240 research outputs found
Nonlinear force-free reconstruction of the global solar magnetic field: methodology
We present a novel numerical method that allows the calculation of nonlinear
force-free magnetostatic solutions above a boundary surface on which only the
distribution of the normal magnetic field component is given. The method relies
on the theory of force-free electrodynamics and applies directly to the
reconstruction of the solar coronal magnetic field for a given distribution of
the photospheric radial field component. The method works as follows: we start
with any initial magnetostatic global field configuration (e.g. zero, dipole),
and along the boundary surface we create an evolving distribution of tangential
(horizontal) electric fields that, via Faraday's equation, give rise to a
respective normal field distribution approaching asymptotically the target
distribution. At the same time, these electric fields are used as boundary
condition to numerically evolve the resulting electromagnetic field above the
boundary surface, modelled as a thin ideal plasma with non-reflecting,
perfectly absorbing outer boundaries. The simulation relaxes to a nonlinear
force-free configuration that satisfies the given normal field distribution on
the boundary. This is different from existing methods relying on a fixed
boundary condition - the boundary evolves toward the a priori given one, at the
same time evolving the three-dimensional field solution above it. Moreover,
this is the first time a nonlinear force-free solution is reached by using only
the normal field component on the boundary. This solution is not unique, but
depends on the initial magnetic field configuration and on the evolutionary
course along the boundary surface. To our knowledge, this is the first time
that the formalism of force-free electrodynamics, used very successfully in
other astrophysical contexts, is applied to the global solar magnetic field.Comment: 18 pages, 5 figures, Solar Physic
Transport and optical response of molecular junctions driven by surface plasmon-polaritons
We consider a biased molecular junction subjected to external time-dependent
electromagnetic field. The field for two typical junction geometries (bowtie
antennas and metal nanospheres) is calculated within finite-difference
time-domain technique. Time-dependent transport and optical response of the
junctions is calculated within non-equilibrium Green's function approach
expressed in a form convenient for description of multi-level systems. We
present numerical results for a two-level (HOMO-LUMO) model, and discuss
influence of localized surface plasmon polariton modes on transport.Comment: 9 pages, 6 figure
Negative refraction and left-handed behavior in two-dimensional photonic crystals
We systematically examine the conditions of obtaining left-handed (LH)
behavior in photonic crystals. Detailed studies of the phase and group
velocities as well as the phase np and group ng refractive index are given. The
existence of negative refraction does not guarantee the existence of negative
index of refraction and so LH behavior. A wedge type of experiment is suggested
that can unambiguously distiguinsh between cases of negative refraction that
occur when left-handed behavior is present, from cases that show negative
refraction without LH behavior.Comment: 4 pages 4 figures, submitted to Phys. Rev. B Rapid Communication
Particle Acceleration in Pulsar Wind Nebulae: PIC modelling
We discuss the role of particle-in-cell (PIC) simulations in unveiling the
origin of the emitting particles in PWNe. After describing the basics of the
PIC technique, we summarize its implications for the quiescent and the flaring
emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be
emerging that, in addition to the standard scenario of particle acceleration
via the Fermi process at the termination shock of the pulsar wind, magnetic
reconnection in the wind, at the termination shock and in the Nebula plays a
major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae"
edited by D. Torres for Springer, based on the invited contributions to the
workshop held in Sant Cugat (Barcelona), June 14-17, 201
Exciton-photon interaction in a quantum dot embedded in a photonic microcavity
We present a detailed analysis of exciton-photon interaction in a microcavity
made out of a photonic crystal slab. Here we have analyzed a disk-like quantum
dot where an exciton is formed. Excitonic eigen-functions in addition to their
eigen-energies are found through direct matrix diagonalization, while wave
functions corresponding to unbound electron and hole are chosen as the basis
set for this procedure. In order to evaluate these wave functions precisely, we
have used Luttinger Hamiltonian in the case of hole while ignoring bands
adjacent to conduction band for electron states. After analyzing Excitonic
states, a photonic crystal based microcavity with a relatively high quality
factor mode has been proposed and its lattice constant has been adjusted to
obtain the prescribed resonant frequency. We use finite-difference time-domain
method in order to simulate our cavity with sufficient precision. Finally, we
formulate the coupling constants for exciton-photon interaction both where
intra-band and inter-band transitions occur. By evaluating a sample coupling
constant, it has been shown that the system can be in strong coupling regime
and Rabi oscillations can occur for Excitonic state population.Comment: Journal of Physics B: Atomic and Molecular Physics (to appear
Alfv\'en Reflection and Reverberation in the Solar Atmosphere
Magneto-atmospheres with Alfv\'en speed [a] that increases monotonically with
height are often used to model the solar atmosphere, at least out to several
solar radii. A common example involves uniform vertical or inclined magnetic
field in an isothermal atmosphere, for which the Alfv\'en speed is exponential.
We address the issue of internal reflection in such atmospheres, both for
time-harmonic and for transient waves. It is found that a mathematical boundary
condition may be devised that corresponds to perfect absorption at infinity,
and, using this, that many atmospheres where a(x) is analytic and unbounded
present no internal reflection of harmonic Alfv\'en waves. However, except for
certain special cases, such solutions are accompanied by a wake, which may be
thought of as a kind of reflection. For the initial-value problem where a
harmonic source is suddenly switched on (and optionally off), there is also an
associated transient that normally decays with time as O(t-1) or O(t-1 ln t),
depending on the phase of the driver. Unlike the steady-state harmonic
solutions, the transient does reflect weakly. Alfv\'en waves in the solar
corona driven by a finite-duration train of p-modes are expected to leave such
transients.Comment: Accepted by Solar Physic
Development of Photonic Crystal Fiber Based Gas/ Chemical Sensors
The development of highly-sensitive and miniaturized sensors that capable of
real-time analytes detection is highly desirable. Nowadays, toxic or colorless
gas detection, air pollution monitoring, harmful chemical, pressure, strain,
humidity, and temperature sensors based on photonic crystal fiber (PCF) are
increasing rapidly due to its compact structure, fast response and efficient
light controlling capabilities. The propagating light through the PCF can be
controlled by varying the structural parameters and core-cladding materials, as
a result, evanescent field can be enhanced significantly which is the main
component of the PCF based gas/chemical sensors. The aim of this chapter is to
(1) describe the principle operation of PCF based gas/ chemical sensors, (2)
discuss the important PCF properties for optical sensors, (3) extensively
discuss the different types of microstructured optical fiber based gas/
chemical sensors, (4) study the effects of different core-cladding shapes, and
fiber background materials on sensing performance, and (5) highlight the main
challenges of PCF based gas/ chemical sensors and possible solutions
Spatial Kramers-Kronig relations and the reflection of waves
Copyright © 2015, Rights Managed by Nature Publishing GroupAuthor version of article. The version of record is available from the publisher via DOI: 10.1038/nphoton.2015.106When a planar dielectric medium has a permittivity profile that is an analytic function in the upper or lower half of the complex position plane x=x'+ix'' then the real and imaginary parts of its permittivity are related by the spatial Kramers-Kronig relations. We find that such a medium will not reflect radiation incident from one side, whatever the angle of incidence. Using the spatial Kramers-Kronig relations, one can derive a real part of a permittivity profile from some given imaginary part (or vice versa) such that the reflection is guaranteed to be zero. This result is valid for both scalar and vector wave theories and may have relevance for designing materials that efficiently absorb radiation or for the creation of a new type of anti-reflection surface.Engineering and Physical Sciences Research Council (EPSRC
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