782 research outputs found
Optical second harmonic generation near a black hole horizon as possible source of experimental information on quantum gravitational effects
Optical second harmonic generation near a black hole horizon is suggested as
a source of experimental information on quantum gravitational effects. While
absent in the framework of general relativity, second harmonic generation
appears in the toy models of sonic and electromagnetic black holes, where
spatial dispersion at high frequencies for waves boosted towards the horizon is
introduced. Localization effects in the light scattering from random
fluctuations of matter fields and space-time metric near the black hole horizon
produce a pronounced peak in the angular distribution of second harmonics of
light in the direction normal to the horizon. Such second harmonic light has
the best chances to escape the vicinity of the black hole. This phenomenon is
similar to the well-known strong enhancement of diffuse second harmonic
emission from a randomly rough metal surface in the direction normal to the
surface.Comment: 4 pages, 1 figur
Electromagnetic effects of neutrinos in an electron gas
We study the electromagnetic properties of a system that consists of an
electron background and a neutrino gas that may be moving or at rest, as a
whole, relative to the background. The photon self-energy for this system is
characterized by the usual transverse and longitudinal polarization functions,
and two additional ones which are the focus of our calculations, that give rise
to birefringence and anisotropic effects in the photon dispersion relations.
Expressions for them are obtained, which depend on the neutrino number
densities and involve momentum integrals over the electron distribution
functions, and are valid for any value of the photon momentum and general
conditions of the electron gas. Those expressions are evaluated explicitly for
several special cases and approximations which are generally useful in
astrophysical and cosmological settings. Besides studying the photon dispersion
relations, we consider the macroscopic electrodynamic equations for this
system, which involve the standard dielectric and permeability constants plus
two additional ones related to the photon self-energy functions. As an
illustration, the equations are used to discuss the evolution of a magnetic
field perturbation in such a medium. This particular phenomena has also been
considered in a recent work by Semikoz and Sokoloff as a mechanism for the
generation of large-scale magnetic fields in the Early Universe as a
consequence of the neutrino-plasma interactions, and allows us to establish
contact with a specific application in a well defined context, with a broader
scope and from a very different point of view.Comment: Revtex 20 page
Spectral Dependence of Coherent Backscattering of Light in a Narrow-Resonance Atomic System
We report a combined theoretical and experimental study of the spectral and
polarization dependence of near resonant radiation coherently backscattered
from an ultracold gas of 85Rb atoms. Measurements in an approximately 6 MHz
range about the 5s^{2}S_{1/2}- 5p^{2}P_{3/2}, F=3 - F'=4 hyperfine transition
are compared with simulations based on a realistic model of the experimental
atomic density distribution. In the simulations, the influence of heating of
the atoms in the vapor, magnetization of the vapor, finite spectral bandwidth,
and other nonresonant hyperfine transitions are considered. Good agreement is
found between the simulations and measurements.Comment: 10 pages, 12 figur
Numerical simulation of concentration over-voltage in a polymer electrolyte fuel cell under low-hydrogen conditions
This article elucidates the effect of low hydrogen concentration fuel gas on polymer electrolyte fuel cell (PEFC) performance, with particular emphasis on the transport of chemical species in the anode separator channel and the electrochemical reactions. A numerical simulation model for PEFCs was developed; the model combined a computational fluid dynamics model for mass transfer in the anode separator and the gas diffusion layer(GDL)as well as a PEFC electrochemical reaction model takinginto account the activation, concentration, and resistance over-voltages. The emphasis in this study is placed on obtaining a basic understanding of how three-dimensional flow and low-hydrogen fuel transport phenomena in the anode separator channelimpactthe electrochemical processes occurring in PEFCs. Comparison of the numerical simulation results with experimental data indicates that the performance degradation in PEFCs is negligible for hydrogen concentrations over 30%, whereas it becomes significant for concentrations below 10%. Furthermore, the numerical simulationresults showthat controlling the fuel supply flow rate stimulates hydrogen transport inthe GDL and the catalyst layer, which consequently enhances PEFC performance under low-hydrogen conditions
Cooperative resonance linewidth narrowing in a planar metamaterial
We theoretically analyze the experimental observations of a spectral line
collapse in a metamaterial array of asymmetric split ring resonators [Fedotov
et al., Phys. Rev. Lett. 104, 223901 (2010)]. We show that the ensemble of
closely-spaced resonators exhibits cooperative response, explaining the
observed system-size dependent narrowing of the transmission resonance
linewidth. We further show that this cooperative narrowing depends sensitively
on the lattice spacing and that significantly stronger narrowing could be
achieved in media with suppressed ohmic losses.Comment: 19 pages, 6 figures, to appear in New Journal of Physic
Self-Averaging Scaling Limits of Two-Frequency Wigner Distribution for Random Paraxial Waves
Two-frequency Wigner distribution is introduced to capture the asymptotic
behavior of the space-frequency correlation of paraxial waves in the radiative
transfer limits. The scaling limits give rises to deterministic transport-like
equations. Depending on the ratio of the wavelength to the correlation length
the limiting equation is either a Boltzmann-like integral equation or a
Fokker-Planck-like differential equation in the phase space. The solutions to
these equations have a probabilistic representation which can be simulated by
Monte Carlo method. When the medium fluctuates more rapidly in the longitudinal
direction, the corresponding Fokker-Planck-like equation can be solved exactly.Comment: typos correcte
Scalar Quantum Field Theory in Disordered Media
A free massive scalar field in inhomogeneous random media is investigated.
The coefficients of the Klein-Gordon equation are taken to be random functions
of the spatial coordinates. The case of an annealed-like disordered medium,
modeled by centered stationary and Gaussian processes, is analyzed. After
performing the averages over the random functions, we obtain the two-point
causal Green's function of the model up to one-loop. The disordered scalar
quantum field theory becomes qualitatively similar to a
self-interacting theory with a frequency-dependent coupling
Propagation of wave packets in randomly stratified media
The propagation of a narrow-band signal radiated by a point source in a
randomly layered absorbing medium is studied asymptotically in the
weak-scattering limit. It is shown that in a disordered stratified medium that
is homogeneous on average a pulse is channelled along the layers in a narrow
strip in the vicinity of the source. The space-time distribution of the pulse
energy is calculated. Far from the source, the shape of wave packets is
universal and independent of the frequency spectrum of the radiated signal.
Strong localization effects manifest themselves also as a low-decaying tail of
the pulse and a strong time delay in the direction of stratification. The
frequency-momentum correlation function in a one-dimensional random medium is
calculated.Comment: 11 pages, 3 figures, Revtex-4. Submitted to Phys. Rev.
Anisotropic multiple scattering in diffuse media
The multiple scattering of scalar waves in diffusive media is investigated by
means of the radiative transfer equation. This approach amounts to a
resummation of the ladder diagrams of the Born series; it does not rely on the
diffusion approximation. Quantitative predictions are obtained, concerning
various observables pertaining to optically thick slabs, such as the mean
angle-resolved reflected and transmitted intensities, and the shape of the
enhanced backscattering cone. Special emphasis is put on the dependence of
these quantities on the anisotropy of the cross-section of the individual
scatterers, and on the internal reflections due to the optical index mismatch
at the boundaries of the sample. The regime of very anisotropic scattering,
where the transport mean free path is much larger than the scattering
mean free path , is studied in full detail. For the first time the
relevant Schwarzschild-Milne equation is solved exactly in the absence of
internal reflections, and asymptotically in the regime of a large index
mismatch. An unexpected outcome concerns the angular width of the enhanced
backscattering cone, which is predicted to scale as
, in contrast with the generally
accepted law, derived within the diffusion approximation.Comment: 53 pages TEX, including 2 tables. The 4 figures are sent at reques
Analysis of Granular Packing Structure by Scattering of THz Radiation
Scattering methods are widespread used to characterize the structure and
constituents of matter on small length scales. This motivates this introductory
text on identifying prospective approaches to scattering-based methods for
granular media. A survey to light scattering by particles and particle
ensembles is given. It is elaborated why the established scattering methods
using X-rays and visible light cannot in general be transferred to granular
media. Spectroscopic measurements using Terahertz radiation are highlighted as
they to probe the scattering properties of granular media, which are sensitive
to the packing structure. Experimental details to optimize spectrometer for
measurements on granular media are discussed. We perform transmission
measurements on static and agitated granular media using Fourier-transform
spectroscopy at the THz beamline of the BessyII storage ring. The measurements
demonstrate the potential to evaluate degrees of order in the media and to
track transient structural states in agitated bulk granular media.Comment: 12 Pages, 9 Figures, 56 Reference
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