7,330 research outputs found
Theory of fishnet negative-index optical metamaterials
We theoretically study fishnet metamaterials at optical frequencies. In
contrast to earlier works, we provide a microscopic description by tracking the
transversal and longitudinal flows of energy through the fishnet mesh composed
of intersecting subwavelength plasmonic waveguides. The analysis is supported
by a semi-analytical model based on surface-plasmon coupled-mode equations,
which provides accurate formulas for the fishnet refractive index, including
the real-negative and imaginary parts. The model simply explains how the
surface plasmons couple at the waveguide intersections and it shines new light
on the fishnet negative-index paradigm at optical frequencies. Extension of the
theory for loss-compensated metamaterials with gain media is also presented.Comment: 4 figure
Homogenization of an ensemble of interacting resonant scatterers
We study theoretically the concept of homogenization in optics using an
ensemble of randomly distributed resonant stationary atoms with density .
The ensemble is dense enough for the usual condition for homogenization, viz.
, to be reached. Introducing the coherent and incoherent
scattered powers, we define two criteria to define the homogenization regime.
We find that when the excitation field is tuned in a broad frequency range
around the resonance, none of the criteria for homogenization is fulfilled,
meaning that the condition is not sufficient to
characterize the homogenized regime around the atomic resonance. We interpret
these results as a consequence of the light-induced dipole-dipole interactions
between the atoms, which implies a description of scattering in terms of
collective modes rather than as a sequence of individual scattering events.
Finally, we show that, although homogenization can never be reached for a dense
ensemble of randomly positioned laser-cooled atoms around resonance, it becomes
possible if one introduces spatial correlations in the positions of the atoms
or non-radiative losses, such as would be the case for organic molecules or
quantum dots coupled to a phonon bath.Comment: 9 pages, 5 figures. Corrected mistakes in reference
Slow-wave effect and mode-profile matching in Photonic Crystal microcavities
Physical mechanisms involved in the light confinement in photonic crystal
slab microcavities are investigated. We first present a full three-dimensional
numerical study of these microcavities. Then, to gain physical insight into the
confinement mechanisms, we develop a Fabry-Perot model. This model provides
accurate predictions and sheds new light on the physics of light confinement.
We clearly identify two mechanisms to enhance the Q factor of these
microcavities. The first one consists in improving the mode-profile matching at
the cavity terminations and the second one in using a slow wave in the cavity.Comment: accepted for publication in Phys. Rev. B, 8 pages, 4 figure
Difference between penetration and damping lengths in photonic crystal mirrors
Different mirror geometries in two-dimensional photonic crystal slabs are
studied with fully-vectorial calculations. We compare their optical properties
and, in particular, we show that, for heterostructure mirrors, the penetration
length associated with the delay induced by distributed reflection is not
correlated to the characteristic damping length of the electromagnetic energy
distribution in the mirror. This unexpected result evidences that the usual
trade-off between short damping lengths and large penetration lengths that is
classically encountered in distributed Bragg reflectors can be overcome with
carefully designed photonic crystal structures.Comment: to be published in Applied Physics Letters, 4 pages, 4 figure
Relevance of d-D interactions on neutron and tritium production in IFMIF-EVEDA accelerator prototype
In the IFMIF-EVEDA accelerator prototype, deuterium is implanted in the components due to beam losses and in the beam dump, where the beam is stopped. The interaction of the deuterons with the deuterium previously implanted leads to the production of neutrons and tritium, which are important issues for radioprotection and safety analysis. A methodology to assess these production pathways in more realistic approach has been developed. The new tools and their main achievement are: (i) an âeffective diffusivity coefficientâ (deduced from available experimental data) that enables simulation of the diffusion phase, and (ii) the MCUNED code (able to handle deuteron transport libraries) allows to simulate the transport-slowdown of deuteron/tritium (to get the concentration profiles) and the neutron/tritium productions from d-Cu and d-D for up to 9 MeV incident deuteron. The results with/without theses tools are presented and their effect on the relevance of d-D sources versus d-Cu is evaluated
Extended sudden approximation model for high-energy nucleon removal reactions
A model based on the sudden approximation has been developed to describe high
energy single nucleon removal reactions. Within this approach, which takes as
its starting point the formalism of Hansen \cite{Anne2}, the nucleon-removal
cross section and the full 3-dimensional momentum distributions of the core
fragments including absorption, diffraction, Coulomb and nuclear-Coulomb
interference amplitudes, have been calculated. The Coulomb breakup has been
treated to all orders for the dipole interaction. The model has been compared
to experimental data for a range of light, neutron-rich psd-shell nuclei. Good
agreement was found for both the inclusive cross sections and momentum
distributions. In the case of C, comparison is also made with the
results of calculations using the transfer-to-the-continuum model. The
calculated 3-dimensional momentum distributions exhibit longitudinal and
transverse momentum components that are strongly coupled by the reaction for
s-wave states, whilst no such effect is apparent for d-waves. Incomplete
detection of transverse momenta arising fromlimited experimental acceptances
thus leads to a narrowing of the longitudinal distributions for nuclei with
significant s-wave valence neutron configurations, as confirmed by the data.
Asymmetries in the longitudinal momentum distributions attributed to
diffractive dissociation are also explored.Comment: 16 figures, submitted to Phys. Rev.
New capabilities for Monte Carlo simulation of deuteron transport and secondary products generation
Several important research programs are dedicated to the development of facilities based on deuteron accelerators. In designing these facilities, the definition of a validated computational approach able to simulate deuteron transport and evaluate deuteron interactions and production of secondary particles with acceptable precision is a very important issue. Current Monte Carlo codes, such as MCNPX or PHITS, when applied for deuteron transport calculations use built-in semi-analytical models to describe deuteron interactions. These models are found unreliable in predicting neutron and photon generated by low energy deuterons, typically present in those facilities.
We present a new computational tool, resulting from an extension of the MCNPX code, which improve significantly the treatment of problems where any secondary product (neutrons, photons, tritons, etc.) generated by low energy deuterons reactions could play a major role. Firstly, it handles deuteron evaluated data libraries, which allow describing better low deuteron energy interactions. Secondly, it includes a reduction variance technique for production of secondary particles by charged particle-induced nuclear interactions, which allow reducing drastically the computing time needed in transport and nuclear response calculations. Verification of the computational tool is successfully achieved. This tool can be very helpful in addressing design issues such as selection of the dedicated neutron production target and accelerator radioprotection analysis. It can be also helpful to test the deuteron cross-sections under development in the frame of different international nuclear data program
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