Properties of Si Nanoclusters Synthesized in Inverse Micelles

Description: Highly crystalline, size-selected silicon (Si) nanocrystals in the size range 2-10 nm were grown in inverse micelles and their optical absorption and photoluminescence (PL) properties were studied. High resolution TEM and electron diffraction results show that these nanocrystals retain their cubic diamond stuctures down to sizes {approximately}4 nm in diameter, and optical absorption data suggest that this structure and bulk-like properties are retained down to the smallest sizes produced ({approximately}1.8 nm diameter containing about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and separate the clusters into pure, monodisperse populations. The optical absorption revealed features associated with both the indirect and direct bandgap transitions, and these transitions exhibited different quantum confinement effects. The indirect bandgap shifts from 1.1 eV in the bulk to {approximately}2.1 eV for nanocrystals {approximately}2 nm in diameter and the direct transition at r(l_"X - r15) blue shifts by 0.4 eV from its 3.4 eV bulk value over the same size range. Tailorable, visible, room temperature PL in the range 700-350 nm (1.8 - 3.5 eV) was observed from these nanocrystals. The most intense PL was in the violet region of the spectrum ({approximately}400 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect bandgap recombination. The results are compared to earlier work on Si clusters grown by other techniques and to the predictions of various model calculations. Currently, the wide variations in the theoretical predictions of the various models along with considerable uncertainties in experimental size determination for clusters less than 3-4 nm, make it difficult to select among competing models.
Date: December 14, 1998
Creator: Provencio, P.N.; Samara, G.A. & Wilcoxon, J.P.
Partner: UNT Libraries Government Documents Department

Laser beam welding of any metal.

Description: The effect of a metal's thermophysical properties on its weldability are examined. The thermal conductivity, melting point, absorptivity and thermal diffusivity of the metal and the laser beam focused diameter and welding speed influence the minimum beam irradiance required for melting and welding. Beam diameter, surface tension and viscosity of the molten metal affect weld pool stability and weld quality. Lower surface tension and viscosity increases weld pool instability. With larger beam diameters causing wider welds, dropout also increases. Effects of focused beam diameter and joint fitup on weldability are also examined. Small beam diameters are sensitive to beam coupling problems in relation to fitup precision in addition to beam alignment to the seam. Welding parameters for mitigating weld pool instability and increasing weld quality are derived from the above considerations. Guidelines are presented for the tailoring of welding parameters to achieve good welds. Weldability problems can also be anticipated from the properties of a metal.
Date: October 1, 1998
Creator: Leong, K. H.
Partner: UNT Libraries Government Documents Department

Determination of Fe{sup 2+} and Fe{sup 3+} concentrations of semi- insulating InP:Fe

Description: Semi-insulating InP is most commonly obtained by doping with the deep acceptor iron to compensate the shallow donors which otherwise render the material n-type conducting. As the Fermi level in semi-insulating InP is closed to the iron acceptor level, both charge states - Fe{sup 2+} as well as Fe{sup 3+} corresponding to the acceptor level occupied unoccupied by an electron - are present. Mayor et al.(1) presented a method based on absorption measurements in the nearbandgap region of InP to determine the concentration of both charge states separately. In this paper we compare iron concentrations obtained by this method with the results from intracenter absorption, DLTS, EPR, Hall effect measurements and glow discharge mass spectroscopy. We present a new calibration for the optical absorption cross sections.
Date: January 1, 1992
Creator: Zach, F. X.; Bourret, E. D.; Bliss, D.; Weber, E. R. & Haller, E. E.
Partner: UNT Libraries Government Documents Department

From Antenna to Assay: Lessons Learned in Lanthanide Luminescence

Description: Ligand-sensitized luminescent lanthanide(III) complexes are of considerable current interest due to their unique photophysical properties (micro- to millisecond lifetimes, characteristic and narrow emission bands, and large Stokes shifts), which make them well suited to serve as labels in fluorescence-based bioassays. The long-lived Ln(III) emission can be temporally resolved from scattered light and background fluorescence, resulting in vastly enhanced measurement sensitivity. One of the challenges in this field is the design of sensitizing ligands that provide highly emissive Ln(III) complexes that also possess sufficient stability and aqueous solubility required for practical applications. In this account we give an overview of some of the general properties of the trivalent lanthanides and follow with a summary of advances made in our laboratory in the development of highly luminescent Tb(III) and Eu(III) complexes for applications in biotechnology. A focus of our research has been the optimization of these compounds as potential commercial agents for use in Homogeneous Time Resolved Fluorescence (HTRF) technology, the requirements and current use of which will be briefly discussed. Our approach involves developing high-stability octadentate Tb(III) and Eu(III) complexes that rely on all-oxygen donor atoms as well as using multi-chromophore chelates to increase molar absorptivity compared to earlier examples that utilize a single pendant antenna chromophore. We have found that ligands based on 2-hydroxyisophthalamide (IAM) provide exceptionally emissive Tb(III) complexes with quantum yield values up to ca. 60%. Solution thermodynamic studies have indicated that these complexes are stable at the nanomolar concentrations required for commercial assays. Through synthetic modification of the IAM-chromophore, in conjunction with time-dependent density functional theory (TD-DFT) calculations, we have developed a method to predict absorption and emission properties of these chromophores as a tool to guide ligand design. Additionally we have investigated chiral IAM ligands that yield Tb(III) complexes possessing both high quantum yield values and ...
Date: September 25, 2008
Creator: Moore, Evan; Samuel, Amanda & Raymond, Kenneth
Partner: UNT Libraries Government Documents Department

Synthesis, electronic and optical properties of Si nanostructures

Description: Silicon and silicon oxide nanostructures have been deposited on solid substrates, in an ultra high vacuum (UHV) chamber, by laser ablation or thermal vaporization. Laser ablation followed by substrate post annealing produced Si clusters with average size of a few nanometers, on highly oriented pyrolytic graphite (HOPG) surfaces. This technique, which is based on surface diffusion, is limited to the production of less than one layer of clusters on a given surface. The low coverage of Si clusters and the possibility of nonradiative decay of excitation in the Si cores to the HOPG substrates in these samples rendered them unsuitable for many optical measurements. Thermal vaporization of Si in an Ar buffer gas, on the contrary, yielded multilayer coverage of Si nanoclusters with a fairly narrow size distribution of about 2 nm, full width at half maximum (FWHM). As a result, further study was performed only on Si nanoclusters synthesized by thermal vaporization in a buffer gas. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) revealed that these nanoclusters were crystalline. However, during synthesis, if oxygen was the buffer gas, a network of amorphous Si oxide nanostructures (an-SiO{sub x}) with occasional embedded Si dots was formed. All samples showed strong infrared and/or visible photoluminescence (PL) with varying decay times from nanoseconds to microseconds depending on synthesis conditions. There were differences in PL spectra for hydrogen and oxygen passivated nc-Si, while many common PL properties between oxygen passivated nc-Si and an SiO{sub x} were observed. The observed experimental results can be best explained by a model involving absorption between quantum confined states in the Si cores and emission for which the decay times are very sensitive to surface and/or interface states.
Date: September 1, 1996
Creator: Dinh, L.N.
Partner: UNT Libraries Government Documents Department

Spatial domain-based parallelism in large scale, participating-media, radiative transport applications

Description: Parallelism for gray participating media radiation heat transfer may be placed in two primary categories: spatial and angular domain-based parallelism. Angular, e.g., ray based, decomposition has received the greatest attention in the open literature for moderate sized applications where the entire geometry may be placed on processor. Angular based decomposition is limited, however, for large scale applications (O(10{sup 6}) to O(10{sup 8}) computational cells) given the memory required to store computational grids of this size on each processor. Therefore, the objective of this work is to examine the application of spatial domain-based parallelism to large scale, three-dimensional, participating-media radiation transport calculations using a massively parallel supercomputer architecture. Both scaled and fixed problem size efficiencies are presented for an application of the Discrete Ordinate method to a three dimensional, non-scattering radiative transport application with nonuniform absorptivity. The data presented shows that the spatial domain-based decomposition paradigm results in some degradation in the parallel efficiency but provides useful speedup for large computational grids.
Date: November 1, 1996
Creator: Burns, S.P. & Christon, M.A.
Partner: UNT Libraries Government Documents Department

Infrared spectroscopic, x-ray, and nanoscale characterization of strontium titanate thin films

Description: Attenuated total reflectance (ATR) measurements were performed using Fourier transform infrared (FTIR) spectroscopy in the ATR mode with a thallium iodobromide (KRS-5) crystal to measure the frequencies of the {nu}{sub 3} and {nu}{sub 4} phonon absorption bands in thin strontium titanate films deposited on single-crystal yttrium-barium copper oxide (YBCO), lanthanum aluminate, magnesium oxide, and strontium titanate substrates. The KRS-5 crystal enabled FTIR-ATR measurements to be made at frequencies above 400 cm{sup {minus}1}. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements were also made to further characterize the films. The measurements were repeated on single-crystal specimens of strontium titanate and the substrates for comparison. Softening in the frequency of the {nu}{sub 4} transverse optical phonon in the lattice- mismatched films below the established value of 544 cm{sup {minus}1} is indicative of the highly textured, polycrystalline ceramic nature of the films and is consistent with the XRD and AFM results.
Date: April 1, 1996
Creator: Webb, J.D.; Moutinho, H.R.; Kazmerski, L.L.; Mueller, C.H.; Rivkin, T.V.; Treece, R.E. et al.
Partner: UNT Libraries Government Documents Department

Sub-gap and band edge optical absorption in a-Si:H by photothermal deflection spectroscopy

Description: Using photothermal deflection spectroscopy, the optical absorption of various a-Si:H films was investigated in the range of 2.1 to 0.6 eV. An absorption shoulder which depends on deposition conditions and on doping was found and was attributed to dangling bonds. The exponential edge broadens with increasing spin density.
Date: July 1, 1981
Creator: Jackson, W.B. & Amer, N.M.
Partner: UNT Libraries Government Documents Department

Aethalometer - an instrument for the real-time measurement of optical absorption by aerosol particles

Description: We describe an instrument that measures the concentration of optically absorbing aerosol particles in real time. This absorption is normally due to black carbon, which is a good tracer for combustion emission. The minimum resolving times range from seconds in urban environments to minutes in remote locations. We present results obtained during operation on an aircraft. Due to the time resolution capability, we can determine the spatial distributions of absorbing aerosol. From the Greek word for to blacken with soot, we have named this instrument the aethalometer.
Date: August 1, 1983
Creator: Hansen, A.D.A.; Rosen, H. & Novakov, T.
Partner: UNT Libraries Government Documents Department

Dependence of the solar absorptance of selective absorber coatings on the angle of incidence

Description: The directional solar absorptances ..cap alpha../sub s/(theta) of samples of a number of selective absorber coatings have been determined. The spectral directional hemispherical reflectances plambda(theta;2..pi..) of each sample was measured over the wavelength range 0.3..mu.. to 2.5..mu.. at angles of incidence theta between 10/sup 0/ and 80/sup 0/. The quantity (1-plambda(theta;2..pi..)) was convoluted over an AM2 solar spectrum to obtain ..cap alpha../sub s/(theta) at each angle of incidence. The solar absorptance at near normal incidence varied from sample to sample and from coating to coating, as expected, given the present state of the art. All the absorptances show similar angular dependences, however. When normalized to unity at normal incidence, the data nearly describe a single curve, for which a power series in theta was found. For comparison, the solar absorptance was also determined for freshly prepared lamp black.
Date: January 1, 1977
Creator: Reed, K A
Partner: UNT Libraries Government Documents Department

Electron-electron interaction effects in quasi-one-dimensional conducting polymers and related systems

Description: We review recent results on the role of electron-electron (e-e) interactions in quasi-one-dimensional conducting polymers and related systems. Within the Peierls-Hubbard model, the effects of both short-range (on-site U and nearest neighbor V/sub 1/) and long-range (V/sub j/, jgreater than or equal to 2) are examined, the former using quantum Monte Carlo and the latter with exact diagonalization of finite size systems and analytic arguments. We also discuss optical absorption properties in the presence of electronic correlation, focusing on the weak and strong coupling limits.
Date: January 1, 1986
Creator: Campbell, D.K.; Baeriswyl, D. & Mazumdar, S.
Partner: UNT Libraries Government Documents Department

A theoretical model for adsorption capacities of charcoal beds: I, Relative humidity effects

Description: Water vapor is the major interferent in the adsorption of other vapors from air when that air is passed through an activated charcoal bed. A limited amount of data (published and unpublished) is available on the magnitudes of capacity (or service life) reduction as a function of relative humidity (water vapor concentration) and preadsorbed water. A simple equilibrium model has been developed which quantitatively explains observed humidity effects and allows extrapolation of data to untested conditions. 9 refs., 4 figs., 2 tabs.
Date: January 1, 1987
Creator: Wood, G.O.
Partner: UNT Libraries Government Documents Department