309 research outputs found
The Effect of Poststenotic Vessel Wall Compliance upon the Pulsus Tardus Phenomenon
Recent studies have investigated the detection of significant arterial stenoses through identification of the "pulsus tardus" phenomenon in Doppler waveforms obtained distal to the stenosis. The etiology of this phenomenon, however, has not yet been determined. Using an in vitro model based upon an electrical circuit analogy, the authors had as their objective to determine whether the compliance of the vessel wall immediately distal to a stenosis, in conjunction with the stenosis, is the cause of pulsus tardus. For a constant stenosis, it was found that the degree of pulsus tardus, as quantitated by the acceleration index, increased as the compliance of the poststenotic segment increased. It is concluded that pulsus tardus distal to an arterial stenosis is likely due to the compliance of the normally distensible artery, in conjunction with the stenosis. Pathological conditions that alter the compliance of the poststenotic segment may affect the degree of pulsus tardus, perhaps limiting its usefulness for upstream stenosis detection.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68136/2/10.1177_000331979404500703.pd
Impact Ionization in ZnS
The impact ionization rate and its orientation dependence in k space is
calculated for ZnS. The numerical results indicate a strong correlation to the
band structure. The use of a q-dependent screening function for the Coulomb
interaction between conduction and valence electrons is found to be essential.
A simple fit formula is presented for easy calculation of the energy dependent
transition rate.Comment: 9 pages LaTeX file, 3 EPS-figures (use psfig.sty), accepted for
publication in PRB as brief Report (LaTeX source replaces raw-postscript
file
Accurate first principles detailed balance determination of Auger recombination and impact ionization rates in semiconductors
The technologically important problem of predicting Auger recombination
lifetimes in semiconductors is addressed by means of a fully first--principles
formalism. The calculations employ highly precise energy bands and wave
functions provided by the full--potential linearized augmented plane wave
(FLAPW) code based on the screened exchange local density approximation. The
minority carrier Auger lifetime is determined by two closely related
approaches: \emph{i}) a direct evaluation of the Auger rates within Fermi's
Golden Rule, and \emph{ii}) an indirect evaluation, based on a detailed balance
formulation combining Auger recombination and its inverse process, impact
ionization, in a unified framework. Calculated carrier lifetimes determined
with the direct and indirect methods show excellent consistency \emph{i})
between them for -doped GaAs and \emph{ii}%) with measured values for GaAs
and InGaAs. This demonstrates the validity and accuracy of the computational
formalism for the Auger lifetime and indicates a new sensitive tool for
possible use in materials performance optimization.Comment: Phys. Rev. Lett. accepte
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The effect of lattice temperature on surface damage in fused silica optics
We examine the effect of lattice temperature on the probability of surface damage initiation for 355nm, 7ns laser pulses for surface temperatures below the melting point to temperatures well above the melting point of fused silica. At sufficiently high surface temperatures, damage thresholds are dramatically reduced. Our results indicate a temperature activated absorption and support the idea of a lattice temperature threshold of surface damage. From these measurements, we estimate the temperature dependent absorption coefficient for intrinsic silica
A novel, aerosol-nanocrystal floating-gate device for non-volatile memory applications
This paper describes the fabrication, and structural and electrical characterization of a new, aerosol-nanocrystal floating-gate FET, aimed at non-volatile memory (NVM) applications. This aerosol-nanocrystal NVM device features program/erase characteristics comparable to conventional stacked gate NVM devices, excellent endurance (>l0^5 P/E cycles), and long-term non-volatility in spite of a thin bottom oxide (55-60Ă…). In addition, a very simple fabrication process makes this aerosol-nanocrystal NVM device a potential candidate for low cost NVM applications
Dilated renal collecting systems:Differentiating obstructive from nonobstructive dilation using duplex doppler ultrasound
Two patients with ileal loop urinary diversions, studied with real-time and Doppler sonogphy ("duplex sonography") of the kidneys, were shown to have dilated intrarenal collecting Resistive index measurements calculated from the Doppler signal correctly identified obstructive dilatation in 1 case and nonobstructive dilatation in the other.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29467/1/0000553.pd
Impact ionization in GaAs: a screened exchange density functional approach
Results are presented of a fully ab-initio calculation of impact ionization
rates in GaAs within the density functional theory framework, using a
screened-exchange formalism and the highly precise all-electron full-potential
linearized augmented plane wave (FLAPW) method. The calculated impact
ionization rates show a marked orientation dependence in {\bf k} space,
indicating the strong restrictions imposed by the conservation of energy and
momentum. This anisotropy diminishes as the impacting electron energy
increases. A Keldysh type fit performed on the energy-dependent rate shows a
rather soft edge and a threshold energy greater than the direct band gap. The
consistency with available Monte Carlo and empirical pseudopotential
calculations shows the reliability of our approach and paves the way to
ab-initio calculations of pair production rates in new and more complex
materials.Comment: 11 pages, 4 figures, Submitted to Phys. Rev.
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Phase modulation in high power optical systems caused by pulsed laser-driven particle ablation events
Results of applying a non-evaporative mitigation technique to laser-initiated surface damage on fused-silica
We present results from a study to determine an acceptable CO{sub 2} laser-based non-evaporative mitigation protocol for use on surface damage sites in fused-silica optics. A promising protocol is identified and evaluated on a set of surface damage sites created under ICF-type laser conditions. Mitigation protocol acceptability criteria for damage re-initiation and growth, downstream intensification, and residual stress are discussed. In previous work, we found that a power ramp at the end of the protocol effectively minimizes the residual stress (<25 MPa) left in the substrate. However, the biggest difficulty in determining an acceptable protocol was balancing between low re-initiation and problematic downstream intensification. Typical growing surface damage sites mitigated with a candidate CO{sub 2} laser-based mitigation protocol all survived 351 nm, 5 ns damage testing to fluences >12.5 J/cm{sup 2}. The downstream intensification arising from the mitigated sites is evaluated, and all but one of the sites has 100% passing downstream damage expectation values. We demonstrate, for the first time, a successful non-evaporative 10.6 {micro}m CO{sub 2} laser mitigation protocol applicable to fused-silica optics used on fusion-class lasers like the National Ignition Facility (NIF)
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Pump and probe damage testing for investigation of transient material modifications associated with laser damage in optical materials
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
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