203 research outputs found
Radiation from elementary sources in a uniaxial wire medium
We investigate the radiation properties of two types of elementary sources
embedded in a uniaxial wire medium: a short dipole parallel to the wires and a
lumped voltage source connected across a gap in a generic metallic wire. It is
demonstrated that the radiation pattern of these elementary sources have quite
anomalous and unusual properties. Specifically, the radiation pattern of a
short vertical dipole resembles that of an isotropic radiator close to the
effective plasma frequency of the wire medium, whereas the radiation from the
lumped voltage generator is characterized by an infinite directivity and a
non-diffractive far-field distribution.Comment: 10 pages, 4 figure
Oblique launching of optical surface waves by a subwavelength slit
The electromagnetic field on the metal surface launched by a subwavelength
slit is analytically studied, for the case when the fundamental mode inside the
slit has a wavevector component along the slit axis (conical mount). Both
near-field and far-field regions are discussed, and the role of surface
plasmon-polaritons and Norton waves is revealed. It is shown that the distance
from the slit at which NW are more intense than surface plasmons decrease with
parallel wavevector, which could help experimental studies on Norton waves.
Additionally, it is found that the s-polarization component, while present for
any non-zero parallel wavevector, only weakly contributes to the NWs.Comment: 8 pages, 5 figure
Dynamics of light propagation in spatiotemporal dielectric structures
Propagation, transmission and reflection properties of linearly polarized
plane waves and arbitrarily short electromagnetic pulses in one-dimensional
dispersionless dielectric media possessing an arbitrary space-time dependence
of the refractive index are studied by using a two-component, highly symmetric
version of Maxwell's equations. The use of any slow varying amplitude
approximation is avoided. Transfer matrices of sharp nonstationary interfaces
are calculated explicitly, together with the amplitudes of all secondary waves
produced in the scattering. Time-varying multilayer structures and
spatiotemporal lenses in various configurations are investigated analytically
and numerically in a unified approach. Several new effects are reported, such
as pulse compression, broadening and spectral manipulation of pulses by a
spatiotemporal lens, and the closure of the forbidden frequency gaps with the
subsequent opening of wavenumber bandgaps in a generalized Bragg reflector
In the diffraction shadow: Norton waves versus surface plasmon-polaritons in the optical region
Surface electromagnetic modes supported by metal surfaces have a great
potential for uses in miniaturised detectors and optical circuits. For many
applications these modes are excited locally. In the optical regime, Surface
Plasmon Polaritons (SPPs) have been thought to dominate the fields at the
surface, beyond a transition region comprising 3-4 wavelengths from the source.
In this work we demonstrate that at sufficiently long distances SPPs are not
the main contribution to the field. Instead, for all metals, a different type
of wave prevails, which we term Norton waves for their reminiscence to those
found in the radio-wave regime at the surface of the Earth. Our results show
that Norton Waves are stronger at the surface than SPPs at distances larger
than 6-9 SPP's absorption lengths, the precise value depending on wavelength
and metal. Moreover, Norton waves decay more slowly than SPPs in the direction
normal to the surface.Comment: 8 pages, 8 figure
Generalized Huygens principle with pulsed-beam wavelets
Huygens' principle has a well-known problem with back-propagation due to the
spherical nature of the secondary wavelets. We solve this by analytically
continuing the surface of integration. If the surface is a sphere of radius
, this is done by complexifying to . The resulting complex sphere
is shown to be a real bundle of disks with radius tangent to the sphere.
Huygens' "secondary source points" are thus replaced by disks, and his
spherical wavelets by well-focused pulsed beams propagating outward. This
solves the back-propagation problem. The extended Huygens principle is a
completeness relation for pulsed beams, giving a representation of a general
radiation field as a superposition of such beams. Furthermore, it naturally
yields a very efficient way to compute radiation fields because all pulsed
beams missing a given observer can be ignored. Increasing sharpens the
focus of the pulsed beams, which in turn raises the compression of the
representation.Comment: 49 pages, 14 figure
Modal Analysis and Coupling in Metal-Insulator-Metal Waveguides
This paper shows how to analyze plasmonic metal-insulator-metal waveguides
using the full modal structure of these guides. The analysis applies to all
frequencies, particularly including the near infrared and visible spectrum, and
to a wide range of sizes, including nanometallic structures. We use the
approach here specifically to analyze waveguide junctions. We show that the
full modal structure of the metal-insulator-metal (MIM) waveguides--which
consists of real and complex discrete eigenvalue spectra, as well as the
continuous spectrum--forms a complete basis set. We provide the derivation of
these modes using the techniques developed for Sturm-Liouville and generalized
eigenvalue equations. We demonstrate the need to include all parts of the
spectrum to have a complete set of basis vectors to describe scattering within
MIM waveguides with the mode-matching technique. We numerically compare the
mode-matching formulation with finite-difference frequency-domain analysis and
find very good agreement between the two for modal scattering at symmetric MIM
waveguide junctions. We touch upon the similarities between the underlying
mathematical structure of the MIM waveguide and the PT symmetric quantum
mechanical pseudo-Hermitian Hamiltonians. The rich set of modes that the MIM
waveguide supports forms a canonical example against which other more
complicated geometries can be compared. Our work here encompasses the microwave
results, but extends also to waveguides with real metals even at infrared and
optical frequencies.Comment: 17 pages, 13 figures, 2 tables, references expanded, typos fixed,
figures slightly modifie
Adaptive Filtering Enhances Information Transmission in Visual Cortex
Sensory neuroscience seeks to understand how the brain encodes natural
environments. However, neural coding has largely been studied using simplified
stimuli. In order to assess whether the brain's coding strategy depend on the
stimulus ensemble, we apply a new information-theoretic method that allows
unbiased calculation of neural filters (receptive fields) from responses to
natural scenes or other complex signals with strong multipoint correlations. In
the cat primary visual cortex we compare responses to natural inputs with those
to noise inputs matched for luminance and contrast. We find that neural filters
adaptively change with the input ensemble so as to increase the information
carried by the neural response about the filtered stimulus. Adaptation affects
the spatial frequency composition of the filter, enhancing sensitivity to
under-represented frequencies in agreement with optimal encoding arguments.
Adaptation occurs over 40 s to many minutes, longer than most previously
reported forms of adaptation.Comment: 20 pages, 11 figures, includes supplementary informatio
Transmutations and spectral parameter power series in eigenvalue problems
We give an overview of recent developments in Sturm-Liouville theory
concerning operators of transmutation (transformation) and spectral parameter
power series (SPPS). The possibility to write down the dispersion
(characteristic) equations corresponding to a variety of spectral problems
related to Sturm-Liouville equations in an analytic form is an attractive
feature of the SPPS method. It is based on a computation of certain systems of
recursive integrals. Considered as families of functions these systems are
complete in the -space and result to be the images of the nonnegative
integer powers of the independent variable under the action of a corresponding
transmutation operator. This recently revealed property of the Delsarte
transmutations opens the way to apply the transmutation operator even when its
integral kernel is unknown and gives the possibility to obtain further
interesting properties concerning the Darboux transformed Schr\"{o}dinger
operators.
We introduce the systems of recursive integrals and the SPPS approach,
explain some of its applications to spectral problems with numerical
illustrations, give the definition and basic properties of transmutation
operators, introduce a parametrized family of transmutation operators, study
their mapping properties and construct the transmutation operators for Darboux
transformed Schr\"{o}dinger operators.Comment: 30 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1111.444
From Gaussian beam to complex-source-point spherical wave
It is shown that the paraxial Gaussian beam becomes the complex-source-point spherical wave when all-order corrections are made according to the method of Lax, Louisell, and McKnight. Apparent contradictions between previously published first-order corrections are also discussed
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