18,985 research outputs found
Localization versus subradiance in three-dimensional scattering of light
We study the scattering modes of light in a three-dimensional disordered
medium, in the scalar approximation and above the critical density for Anderson
localization. Localized modes represent a minority of the total number of
modes, even well above the threshold density, whereas spatially extended
subradiant modes predominate. For specific energy ranges however, almost all
modes are localized, yet adjusting accordingly the probe frequency does not
allow to address these only in the regime accessible numerically. Finally,
their lifetime is observed to be dominated by finite-size effects, and more
specifically by the ratio of the localization length to their distance to the
system boundaries.Comment: Add figure comparing localization percentage via frequency, fixed
text, addition of Ioffe-Regel criterion limits, figure axis were normalize
Making electromagnetic wavelets
Electromagnetic wavelets are constructed using scalar wavelets as
superpotentials, together with an appropriate polarization. It is shown that
oblate spheroidal antennas, which are ideal for their production and reception,
can be made by deforming and merging two branch cuts. This determines a unique
field on the interior of the spheroid which gives the boundary conditions for
the surface charge-current density necessary to radiate the wavelets. These
sources are computed, including the impulse response of the antenna.Comment: 29 pages, 4 figures; minor corrections and addition
The cosomological evolution of the environments of powerful radio galaxies
We present the results from the analysis of 26 extragalactic radio sources of
type FRII which were observed with the VLA at 5 GHz and around the 1.4 GHz
band. The sources were selected to have redshifts in the range ,
radio powers between and angular size . We found
that the depolarisation and the rms variations in the rotation measure
increased with redshift. The flux values obtained from the observations were
used to derive by means of analytical modelling the jet--power, density of the
central environment, age of the source and its lobe pressure and the results
were then compared with the observations. We find no significant correlations
with the density parameter suggesting that the depolarisation and the rms
variations in the rotation measure are indicative of the environment becoming
more disordered rather than denser. The age and size of a source are correlated
and both were found to be independent of redshift and radio--power. Jet--power
strongly correlated with the radio--power. The lobe pressure was found to be
anti--correlated with size which could explain why there are no sources beyond
a few Mpc in size. We found no significant correlation between size and density
which demonstrates that the sample is a fair representation of the population.Comment: 2 pages, Cozumel AGN 2003 conference proceeding
Fluctuation properties of laser light after interaction with an atomic system: comparison between two-level and multilevel atomic transitions
The complex internal atomic structure involved in radiative transitions has
an effect on the spectrum of fluctuations (noise) of the transmitted light. A
degenerate transition has different properties in this respect than a pure
two-level transition. We investigate these variations by studying a certain
transition between two degenerate atomic levels for different choices of the
polarization state of the driving laser. For circular polarization,
corresponding to the textbook two-level atom case, the optical spectrum shows
the characteristic Mollow triplet for strong laser drive, while the
corresponding noise spectrum exhibits squeezing in some frequency ranges. For a
linearly polarized drive, corresponding to the case of a multilevel system,
additional features appear in both optical and noise spectra. These differences
are more pronounced in the regime of a weakly driven transition: whereas the
two-level case essentially exhibits elastic scattering, the multilevel case has
extra noise terms related to spontaneous Raman transitions. We also discuss the
possibility to experimentally observe these predicted differences for the
commonly encountered case where the laser drive has excess noise in its phase
quadrature.Comment: New version. Accepted for publication in Physical Review
Cooperative effects and disorder: A scaling analysis of the spectrum of the effective atomic Hamiltonian
We study numerically the spectrum of the non-Hermitian effective Hamiltonian
that describes the dipolar interaction of a gas of atoms with the
radiation field. We analyze the interplay between cooperative effects and
disorder for both scalar and vectorial radiation fields. We show that for dense
gases, the resonance width distribution follows, both in the scalar and
vectorial cases, a power law that originates
from cooperative effects between more than two atoms. This power law is
different from the behavior, which has been
considered as a signature of Anderson localization of light in random systems.
We show that in dilute clouds, the center of the energy distribution is
described by Wigner's semicircle law in the scalar and vectorial cases. For
dense gases, this law is replaced in the vectorial case by the Laplace
distribution. Finally, we show that in the scalar case the degree of resonance
overlap increases as a power law of the system size for dilute gases, but
decays exponentially with the system size for dense clouds.Comment: 11 pages, 12 figure
Low and high intensity velocity selective coherent population trapping in a two-level system
An experimental investigation is made of sub-recoil cooling by velocity
selective coherent population trapping in a two-level system in Sr. The
experiment is carried out using the narrow linewidth intercombination line at
689 nm. Here, the ratio between the recoil shift and the linewidth is as high
as 0.64. We show that, on top of a broader momentum profile, subrecoil features
develop, whose amplitude is strongly dependent on the detuning from resonance.
We attribute this structure to a velocity selective coherent population
trapping mechanism. We also show that the population trapping phenomenon leads
to complex momentum profiles in the case of highly saturated transitions,
displaying a multitude of subrecoil features at integer multiples of the recoil
momentum.Comment: 6 pages and 7 figure
- …