95,157 research outputs found
Transformation elastodynamics and active exterior acoustic cloaking
This chapter consists of three parts. In the first part we recall the
elastodynamic equations under coordinate transformations. The idea is to use
coordinate transformations to manipulate waves propagating in an elastic
material. Then we study the effect of transformations on a mass-spring network
model. The transformed networks can be realized with "torque springs", which
are introduced here and are springs with a force proportional to the
displacement in a direction other than the direction of the spring terminals.
Possible homogenizations of the transformed networks are presented, with
potential applications to cloaking. In the second and third parts we present
cloaking methods that are based on cancelling an incident field using active
devices which are exterior to the cloaked region and that do not generate
significant fields far away from the devices. In the second part, the exterior
cloaking problem for the Laplace equation is reformulated as the problem of
polynomial approximation of analytic functions. An explicit solution is given
that allows to cloak larger objects at a fixed distance from the cloaking
device, compared to previous explicit solutions. In the third part we consider
the active exterior cloaking problem for the Helmholtz equation in 3D. Our
method uses the Green's formula and an addition theorem for spherical outgoing
waves to design devices that mimic the effect of the single and double layer
potentials in Green's formula.Comment: Submitted as a chapter for the volume "Acoustic metamaterials:
Negative refraction, imaging, lensing and cloaking", Craster and Guenneau
ed., Springe
Accurate Transfer Maps for Realistic Beamline Elements: Part I, Straight Elements
The behavior of orbits in charged-particle beam transport systems, including
both linear and circular accelerators as well as final focus sections and
spectrometers, can depend sensitively on nonlinear fringe-field and
high-order-multipole effects in the various beam-line elements. The inclusion
of these effects requires a detailed and realistic model of the interior and
fringe fields, including their high spatial derivatives. A collection of
surface fitting methods has been developed for extracting this information
accurately from 3-dimensional field data on a grid, as provided by various
3-dimensional finite-element field codes. Based on these realistic field
models, Lie or other methods may be used to compute accurate design orbits and
accurate transfer maps about these orbits. Part I of this work presents a
treatment of straight-axis magnetic elements, while Part II will treat bending
dipoles with large sagitta. An exactly-soluble but numerically challenging
model field is used to provide a rigorous collection of performance benchmarks.Comment: Accepted to PRST-AB. Changes: minor figure modifications, reference
added, typos corrected
A Surface Admittance Equivalence Principle for Non-Radiating and Cloaking Problems
In this paper, we address non-radiating and cloaking problems exploiting the
surface equivalence principle, by imposing at any arbitrary boundary the
control of the admittance discontinuity between the overall object (with or
without cloak) and the background. After a rigorous demonstration, we apply
this model to a non-radiating problem, appealing for anapole modes and
metamolecules modeling, and to a cloaking problem, appealing for non-Foster
metasurface design. A straightforward analytical condition is obtained for
controlling the scattering of a dielectric object over a surface boundary of
interest. Previous quasi-static results are confirmed and a general closed-form
solution beyond the subwavelength regime is provided. In addition, this
formulation can be extended to other wave phenomena once the proper admittance
function is defined (thermal, acoustics, elastomechanics, etc.).Comment: 7 page
Quantum Conductivity for Metal-Insulator-Metal Nanostructures
We present a methodology based on quantum mechanics for assigning quantum
conductivity when an ac field is applied across a variable gap between two
plasmonic nanoparticles with an insulator sandwiched between them. The quantum
tunneling effect is portrayed by a set of quantum conductivity coefficients
describing the linear ac conductivity responding at the frequency of the
applied field and nonlinear coefficients that modulate the field amplitude at
the fundamental frequency and its harmonics. The quantum conductivity,
determined with no fit parameters, has both frequency and gap dependence that
can be applied to determine the nonlinear quantum effects of strong applied
electromagnetic fields even when the system is composed of dissimilar metal
nanostructures. Our methodology compares well to results on quantum tunneling
effects reported in the literature and it is simple to extend it to a number of
systems with different metals and different insulators between them
Restoring the full velocity field in the gaseous disk ofthe spiral galaxy NGC 157
We analyse the line-of-sight velocity field of ionized gas in the spiral
galaxy NGC 157 which has been obtained in the H\alpha emission at the 6m
telescope of SAO RAS. The existence of systematic deviations of the observed
gas velocities from pure circular motion is shown. A detailed investigation of
these deviations is undertaken by applying a Fourier analysis of the azimuthal
distributions of the line-of-sight velocities at different distances from the
galactic center. As a result of the analysis, all the main parameters of the
wave spiral pattern are determined: the corotation radius, the amplitudes and
phases of the gas velocity perturbations at different radii, and the velocity
of circular rotation of the disk corrected for the velocity perturbations due
to spiral arms. At a high confidence level, the presence of the two giant
anticyclones in the reference frame rotating with the spiral pattern is shown;
their sizes and the localization of their centers are consistent with the
results of the analytic theory and of numerical simulations. Besides the
anticyclones, the existence of cyclones in residual velocity fields of spiral
galaxies is predicted. In the reference frame rotating with the spiral pattern
these cyclones have to reveal themselves in galaxies where a radial gradient of
azimuthal residual velocity is steeper than that of the rotation velocity
(abridged).Comment: 23 pages including 25 eps-figures. Accepted for publication in A&
Spectroscopy studies of straincompensated mid-infrared QCL active regions on misoriented substrates
In this work, we perform spectroscopic studies of AlGaAs/InGaAs quantum cascade laser structures that demonstrate frequency mixing using strain-compensated active regions. Using a three-quantum well design based on diagonal transitions, we incorporate strain in the active region using single and double well configurations on various surface planes (100) and (111). We observe the influence of piezoelectric properties in molecular beam epitaxy grown structures, where the addition of indium in the GaAs matrix increases the band bending in between injector regions and demonstrates a strong dependence on process conditions that include sample preparation, deposition rates, mole fraction, and enhanced surface diffusion lengths. We produced mid-infrared structures under identical deposition conditions that differentiate the role of indium(strain) in intracavity frequency mixing and show evidence that this design can potentially be implemented using other material systems
Science Requirements and Conceptual Design for a Polarized Medium Energy Electron-Ion Collider at Jefferson Lab
This report presents a brief summary of the science opportunities and program
of a polarized medium energy electron-ion collider at Jefferson Lab and a
comprehensive description of the conceptual design of such a collider based on
the CEBAF electron accelerator facility.Comment: 160 pages, ~93 figures This work was supported by the U.S. Department
of Energy, Office of Nuclear Physics, under Contract No. DE-AC05-06OR23177,
DE-AC02-06CH11357, DE-AC05-060R23177, and DESC0005823. The U.S. Government
retains a non-exclusive, paid-up, irrevocable, world-wide license to publish
or reproduce this manuscript for U.S. Government purpose
Surface Integral Method for the Second Harmonic Generation in Metal Nanoparticles
Second harmonic (SH) radiation in metal nanoparticles is generated by both
nonlocal-bulk and local-surface SH sources, induced by the electromagnetic
field at the fundamental frequency. We propose a surface integral equation
(SIE) method for evaluating the SH radiation generated by metal nanoparticles
with arbitrary shapes, considering all SH sources. We demonstrate that the
contribution of the nonlocal-bulk SH sources to the SH electromagnetic field
can be taken into account through equivalent surface electric and magnetic
currents. We numerically solve the SIE problem by using the Galerkin method and
the Rao-Wilton-Glisson basis functions in the framework of the distribution
theory. The accuracy of the proposed method is verified by comparison with the
SH-Mie analytical solution. As an example of a complex-shaped particle, we
investigate the SH scattering by a triangular nano-prism. This method paves the
way for a better understanding of the SH generation process in arbitrarily
shaped nanoparticles and can also have a high impact in the design of novel
nanoplasmonic devices with enhanced SH emission
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