12,419 research outputs found
Coherent Perfect Absorbers: Time-reversed Lasers
We show that an arbitrary body or aggregate can be made perfectly absorbing
at discrete frequencies if a precise amount of dissipation is added under
specific conditions of coherent monochromatic illumination. This effect arises
from the interaction of optical absorption and wave interference, and
corresponds to moving a zero of the elastic S-matrix onto the real wavevector
axis. It is thus the time-reversed process of lasing at threshold. The effect
is demonstrated in a simple Si slab geometry illuminated in the 500-900 nm
range. Coherent perfect absorbers are novel linear optical elements, absorptive
interferometers, which may be useful for controlled optical energy transfer.Comment: 4 pages, 4 figure
Noise Properties of Coherent Perfect Absorbers and Critically-coupled Resonators
The performance of a coherent perfect absorber (time-reversed laser) is
limited by quantum and thermal noise. At zero temperature, the quantum shot
noise dominates the signal for frequencies close to the resonance frequency,
and both vanish exactly at the resonance frequency. We compute the sensitivity
of the absorbing cavity as a background-free detector, limited by finite signal
or detector bandwidth.Comment: 6 pages, 3 figure
Semi-leptonic and Non-leptonic meson decays to charmed mesons
We study the semi-leptonic and non-leptonic weak decays which are
governed by the transitions. The branching ratios, CP
asymmetries (CPA) and polarization fractions (FA) of non-leptonic decays are
investigated in the factorization approximation. The
form factors are estimated in the Salpeter method. Our estimation on branching
ratios generally agree with the existent experimental data. For CPA and
polarizations, comparisons among the FA results, the perturbative QCD
predictions and experimental data are made.Comment: 8 pages, 1 figures, 5 table
Scale-free networks with tunable degree distribution exponents
We propose and study a model of scale-free growing networks that gives a
degree distribution dominated by a power-law behavior with a model-dependent,
hence tunable, exponent. The model represents a hybrid of the growing networks
based on popularity-driven and fitness-driven preferential attachments. As the
network grows, a newly added node establishes new links to existing nodes
with a probability based on popularity of the existing nodes and a
probability based on fitness of the existing nodes. An explicit form of
the degree distribution is derived within a mean field approach. For
reasonably large , , where the
function is dominated by the behavior of for small
values of and becomes -independent as , and is a
model-dependent exponent. The degree distribution and the exponent
are found to be in good agreement with results obtained by extensive numerical
simulations.Comment: 12 pages, 2 figures, submitted to PR
The Origin of Gamma-Rays from Globular Clusters
Fermi has detected gamma-ray emission from eight globular clusters. We
suggest that the gamma-ray emission from globular clusters may result from the
inverse Compton scattering between relativistic electrons/positrons in the
pulsar wind of MSPs in the globular clusters and background soft photons
including cosmic microwave/relic photons, background star lights in the
clusters, the galactic infrared photons and the galactic star lights. We show
that the gamma-ray spectrum from 47 Tuc can be explained equally well by upward
scattering of either the relic photons, the galactic infrared photons or the
galactic star lights whereas the gamma-ray spectra from other seven globular
clusters are best fitted by the upward scattering of either the galactic
infrared photons or the galactic star lights. We also find that the observed
gamma-ray luminosity is correlated better with the combined factor of the
encounter rate and the background soft photon energy density. Therefore the
inverse Compton scattering may also contribute to the observed gamma-ray
emission from globular clusters detected by Fermi in addition to the standard
curvature radiation process. Furthermore, we find that the emission region of
high energy photons from globular cluster produced by inverse Compton
scattering is substantially larger than the core of globular cluster with a
radius >10pc. The diffuse radio and X-rays emitted from globular clusters can
also be produced by synchrotron radiation and inverse Compton scattering
respectively. We suggest that future observations including radio, X-rays, and
gamma-rays with energy higher than 10 GeV and better angular resolution can
provide better constraints for the models.Comment: Accepted by ApJ, Comments may send to Prof. K.S. Cheng:
[email protected]
Future cereal production in China: Modelling the interaction of climate change, water availability and socio-economic scenarios. The Impacts of Climate Change on Chinese Agriculture – Phase II Final Report
The Fundamental Plane of Gamma-ray Globular Clusters
We have investigated the properties of a group of -ray emitting
globular clusters (GCs) which have recently been uncovered in our Galaxy. By
correlating the observed -ray luminosities with various
cluster properties, we probe the origin of the high energy photons from these
GCs. We report is positively correlated with the encounter rate
and the metalicity which place an
intimate link between the gamma-ray emission and the millisecond pulsar
population. We also find a tendency that increase with the energy
densities of the soft photon at the cluster location. Furthermore, the
two-dimensional regression analysis suggests that , soft photon
densities, and / possibly span fundamental
planes which potentially provide better predictions for the -ray
properties of GCs.Comment: 17 pages, 4 figures, 3 tables, published in Ap
SU(3) Quantum Interferometry with single-photon input pulses
We develop a framework for solving the action of a three-channel passive
optical interferometer on single-photon pulse inputs to each channel using
SU(3) group-theoretic methods, which can be readily generalized to higher-order
photon-coincidence experiments. We show that features of the coincidence plots
vs relative time delays of photons yield information about permanents,
immanants, and determinants of the interferometer SU(3) matrix
Economic evaluation of cochlear implantation
Conference Theme: Challenges to specialists in the 21st centurypublished_or_final_versio
Discovery of X-ray pulsations from "next Geminga" - PSR J1836+5925
We report the X-ray pulsation of ~173.3 ms for the "next Geminga", PSR
J1836+5925, with recent XMM-Newton investigations. The X-ray periodicity is
consistent wtih the gamma-ray ephemeris at the same epoch. The X-ray folded
light curve has a sinusoidal structure which is different from the
double-peaked gamma-ray pulse profile. We have also analysed the X-ray
phase-averaged spectra which shows the X-ray emission from PSR J1836+5925 is
thermal dominant. This suggests the X-ray pulsation mainly originates from the
modulated hot spot on the stellar surface.Comment: 7 pages, 3 figures, 1 table, accepted for publication in ApJ Lette
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