306 research outputs found
Experimental demonstration of a mu=-1 metamaterial lens for magnetic resonance imaging
In this work a mu=-1 metamaterial (MM) lens for magnetic resonance imaging
(MRI) is demonstrated. MRI uses surface coils to detect the radiofrequency(RF)
energy absorbed and emitted by the nuclear spins in the imaged object. The
proposed MM lens manipulates the RF field detected by these surface coils, so
that the coil sensitivity and spatial localization is substantially improved.
Beyond this specific application, we feel that the reported results are the
experimental confirmation of a new concept for the manipulation of RF field in
MRI, which paves the way to many other interesting applications.Comment: 9 pages, 3 figure
On effective electromagnetic parameters of artificial nanostructured magnetic materials
In this paper we discuss effective material parameter description of new
nanostructures designed to perform as artificial magnetic materials for vis-
ible light. Among these structures there are various split-ring resonators,
dual-bar structures, fishnet layers and other geometries. Artificial magnetic
response in these structures appears due to weak spatial dispersion effects,
and it is important to study the conditions under which the magnetic re- sponse
can be adequately measured with effective permeability tensor. On the examples
of dual bars and split rings we show that this is possible only under some
quite restrictive conditions. In the general case, more compli- cated
constitutive relations with more effective material parameters need to be
developed.Comment: 20 pages, 2 figure
How an antenna launches its input power into radiation: the pattern of the Poynting vector at and near an antenna
In this paper I first address the question of whether the seat of the power
radiated by an antenna made of conducting members is distributed over the
``arms'' of the antenna according to -J . E, where J is the specified current
density and E is the electric field produced by that source. Poynting's theorem
permits only a global identification of the total input power, usually from a
localized generator, with the total power radiated to infinity, not a local
correspondence of -J . E dv with some specific radiated power, r^2 S . n dO. I
then describe a model antenna consisting of two perfectly conducting
hemispheres of radius a separated by a small equatorial gap across which occurs
the driving oscillatory electric field. The fields and surface current are
determined by solution of the boundary value problem. In contrast to the first
approach (not a boundary value problem), the tangential electric field vanishes
on the metallic surface. There is no radial Poynting vector normal to the
surface. Numerical examples are shown to illustrate how the energy flows from
the input region of the gap and is guided near the antenna by its ``arms''
until it is launched at larger r/a into the radiation pattern determined by the
value of ka.Comment: 24pages, 8 figures, submitted for publicatio
Sub-wavelength imaging: Resolution enhancement using metal wire gratings
An experimental evidence of subwavelength imaging with a "lens", which is a
uniaxial negative permittivity wire medium slab, is reported. The slab is
formed by gratings of long thin parallel conducting cylinders. Taking into
account the anisotropy and spatial dispersion in the wire medium we
theoretically show that there are no usual plasmons that could be exited on
surfaces of such a slab, and there is no resonant enhancement of evanescent
fields in the slab. The experimentally observed clear improvement of the
resolution in the presence of the slab is explained as filtering out the
harmonics with small wavenumbers. In other words, the wire gratings (the wire
medium) suppress strong traveling-mode components increasing the role of
evanescent waves in the image formation. This effect can be used in near-field
imaging and detection applications.Comment: 12 pages, 6 figure
Heating mechanisms in radio frequency driven ultracold plasmas
Several mechanisms by which an external electromagnetic field influences the
temperature of a plasma are studied analytically and specialized to the system
of an ultracold plasma (UCP) driven by a uniform radio frequency (RF) field.
Heating through collisional absorption is reviewed and applied to UCPs.
Furthermore, it is shown that the RF field modifies the three body
recombination process by ionizing electrons from intermediate high-lying
Rydberg states and upshifting the continuum threshold, resulting in a
suppression of three body recombination. Heating through collisionless
absorption associated with the finite plasma size is calculated in detail,
revealing a temperature threshold below which collisionless absorption is
ineffective.Comment: 14 pages, 7 figure
Strong asymmetry of microwave absorption by bi-layer conducting ferromagnetic films in the microstrip-line based broadband ferromagnetic resonance
Peculiarities of ferromagnetic resonance response of conducting magnetic
bi-layer films of nanometric thicknesses excited by microstrip microwave
transducers have been studied theoretically. Strong asymmetry of the response
has been found. Depending on the order of layers with respect to the transducer
either the first higher-order standing spin wave mode, or the fundamental mode
shows the largest response.
Film conductivity and lowered symmetry of microwave fields of such
transducers are responsible for this behavior. Amplitude of which mode is
larger also depends on the driving frequency. This effect is explained as
shielding of the asymmetric transducer field by eddy currents in the films.
This shielding remains very efficient for films with thicknesses well below the
microwave skin depth. This effect may be useful for studying buried magnetic
interfaces and should be accounted for in future development of broadband
inductive ferromagnetic resonance methods.Comment: 21 Page, 4 figure
Dipolar resonances in conductive carbon micro-fibers probed by near-field terahertz spectroscopy
We observe dipole resonances in thin conductive carbon micro-fibers by detecting an enhanced electric field in the near-field of a single fiber at terahertz (THz) frequencies. Time-domain analysis of the electric field shows that each fiber sustains resonant current oscillations at the frequency defined by the fiber's length. Strong dependence of the observed resonance frequency and degree of field enhancement on the fibers' conductive properties enable direct non-contact probing of the THz conductivity in single carbon micro-fibers. We find the conductivity of the fibers to be within the range of 1– 5×104 S/m. This approach is suitable for experimental characterization of individual doped semiconductor resonators for THz metamaterials and devices
Generalized surface current method in the macroscopic theory of diffraction radiation
The surface current method known in the theory of electromagnetic waves
diffraction is generalized to be applied for the problems of diffraction
radiation generated by a charged particle moving nearby an ideally-conducting
screen in vacuum. An expression for induced surface current density leading to
the exact results in the theory of transition radiation is derived, and by
using this expression several exact solutions of diffraction radiation problems
are found. Limits of applicability for the earlier known models based on the
surface current conception are indicated. Properties of radiation from a
semi-plane and from a slit in cylinder are investigated at the various
distances to observer.Comment: 8 pages, 8 figure
Diffraction of light by a planar aperture in a metallic screen
We present a complete derivation of the formula of Smythe [Phys.Rev.72, 1066
(1947)] giving the electromagnetic field diffracted by an aperture created in a
perfectly conducting plane surface. The reasoning, valid for any excitating
field and any hole shape, makes use only of the free scalar Green function for
the Helmoltz equation without any reference to a Green dyadic formalism. We
compare our proof with the one previously given by Jackson and connect our
reasoning to the general Huygens Fresnel theorem.Comment: J. Math. Phys. 47, 072901 (2006
Realizability of metamaterials with prescribed electric permittivity and magnetic permeability tensors
We show that any pair of real symmetric tensors \BGve and \BGm can be
realized as the effective electric permittivity and effective magnetic
permeability of a metamaterial at a given fixed frequency. The construction
starts with two extremely low loss metamaterials, with arbitrarily small
microstructure, whose existence is ensured by the work of Bouchitt{\'e} and
Bourel and Bouchitt\'e and Schweizer, one having at the given frequency a
permittivity tensor with exactly one negative eigenvalue, and a positive
permeability tensor, and the other having a positive permittivity tensor, and a
permeability tensor having exactly one negative eigenvalue. To achieve the
desired effective properties these materials are laminated together in a
hierarchical multiple rank laminate structure, with widely separated length
scales, and varying directions of lamination, but with the largest length scale
still much shorter than the wavelengths and attenuation lengths in the
macroscopic effective medium.Comment: 12 pages, no figure
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