484 research outputs found
Application of the discrete dipole approximation to very large refractive indices: Filtered coupled dipoles revived
Application of the discrete dipole approximation to very large refractive indices: Filtered coupled dipoles revived
Application of the discrete dipole approximation to very large refractive indices: Filtered coupled dipoles revived
On the Spectrum of Volume Integral Operators in Acoustic Scattering
Volume integral equations have been used as a theoretical tool in scattering
theory for a long time. A classical application is an existence proof for the
scattering problem based on the theory of Fredholm integral equations. This
approach is described for acoustic and electromagnetic scattering in the books
by Colton and Kress [CoKr83, CoKr98] where volume integral equations appear
under the name "Lippmann-Schwinger equations". In electromagnetic scattering by
penetrable objects, the volume integral equation (VIE) method has also been
used for numerical computations. In particular the class of discretization
methods known as "discrete dipole approximation" [PuPe73, DrFl94] has become a
standard tool in computational optics applied to atmospheric sciences,
astrophysics and recently to nano-science under the keyword "optical tweezers",
see the survey article [YuHo07] and the literature quoted there. In sharp
contrast to the abundance of articles by physicists describing and analyzing
applications of the VIE method, the mathematical literature on the subject
consists only of a few articles. An early spectral analysis of a VIE for
magnetic problems was given in [FrPa84], and more recently [Ki07, KiLe09] have
found sufficient conditions for well-posedness of the VIE in electromagnetic
and acoustic scattering with variable coefficients. In [CoDK10, CoDS12], we
investigated the essential spectrum of the VIE in electromagnetic scattering
under general conditions on the complex-valued coefficients, finding necessary
and sufficient conditions for well-posedness in the sense of Fredholm in the
physically relevant energy spaces. A detailed presentation of these results can
be found in the thesis [Sa14]. Publications based on the thesis are in
preparation. Curiously, whereas the study of VIE in electromagnetic scattering
has thus been completed as far as questions of Fredholm properties are
concerned, the simpler case of acoustic scattering does not seem to have been
covered in the same depth. It is the purpose of the present paper to close this
gap
Plasmonic nanoparticle monomers and dimers: From nano-antennas to chiral metamaterials
We review the basic physics behind light interaction with plasmonic
nanoparticles. The theoretical foundations of light scattering on one metallic
particle (a plasmonic monomer) and two interacting particles (a plasmonic
dimer) are systematically investigated. Expressions for effective particle
susceptibility (polarizability) are derived, and applications of these results
to plasmonic nanoantennas are outlined. In the long-wavelength limit, the
effective macroscopic parameters of an array of plasmonic dimers are
calculated. These parameters are attributable to an effective medium
corresponding to a dilute arrangement of nanoparticles, i.e., a metamaterial
where plasmonic monomers or dimers have the function of "meta-atoms". It is
shown that planar dimers consisting of rod-like particles generally possess
elliptical dichroism and function as atoms for planar chiral metamaterials. The
fabricational simplicity of the proposed rod-dimer geometry can be used in the
design of more cost-effective chiral metamaterials in the optical domain.Comment: submitted to Appl. Phys.
Two years of flight of the Pamela experiment: results and perspectives
PAMELA is a satellite borne experiment designed to study with great accuracy
cosmic rays of galactic, solar, and trapped nature in a wide energy range
(protons: 80 MeV-700 GeV, electrons 50 MeV-400 GeV). Main objective is the
study of the antimatter component: antiprotons (80 MeV-190 GeV), positrons (50
MeV-270 GeV) and search for antinuclei with a precision of the order of
). The experiment, housed on board the Russian Resurs-DK1 satellite,
was launched on June, 2006 in a orbit with an
inclination of 70 degrees. In this work we describe the scientific objectives
and the performance of PAMELA in its first two years of operation. Data on
protons of trapped, secondary and galactic nature - as well as measurements of
the December 2006 Solar Particle Event - are also provided.Comment: To appear on J. Phys. Soc. Jpn. as part of the proceedings of the
International Workshop on Advances in Cosmic Ray Science March, 17-19, 2008
Waseda University, Shinjuku, Tokyo, Japa
Optical detection of single non-absorbing molecules using the surface plasmon of a gold nanorod
Current optical detection schemes for single molecules require light
absorption, either to produce fluorescence or direct absorption signals. This
severely limits the range of molecules that can be detected, because most
molecules are purely refractive. Metal nanoparticles or dielectric resonators
detect non-absorbing molecules by a resonance shift in response to a local
perturbation of the refractive index, but neither has reached single-protein
sensitivity. The most sensitive plasmon sensors to date detect single molecules
only when the plasmon shift is amplified by a highly polarizable label or by a
localized precipitation reaction on the particle's surface. Without
amplification, the sensitivity only allows for the statistical detection of
single molecules. Here we demonstrate plasmonic detection of single molecules
in realtime, without the need for labeling or amplification. We monitor the
plasmon resonance of a single gold nanorod with a sensitive photothermal assay
and achieve a ~ 700-fold increase in sensitivity compared to state-of-the-art
plasmon sensors. We find that the sensitivity of the sensor is intrinsically
limited due to spectral diffusion of the SPR. We believe this is the first
optical technique that detects single molecules purely by their refractive
index, without any need for photon absorption by the molecule. The small size,
bio-compatibility and straightforward surface chemistry of gold nanorods may
open the way to the selective and local detection of purely refractive proteins
in live cells
A new measurement of the antiproton-to-proton flux ratio up to 100 GeV in the cosmic radiation
A new measurement of the cosmic ray antiproton-to-proton flux ratio between 1
and 100 GeV is presented. The results were obtained with the PAMELA experiment,
which was launched into low-earth orbit on-board the Resurs-DK1 satellite on
June 15th 2006. During 500 days of data collection a total of about 1000
antiprotons have been identified, including 100 above an energy of 20 GeV. The
high-energy results are a ten-fold improvement in statistics with respect to
all previously published data. The data follow the trend expected from
secondary production calculations and significantly constrain contributions
from exotic sources, e.g. dark matter particle annihilations.Comment: 10 pages, 4 figures, 1 tabl
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