204 research outputs found
The onset of tree-like patterns in negative streamers
We present the first analytical and numerical studies of the initial stage of
the branching process based on an interface dynamics streamer model in the
fully 3-D case. This model follows from fundamental considerations on charge
production by impact ionization and balance laws, and leads to an equation for
the evolution of the interface between ionized and non-ionized regions. We
compare some experimental patterns with the numerically simulated ones, and
give an explicit expression for the growth rate of harmonic modes associated
with the perturbation of a symmetrically expanding discharge. By means of full
numerical simulation, the splitting and formation of characteristic tree-like
patterns of electric discharges is observed and described
Contour dynamics model for electric discharges
A contour dynamics model for electrical discharges is obtained and analyzed.
The model is deduced as the asymptotic limit of the minimal streamer model for
the propagation of electric discharges, in the limit of small electron
diffusion. The dispersion relation for a non planar 2-D discharge is
calculated. The development and propagation of finger-like patterns are studied
and their main features quantified.Comment: 4 pages, 2 fi
Impact ionization fronts in Si diodes: Numerical evidence of superfast propagation due to nonlocalized preionization
We present numerical evidence of a novel propagation mode for superfast
impact ionization fronts in high-voltage Si -- structures. In
nonlinear dynamics terms, this mode corresponds to a pulled front propagating
into an unstable state in the regime of nonlocalized initial conditions. Before
the front starts to travel, field-ehanced emission of electrons from deep-level
impurities preionizes initially depleted base creating spatially nonuniform
free carriers profile. Impact ionization takes place in the whole high-field
region. We find two ionizing fronts that propagate in opposite directions with
velocities up to 10 times higher than the saturated drift velocity.Comment: 3 pages, 4 figure
Electric discharge contour dynamics model: the effects of curvature and finite conductivity
In this paper we present the complete derivation of the effective contour
model for electrical discharges which appears as the asymptotic limit of the
minimal streamer model for the propagation of electric discharges, when the
electron diffusion is small. It consists of two integro-differential equations
defined at the boundary of the plasma region: one for the motion and a second
equation for the net charge density at the interface. We have computed explicit
solutions with cylindrical symmetry and found the dispersion relation for small
symmetry-breaking perturbations in the case of finite resistivity. We implement
a numerical procedure to solve our model in general situations. As a result we
compute the dispersion relation for the cylindrical case and compare it with
the analytical predictions. Comparisons with experimental data for a 2-D
positive streamers discharge are provided and predictions confirmed.Comment: 23 pages, 3 figure
Optomagnetic composite medium with conducting nanoelements
A new type of metal-dielectric composites has been proposed that is
characterised by a resonance-like behaviour of the effective permeability in
the infrared and visible spectral ranges. This material can be referred to as
optomagnetic medium. The analytical formalism developed is based on solving the
scattering problem for considered inclusions with impedance boundary condition,
which yields the current and charge distributions within the inclusions. The
presence of the effective magnetic permeability and its resonant properties
lead to novel optical effects and open new possible applications.Comment: 48 pages, 13 figures. accepted to Phys. Rev. B; to appear vol. 66,
200
Spontaneous emission rate of an excited atom placed near a nanofiber
The spontaneous decay rates of an excited atom placed near a dielectric
cylinder are investigated. A special attention is paid to the case when the
cylinder radius is small in comparison with radiation wavelength (nanofiber or
photonic wire). In this case, the analytical expressions of the transition
rates for different orientations of dipole are derived. It is shown that the
main contribution to decay rates is due to quasistatic interaction of atom
dipole momentum with nanofiber and the contributions of guided modes are
exponentially small. On the contrary, in the case when the radius of fiber is
only slightly less than radiation wavelength, the influence of guided modes can
be substantial. The results obtained are compared with the case of dielectric
nanospheroid and ideally conducting wire.Comment: 19 pages, 16 Postscript figure
Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials
We study the optical properties of metamaterials made from cut-wire pairs or
plate pairs. We obtain a more pronounced optical response for arrays of plate
pairs -- a geometry which also eliminates the undesired polarization anisotropy
of the cut-wire pairs. The measured optical spectra agree with simulations,
revealing negative magnetic permeability in the range of telecommunications
wavelengths. Thus, nanoscopic plate pairs might serve as an alternative to the
established split-ring resonator design.Comment: 3 pages, 4 figures, submitted to Opt. Let
Opaque perfect lenses
The response of the ``perfect lens'', consisting of a slab of lossless
material of thickness with at one frequency is
investigated. It is shown that as time progresses the lens becomes increasingly
opaque to any physical TM line dipole source located a distance from
the lens and which has been turned on at time . Here a physical source is
defined as one which supplies a bounded amount of energy per unit time. In fact
the lens cloaks the source so that it is not visible from behind the lens
either. For sources which are turned on exponentially slowly there is an exact
correspondence between the response of the perfect lens in the long time
constant limit and the response of lossy lenses in the low loss limit. Contrary
to the usual picture where the field intensity has a minimum at the front
interface we find that the field diverges to infinity there in the long time
constant limit.Comment: The 7th International Conference on the Electrical transport and
Optical Properties of Inhomogenous Media (ETOPIM7
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