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Search Results: 1 - 10 of 469213 matches for " Keith A. Nelson "
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Finite Difference Time Domain (FDTD) Simulations of Electromagnetic Wave Propagation Using a Spreadsheet
David W. Ward,Keith A. Nelson
Physics , 2004,
Abstract: We describe a simple and intuitive implementation of the method of finite difference time domain simulations for propagating electromagnetic waves using the simplest possible tools available in Microsoft Excel. The method overcomes the usual obstacles of familiarity with programming languages as it relies on little more than the cut and paste features that are standard in Excel. Avenues of exploration by students are proposed and sample graphs are included. The pedagogical effectiveness of the implementation was tested during an Independent Activities Period class, composed of 80% freshmen, at MIT, and yielded positive results.
Onset of non-diffusive phonon transport in transient thermal grating decay
A. A. Maznev,Jeremy A. Johnson,Keith A. Nelson
Physics , 2011, DOI: 10.1103/PhysRevB.84.195206
Abstract: The relaxation of a spatially sinusoidal temperature perturbation in a dielectric crystal at a temperature comparable to or higher than the Debye temperature is investigated theoretically. We assume that most phonons contributing to the specific heat have mean free path (MFP) much shorter than the thermal transport distance and can be described by the thermal diffusion model. Low-frequency phonons that may have MFP comparable to or longer than the grating period are described by the Boltzmann transport equation. These low-frequency phonons are assumed to interact with the thermal reservoir of high frequency phonons but not with each other. Within the single mode relaxation time approximation, an analytical expression for the thermal grating relaxation rate is obtained. We show that the contribution of "ballistic" phonons with long MFP to the effective thermal conductivity governing the grating decay is suppressed compared to their contribution to thermal transport at long distances. The reduction in the effective thermal conductivity in Si at room temperature is found to be significant at grating periods as large as 10 microns.
$α$-Scale Decoupling of the Mechanical Relaxation and Diverging Shear Wave Propagation Lengthscale in Triphenylphosphite
Darius H. Torchinsky,Jeremy A. Johnson,Keith A. Nelson
Physics , 2011, DOI: 10.1063/1.3700756
Abstract: We have performed depolarized Impulsive Stimulated Scattering experiments to observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from $\sim$10 - 500 MHz. These measurements, in tandem with previously performed longitudinal and shear measurements, permit further analyses of the relaxation dynamics of TPP within the framework of the mode coupling theory (MCT). Our results provide evidence of $\alpha$ coupling between the shear and longitudinal degrees of freedom up to a decoupling temperature $T_c$ = 231 K. A lower bound length scale of shear wave propagation in liquids verified the exponent predicted by theory in the vicinity of the decoupling temperature.
Nanoconcentration of Terahertz Radiation in Plasmonic Waveguides
Anastasia Rusina,Maxim Durach,Keith A. Nelson,Mark I. Stockman
Physics , 2008, DOI: 10.1364/OE.16.018576
Abstract: Recent years have seen an explosive research and development of nanoplasmonics in the visible and near-infrared (near-ir) frequency regions. One of the most fundamental effects in nanoplasmonics is nano-concentration of optical energy. Plasmonic nanofocusing has been predicted and experimentally achieved. It will be very beneficial for the fundamental science, engineering, environmental, and defense applications to be able to nano-concentrate terahertz radiation (frequency 1 - 10 THz or vacuum wavelength 300 - 30 microns). This will allow for the nanoscale spatial resolution for THz imaging and introduce the THz spectroscopy on the nanoscale, taking full advantage of the rich THz spectra and submicron to nanoscale structures of many engineering, physical, and biological objects of wide interest: electronic components (integrated circuits, etc.), bacteria, their spores, viruses, macromolecules, carbon clusters and nanotubes, etc. In this Letter we establish the principal limits for the nanoconcentration of the THz radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration We predict that the adiabatic compression of THz radiation from the initial spot size of light wavelength to the final size of R = 100 - 250 nm can be achieved with the THz radiation intensity increased by a factor of 10 to 250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for the THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses will allow the observation of nonlinear THz effects and a carrying out a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative.
From populations to coherences and back again: a new insight about rotating dipoles
Sharly Fleischer,Robert W. Field,Keith A. Nelson
Physics , 2014,
Abstract: The process in which light is absorbed by an ensemble of molecules obeys the fundamental law of conservation of energy - the energy removed from the light resides in the molecular degrees of freedom. In the process of coherent emission from excited molecules known as free-induction decay (FID), spectroscopic measurements of the emitted radiation are often conducted in order to gain insight into molecular structure and behavior. However, the direct influence of the FID emission on its molecular source is not measured directly. In this work we present experimental evidence from the molecular rotor perspective of the consequences of terahertz-frequency FID emission from rotationally excited molecules. We show that when gas phase molecules transiently orient under field-free conditions, the energy radiated via FID is manifest as an abrupt reduction in excited rotational populations. The connection between coherent FID emission and stored energy leaves a particularly distinct signature in our measurements, but the results are generalizable throughout coherent spectroscopy and coherent control.
On the physical origins of the negative index of refraction
David W. Ward,Keith A. Nelson,Kevin J. Webb
Physics , 2004, DOI: 10.1088/1367-2630/7/1/213
Abstract: The physical origins of negative refractive index are derived from a dilute microscopic model, producing a result that is generalized to the dense condensed phase limit. In particular, scattering from a thin sheet of electric and magnetic dipoles driven above resonance is used to form a fundamental description for negative refraction. Of practical significance, loss and dispersion are implicit in the microscopic model. While naturally occurring negative index materials are unavailable, ferromagnetic and ferroelectric materials provide device design opportunities.
Nonlinear Acoustics at GHz Frequencies in a Viscoelastic Fragile Glass Former
Christoph Klieber,Vitalyi E. Gusev,Thomas Pezeril,Keith A. Nelson
Physics , 2014, DOI: 10.1103/PhysRevLett.114.065701
Abstract: Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost ten percent in acoustic pulse propagation speed of its leading shock front at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement we deduce the nonlinear acoustic parameter of the glass former in the GHz frequency range across the glass transition temperature.
Molecular Orientation by Intense Single Cycle THz Pulses
Sharly Fleischer,Yan Zhou,Robert W. Field,Keith A. Nelson
Physics , 2011, DOI: 10.1103/PhysRevLett.107.163603
Abstract: Intense single-cycle THz pulses resonantly interacting with molecular rotations are shown to induce significant field-free orientation under ambient conditions. We calculate and measure the angular distribution associated with THz-driven rotational motion and correlate the THz-induced orientation and alignment in an OCS gas sample.
Simulation of Phonon-Polariton Generation and Propagation in Ferroelectric LiNbO3 Crystals
David W. Ward,Eric Statz,Nikolay Stoyanov,Keith A. Nelson
Physics , 2004,
Abstract: We simulate propagation of phonon-polaritons (admixtures of polar lattice vibrations and electromagnetic waves) in ferroelectric LiNbO3 with a model that consists of a spatially periodic array of harmonic oscillators coupled to THz electromagnetic waves through an electric dipole moment. We show that when this model is combined with the auxiliary differential equation method of finite difference time domain (FDTD) simulations, the salient features of phonon-polaritons may be illustrated. Further, we introduce second order nonlinear coupling to an optical field to demonstrate phonon-polariton generation by impulsive stimulated Raman scattering (ISRS). The phonon-polariton dispersion relation in bulk ferroelectric LiNbO3 is determined from simulation.
Assessment of knowledge retention and the value of proctored ultrasound exams after the introduction of an emergency ultrasound curriculum
Vicki E Noble, Bret P Nelson, A Nicholas Sutingco, Keith A Marill, Hilarie Cranmer
BMC Medical Education , 2007, DOI: 10.1186/1472-6920-7-40
Abstract: To compare different training regimens for both attending physicians and first year residents (interns), a prospective study was undertaken to assess knowledge retention six months after an introductory ultrasound course. Eighteen emergency physicians and twelve emergency medicine interns were assessed before and 6 months after an introductory ultrasound course using a standardized, image-based ultrasound test. In addition, the twelve emergency medicine interns were randomized to a group which received additional proctored ultrasound hands-on instruction from qualified faculty or to a control group with no hands-on instruction to determine if proctored exam training impacts ultrasound knowledge. Paired and unpaired estimates of the median shift in test scores between groups were made with the Hodges-Lehmann extension of the Wilcoxon-Mann-Whitney test.Six months after the introductory course, test scores (out of a 24 point test) were a median of 2.0 (95% CI 1.0 to 3.0) points higher for residents in the control group, 5.0 (95% CI 3.0 to 6.0) points higher for residents in the proctored group, and 2.5 (95% CI 1.0 to 4.0) points higher for the faculty group. Residents randomized to undergo proctored ultrasound examinations exhibited a higher score improvement than their cohorts who were not with a median difference of 3.0 (95% CI 1.0 to 5.0) points.We conclude that significant improvement in knowledge persists six months after a standard introductory ultrasound course, and incorporating proctored ultrasound training into an emergency ultrasound curriculum may yield even higher knowledge retention.Bedside ultrasound (US) has been utilized in the emergency department (ED) setting for nearly two decades [1-6]. With increased use by emergency physicians (EPs) has come mandated training during residency, and many guidelines have been proposed regarding training residents in the use of bedside US. In 1994, the Society for Academic Emergency Medicine (SAEM) proposed a bedside
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