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Analysis of high resolution scatter images from laser damage experiments performed on KDP

Description: Interest in producing high damage threshold KH{sub 2}PO{sub 4} (KDP) and (D{sub x}H{sub 1-x}){sub 2}PO{sub 4} (KD*P, DKDP) for optical switching and frequency conversion applications is being driven by the system requirements for the National Ignition Facility (NIF) at Lawrence Livermore National Lab (LLNL). Historically, the path to achieving higher damage thresholds has been to improve the purity of crystal growth solutions. Application of advanced filtration technology has increased the damage threshold, but gives little insight into the actual mechanisms of laser damage. We have developed a laser scatter diagnostic to better study bulk defects and laser damage mechanisms in KDP and KD*P crystals. This diagnostic consists of a cavity doubled, kilohertz class, Nd:YLF laser (527 nm) and high dynamic range CCD camera which allows imaging of bulk scatter signals. With it, we have performed damage tests at 355 nm on four different {open_quotes}vintages{close_quotes} of KDP crystals, concentrating on crystals produced via fast growth methods. We compare the diagnostic`s resolution to LLNL`s standard damage detection method of 100X darkfield microscopy and discuss its impact on damage threshold determination. We have observed the disappearance of scatter sites upon exposure to subthreshold irradiation. In contrast, we have seen scatterers appear where none previously existed. This includes isolated, large (high signal) sites as well as multiple small scatter sites which appear at fluences above 7 J/cm{sup 2} (fine tracking). However, we have not observed a strong correlation of preexisting scatter sites and laser damage sites. We speculate on the connection between the laser-induced disappearance of scatter sites and the observed increase in damage threshold with laser conditioning.
Date: January 5, 1996
Creator: Runkel, M.; Woods, B. & Yan, M.
Partner: UNT Libraries Government Documents Department

Optical scatter as a diagnostic tool for studying bulk defects which cause laser damage in conventional and rapid growth KP and DKDP

Description: Single crystals of KH{sub 2}PO{sub 4} (KDP) and (D{sub x}H{sub 1-x}){sub 2}PO{sub 4} (DKDP) will be used for frequency conversion and as part of a large aperture optical switch in the proposed National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). These crystals must have good optical properties and high laser damage thresholds. Currently these crystals have a lower laser damage threshold than other optical materials in the laser chain which has forced designers to limit the output fluence of the NIF in order to avoid damaging the crystals. Furthermore, while more efficient frequency conversion schemes are being explored both theoretically and experimentally, the advantages of these schemes can not be fully realized unless the damage thresholds of the conversion crystals are increased. Over the past decade, LLNL has generated an extensive data base on the laser damage in KDP and DKDP crystals both at the first and third harmonics of Nd-YAG. While the damage thresholds of these crystals have increased over this time period due, in part, to better filtration of the growth solution, the damage thresholds of the best crystals are still far below what is expected from theoretical limits calculated from the band structure of perfect crystals. Thus damage in KDP and DKDP is caused by defects in the crystals. The authors also rely on a process called laser conditioning to improve the damage thresholds of the crystals. Unfortunately, little is understood about the mechanism of laser induced damage, the conditioning process in the crystals, or the defects which are responsible for damage. The authors have recently implemented a scatter diagnostic for locating and studying defects in crystals and as a tool for studying the mechanism of laser damage and laser conditioning.
Date: July 10, 1995
Creator: Woods, B.; Runkel, M. & Yan, M.
Partner: UNT Libraries Government Documents Department

Reconstruction of surfaces in NiO

Description: We studied the reconstructions of surfaces in NiO by atomistic simulations which utilize Buckingham short range potentials and the shell model. It was found that (hk0) surfaces prefer to reconstruct into (100) surface facets which has the lowest energy. The interaction between these (100) facets is repulsive and converges to zero as size of facets grows. The (111) surface can be stabilized by reconstruction into (100) micro-facets.
Date: July 1, 1995
Creator: Yan, M. & Chen, S.P.
Partner: UNT Libraries Government Documents Department

Characterization of surface and sub-surface defects in optical materials using the near field evanescent wave

Description: Optical properties of sub-micron defects at and near the surface are of interest in many applications, e.g. in high power laser systems where initiation of laser induced damage is a critical issue. In-situ scanning atomic force microscopy (AFM) has been used previously to establish a direct correlation between a particular structural surface inhomogeneity (such as a nodular defect in coatings) and the initiation of local laser damage at that inhomogeneity. Near field scanning optical microscopy (NSOM) make it now possible to provide information on both morphology and optical properties of localized defects at and near the surface. The authors have measured the amplitude variation of the near field evanescent wave around nodular defects and sub-surface inclusions in optical coatings and thus detected local laser field intensification. The observed intensity variation of the evanescent wave agrees with theoretical calculations of laser amplification around the inclusion. These findings support the theory that laser damage may be induced by local electrical field enhancement associated with micron and sub-micron defects.
Date: December 31, 1998
Creator: Yan, M.; Oberhelman, S.; Wang, L.; Siekhaus, W. & Kozlowski, M.
Partner: UNT Libraries Government Documents Department

Defect study in fused silica using near field scanning optical microscopy

Description: Surface defects in fused silica have been characterized using Near Field Scanning Optical Microscopy (NSOM). Using total internal reflection of a p- or s- polarized laser beam, optical scattering from defects located on the surface itself as well as in the subsurface layer of polished fused silica has been measured by NSOM. The local scattering intensity has been compared with simultaneously measured surface topography. In addition, surface defects intentionally created on a fused silica surface by nano-indentation have been used to establish a correlation between optical scattering of s- and p- polarized light, surface morphology and the well known subsurface stress-field associated with nano-indentation.
Date: January 21, 1998
Creator: Yan, M.; Wang, L.; Siekhaus, W.; Kozlowski, M.; Yang, J. & Mohideen, U.
Partner: UNT Libraries Government Documents Department

Photothermal mapping of defects in the study of builk damage in KDP

Description: Interest in producing high-damage-threshold KH{sub 2}PO{sub 4} (KDP) and (D{sub x}H{sub 1-x}){sub 2}PO{sub 4} (DKDP) for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). At present only the best crystals meet the NIF system requirements at the third harmonic (351 nm) and only after a laser conditioning process. Neither the mechanism for damage in bulk KDP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of KDP and DKDP, we have been developing a diagnostic tool that will find these locations, we will use other measurement techniques to determine how these locations differ from the surrounding material and why they cause damage. This will allow crystal growers to focus their efforts during the growth process in improving damage thresholds.
Date: December 20, 1997
Creator: Woods, B.; Yan, M.; DeYoreo, J.; Kozlowski, M.; Radouski, H. & Wu, Z.
Partner: UNT Libraries Government Documents Department

Defects and morphological concerns in electroluminescent polymers

Description: The degradation of luminescence in phenylenevinylene polymers is due to exciton diffusion to quenching defects. The microscopic structure of these defects is identified by in-situ vibrational spectroscopy. The authors present evidence that the defect quenching is due to charge transfer by studies on model phenylenevinylene oligomer. In the absence of defect quenchers, the authors have achieved nearly exponential photoluminescence decay with observed lifetimes > 1 ns and a fourfold increase in electroluminescence. They have also utilized picosecond laser spectroscopy to study the formation yield of emissive excitons in the polymer PPVs with different morphology. They have found that increasing polymer chain separation would greatly increases the luminescent efficiency due to avoiding the interchain excitons (exciplexes). Clarification of the nature of photophysics of conjugated polymers suggests avenues for improvement in fabrication of emissive polymers and electroluminescent polymers devices.
Date: July 24, 1995
Creator: Yan, M.; Rothberg, L. & Galvin, M.E.
Partner: UNT Libraries Government Documents Department

Defects in Ga, Cr, and In-doped CoO

Description: From simulation, trivalent cations, Ga(3+), Cr(3+), Co(3+) and In(3+), bind with Co vacancy to form singly pairs with binding energies of about 0.7 to 0.8 eV. These binding energies are in reasonable agreement with experimental measurement of about 0.5 eV. In ion prefers the second nearest neighbor position from a Co vacancy, while other cations prefer the third nearest neighbor sites. Two cations can also forma triplet with a Co vacancy with binding energies of about 1.2 to 1.5 eV. These valves are in fair agreement with the 0.8 to 1.1 eV measured from the tracer diffusion experiments.
Date: July 1, 1995
Creator: Chen, S.P.; Yan, M.; Grimes, R.W. & Vyas, S.
Partner: UNT Libraries Government Documents Department

Atomistic studies in grain boundaries in NiAl

Description: This paper presents the results of atomistic studies of grain boundaries in NiAl B2 alloy. The interatomic forces are described by Finnis-Sinclair type N-body potentials, and are fitted to properties of NiAl. The results show that the structure, energy and cohesive strength of a grain boundary depend strongly on its chemical configurations, and a grain boundary with surplus of Al is the weakest. Energies of antisite defects at the grain boundary {Sigma}5 (210) are also calculated, and the results suggest that Al has much larger tendency to segregate at a grain boundary than Ni does.
Date: January 1, 1995
Creator: Yan, M.; Chen, S. P. & Vitek, V.
Partner: UNT Libraries Government Documents Department

Atomistic study of energy and structure of surfaces in NiO

Description: The structure and energy of surfaces in NiO have been studied by atomistic calculations employing short range Buckingham potentials fitted to properties of NiO. The polarizability of lattice anions is included by using the shell model. The results show that the surface energy depends strongly on surface orientation, and the 100 surface has the lowest energy. Surfaces with higher energy prefer to reconstruct into 100 facets to lower their energy and to stabilize their structure.
Date: December 31, 1994
Creator: Yan, M.; Chen, S. P.; Voter, A. F. & Mitchell, T. E.
Partner: UNT Libraries Government Documents Department

6 Batch Injection and Slipped Beam Tune Measurements in Fermilab?s Main Injector

Description: During NOVA operations it is planned to run the Fermilab Recycler in a 12 batch slip stacking mode. In preparation for this, measurements of the tune during a six batch injection and then as the beam is decelerated by changing the RF frequency have been carried out in the Main Injector. The coherent tune shifts due to the changing beam intensity were measured and compared well with the theoretically expected tune shift. The tune shifts due to changing RF frequency, required for slip stacking, also compare well with the linear theory, although some nonlinear affects are apparent at large frequency changes. These results give us confidence that the expected tunes shifts during 12 batch slip stacking Recycler operations can be accommodated.
Date: May 1, 2012
Creator: Scott, D. J.; Capista, D.; Kourbanis, I.; Seiya, K. & Yan, M.-J.
Partner: UNT Libraries Government Documents Department

Impurity and laser-induced damage in the growth sectors of rapidly grown DKP crystals

Description: We report the experimental results of impurity contamination and laser-induced damage investigations on rapidly grown potassium dihydrogen phosphate (KDP) crystals. Using absorption spectroscopy and chemical analysis, we determined the impurity distribution in the different growing sectors of KDP single crystals. The level of impurity was dependent on starting materials and growth rate. We also studied influence of impurities on laser-induced damage in fast grown KDP. The laser damage threshold (LDT) in the impurity-rich prismatic sector is same as in the high purity pyramidal sector within experimental error. Meanwhile, the LDT at the boundary of the prismatic and pyramidal sectors is less than half of that in the bulk. Furthermore, we found that the thermal annealing of the crystal eliminated the weakness of this sector boundary and increased its LDT to the same level as in the bulk of the crystal. Result suggests that the laser damage occurred in the vicinity of a high; localized strain field.
Date: February 13, 1997
Creator: Yan, M.; Torres, R.; Runkel, M.; Woods, B.; Hutcheon, I.; Zaitseva, N. et al.
Partner: UNT Libraries Government Documents Department

Investigation of damage in KDP using scattering techniques

Description: Interest in producing high damage threshold KH{sub 2}PO{sub 4} (KDP) and (D{sub x}H{sub 1-x}){sub 2}PO{sub 4} (DKDP)(also called KD*P) for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF). Presently only the best crystals meet the NIF system requirements at the third harmonic (351 nm) and only after a laser conditioning process. Neither the mechanism for damage in bulk KDP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of KDP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After we find these locations we will use other measurement techniques to determine how these locations differ from the other surrounding material and why they cause damage. This will allow crystal growers to focus their efforts to improve damage thresholds. Historically damage thresholds have increased it is believed as a consequence of increased purity of the growth solution and through the use of constant filtration during the growth process. As a result we believe that damage is caused by defects in the crystals and have conducted a series of experiments using light scatter to locate these defects and to determine when and where damage occurs. In this paper we present results which show a low correlation between light scatter from bulk defects in KDP and the initiation sites for damage. We have also studied the effects of thermal conditioning on light scatter, strain induced birefringence and damage threshold. We have seen evidence that regions of high strain also exhibit lower damage threshold than the surrounding lower strain material. When thermally conditioned, these crystals show a decrease in some of the strong linear scattering features and a decrease in the strain birefringence while the ...
Date: February 12, 1997
Creator: Woods, B.; Runkel, M.; Yan, M.; Staggs, M.; Zaitseva, N.; Kozlowski, M. et al.
Partner: UNT Libraries Government Documents Department

Time-Resolved Studies of Laser Damage Processes in DKDP Crystals

Description: The authors have used a 3-color imaging technique to obtain time-resolved series of images during nanosecond laser damage in bulk DKDP crystals. In contrast to single-pump, single-probe time-resolved imaging techniques, they are able to correlate behavior during single damage events. This enables us to observe a range of morphological dynamics that is otherwise difficult to study, including: the propagation of elastic sound waves and the liquid/solid melt front from the damage nucleation site and the dynamics of crack formation and propagation.
Date: February 12, 2000
Creator: Jiang, H; McNary, R; Tom, H; Yan, M; Radousky, H & Demos, S
Partner: UNT Libraries Government Documents Department

Depth profiling of polishing-induced contamination on fused silica surfaces

Description: Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. Here secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finished process contamination on fused silica surfaces. Contaminants detected include the major polishing compound components (Ce or Zr from CeO2 or ZrO2), Al presently largely because of the use of Al2O3 in the final cleaning process (Fe, Cu,Cr) incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-100 ppm range, except for Al with exceeds 1000 ppm. A strong correlation has been shown between the presence of a gray haze damage morphology and the use of CeO2 polishing compound. No strong correlation was found however between high levels of Ce, or any other contaminant and the low damage threshold was observed. In fact one of the strongest indications of a correlation is between increased damage thresholds and increased Zr contamination. This suggests that the correlation between redeposition layer and laser damage threshold is not simple an absorbing contaminant issue.
Date: December 20, 1997
Creator: Kozlowski, M.R.; Carr, J.; Hutcheon, I,; Torres, R.; Sheehan, L. Camp, D. & Yan, M.
Partner: UNT Libraries Government Documents Department

Defects and metastable structures of MgAl{sub 2}O{sub 4}

Description: This paper presents calculated properties of normal and inverse spinel structures of MgAl{sub 2}O{sub 4} and of point defects in the spinel structure. These results provide information for further study of possible metastable states. Calculated properties of ``amorphous`` structure are also presented. Atomistic simulations show that in MgAl{sub 2}O{sub 4} spinel structure, the exchange of an Mg{sup 2+} ion with an Al{sup 3+} ion has the lowest energy increase, 0. 92eV/atom. The Schottky defect increases the energy by 3.71 eV/atom. Frenkel defects are difficult to form, increasing the energy at least 4.59eV/atom for the Mg{sup 2+} Frenkel defect. Proposed rock salt structure of MgAl{sub 2}O{sub 4} has smaller volume and larger Young modulus, and the amorphosu state has larger volume and smaller Young modulus than the MgAl{sub 2}O{sub 4} spinel.
Date: July 1, 1995
Creator: Chen, S.P.; Yan, M.; Grimes, R.W.; Vyas, S. & Gale, J.D.
Partner: UNT Libraries Government Documents Department

Characterization of surface and sub-surface defects in optical materials using the near field evanescent wave

Description: In this paper, we characterize surface and sub-surface defects in fused silica and optical coatings using the surface evanescent wave measured by NSOM implemented on a large-stage AFM. A laser irradiates the sample surface in a total internal reflection configuration. The evanescent wave from the surface is collected by an apertured fiber probe of the NSOM. The amplitude of the surface evanescent wave is proportional to the laser intensity at the surface and therefore sensitive to surface as well as sub-surface defects located in the near-field range (~ 100nm). This subsurface region is thought to contain the great majority of polishing-induced defects. The apertured near field fiber probe provides a spatial resolution of ~100nm and the large stage AFM makes it possible to locate defects in samples with diameters up to 6". We are thus able to map out surface- as well as near-surface optical defects in a large optic as a first step in understanding laser damage mechanisms. These observed defects will be exposed in situ to high fluence laser light to correlate with the initiation of laser damage. We have also used NSOM to measure in situ the effect of chemical etching on optical properties of defects. Post-processing such as chemical etching performed after mechanical polishing is frequently used, and sometimes increases laser damage thresholds. We have measured the amplitude variation of the near field evanescent wave around nodular defects and sub-surface inclusions in optical coatings and thus detected local laser field intensification. The observed intensity variation of the evanescent wave agrees with theoretical calculations of laser amplification around the inclusion. These findings support the theory that laser damage may be induced by local electrical field enhancement associated with micron and sub-micron defects.
Date: September 17, 1998
Creator: Kozlowski, M; Oberhelman, S; Siekhaus, W; Wang, L & Yan, M
Partner: UNT Libraries Government Documents Department

Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation

Description: Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 10<sup>4</sup>-10<sup>6</sup> per mm<sup>3</sup> depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 10<sup>2</sup> or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects.
Date: December 15, 1998
Creator: De Yoreo, J. J.; Demos, S. G.; Radousky, H. B.; Staggs, M. & Yan, M.
Partner: UNT Libraries Government Documents Department

Single-beam photothermal microscopy - a new diagnostic tool for optical materials

Description: A novel photothermal microscopy (PTM) is developed which uses only one laser beam, working as both the pump and the probe. The principle of this single-beam PTM is based on the detection of the second harmonic component of the laser modulated scattering (LMS) signal. This component has a linear dependence on the optical absorptance of the tested area and a quadratic dependence on the pump laser power. Using a pump laser at the wavelengths of 514.5- and 532-nm high-resolution photothermal scans are performed for polished fused silica surfaces and a HfO{sub 2}/SiO{sub 2} multilayer coatings. The results are compared with those from the traditional two-beam PTM mapping. It is demonstrated that the single-beam PTM is more user-friendly (i.e. no alignment is needed) than conventional two-beam PTM and, offers a higher spatial resolution for defect detection.
Date: December 22, 1998
Creator: Feit, M. D.; Kozlowski, M.; Natoli, J. Y.; Rubenchik, A. M.; Sheehan, L.; Wu, Z. L. et al.
Partner: UNT Libraries Government Documents Department

Single Fluorescent Molecule Confocal Microscopy: A New Tool for Molecular Biology Research and Biosensor Development

Description: Our original proposal was presented to the LDRD committee on February 18, 1999. The revised proposal that followed incorporated changes that addressed the issues, concerns, and suggestions put forth by the committee members both during the presentation and in subsequent discussions we've had with individual committee members. The goal of the proposal was to establish an SMD confocal microscopy capability and technology base at LLNL. Here we report on our progress during the 6-month period for which funding was available.
Date: March 9, 2000
Creator: Darrow, C.; Huser, T.; Campos, C.; Yan, M.; Lane, S. & Balhorn, R.
Partner: UNT Libraries Government Documents Department

Confocal Imaging of Biological Tissues Using Second Harmonic Generation

Description: A confocal microscopy imaging system was devised to selectively detect Second harmonic signals generated by biological tissues. Several types of biological tissues were examined using this imaging system, including human teeth, bovine blood vessels, and chicken skin. All these tissues generated strong second harmonic signals. There is considerable evidence that the source of these signals in tissue is collagen. Collagen, the predominant component of most tissues, is known to have second order nonlinear susceptibility. This technique may have diagnostic usefulness in pathophysiological conditions characterized by changes in collagen structure including malignant transformation of nevi, progression of diabetic complications, and abnormalities in wound healing.
Date: March 6, 2000
Creator: Kim, B-M.; Stoller, P.; Reiser, K.; Eichler, J.; Yan, M.; Rubenchik, A. et al.
Partner: UNT Libraries Government Documents Department

Producing KDP and DKDP crystals for the NIF laser

Description: The cost and physics requirements of the NIF have established two important roles for potassium dihydrogen phosphate (KDP) crystals. 1. To extract more laser energy per unit of flashlamp light and laser glass, the NIF has adopted a multipass architecture as shown in Figure 1. Light is injected in the transport spatial filter, first traverses the power amplifiers, and then is directed to main amplifiers, where it makes four passes before being redirected through the power amplifiers towards the target. To enable the multipass of the main amplifiers, a KDP-containing Pockels cell rotates the polarization of the beam to make it either transmit through or reflect off a polarizer held at Brewster's angle within the main laser cavity. If transmitted, the light reflects off a mirror and makes another pass through the cavity. If reflected, it proceeds through the power amplifier to the target. the original seed crystal as the pyramid faces grow. Unfortunately, this pyramidal growth is very slow, and it takes about two years to grow a crystal to NIF size. To provide more programmatic flexibility and reduce costs in the long run, we have developed an alternative technology commonly called rapid growth. Through a combination of higher temperatures and higher supersaturation of the growth solution, a NIF-size boule can be grown in 1 to 2 months from a small ''point'' seed. However, growing boules of adequate size is not sufficient. Care must be taken to prevent inclusions of growth solution and incorporation of atomically substituted 2. Implosions for ICF work far better at shorter wavelengths due to less generation of hot electrons, which preheat the fuel and make it harder to compress. Compromising between optic lifetime and implosion efficiency, both Nova and the NIF operate at a tripled frequency of the 1053-nm fundamental frequency of a neodymium ...
Date: September 2, 1999
Creator: Atherton, L J; Burnham, A K; Combs, R C; Couture, S A; De Yoreo, J J; Hawley-Fedder, R A et al.
Partner: UNT Libraries Government Documents Department