194 research outputs found
A new condition to identify isotropic dielectric-magnetic materials displaying negative phase velocity
The derivation of a new condition for characterizing isotropic
dielectric-magnetic materials exhibiting negative phase velocity, and the
equivalence of that condition with previously derived conditions, are
presented.Comment: 4 page
Polarization--universal rejection filtering by ambichiral structures made of indefinite dielectric--magnetic materials
An ambichiral structure comprising sheets of an anisotropic dielectric
material rejects normally incident plane waves of one circular polarization
(CP) state but not of the other CP state, in its fundamental Bragg regime.
However, if the same structure is made of an dielectric--magnetic material with
indefinite permittivity and permeability dyadics, it may function as a
polarization--universal rejection filter because two of the four planewave
components of the electromagnetic field phasors in each sheet are of the
positive--phase--velocity type and two are of the negative--phase--velocity
type.Comment: Cleaned citations in the tex
Thin-Film Metamaterials called Sculptured Thin Films
Morphology and performance are conjointed attributes of metamaterials, of
which sculptured thin films (STFs) are examples. STFs are assemblies of
nanowires that can be fabricated from many different materials, typically via
physical vapor deposition onto rotating substrates. The curvilinear--nanowire
morphology of STFs is determined by the substrate motions during fabrication.
The optical properties, especially, can be tailored by varying the morphology
of STFs. In many cases prototype devices have been fabricated for various
optical, thermal, chemical, and biological applications.Comment: to be published in Proc. ICTP School on Metamaterials (Augsut 2009,
Sibiu, Romania
Blending of nanoscale and microscale in uniform large-area sculptured thin-film architectures
The combination of large thickness ( m), large--area uniformity (75
mm diameter), high growth rate (up to 0.4 m/min) in assemblies of
complex--shaped nanowires on lithographically defined patterns has been
achieved for the first time. The nanoscale and the microscale have thus been
blended together in sculptured thin films with transverse architectures.
SiO () nanowires were grown by electron--beam evaporation onto
silicon substrates both with and without photoresist lines (1--D arrays) and
checkerboard (2--D arrays) patterns. Atomic self--shadowing due to
oblique--angle deposition enables the nanowires to grow continuously, to change
direction abruptly, and to maintain constant cross--sectional diameter. The
selective growth of nanowire assemblies on the top surfaces of both 1--D and
2--D arrays can be understood and predicted using simple geometrical shadowing
equations.Comment: 17 pages, 9 figure
Electromagnetic waves with negative phase velocity in Schwarzschild-de Sitter spacetime
The propagation of electromagnetic plane waves with negative phase velocity
(NPV) is considered in Schwarzschild-(anti-)de Sitter spacetime. It is
demonstrated that NPV propagation occurs in Schwarzschild-de Sitter spacetime
at lower values of the cosmological constant than is the case for de Sitter
spacetime. Furthermore, we report that neither is NPV propagation observed in
Schwarzschild-anti-de Sitter spacetime, nor is it possible outside the event
horizon of a Schwarzschild blackhole.Comment: Typographical error in eq (34) of EPL version is corrected in arxiv
version. Europhysics Letters (accepted for publication
Depolarization regions of nonzero volume in bianisotropic homogenized composites
In conventional approaches to the homogenization of random particulate
composites, the component phase particles are often treated mathematically as
vanishingly small, point-like entities. The electromagnetic responses of these
component phase particles are provided by depolarization dyadics which derive
from the singularity of the corresponding dyadic Green functions. Through
neglecting the spatial extent of the depolarization region, important
information may be lost, particularly relating to coherent scattering losses.
We present an extension to the strong-property-fluctuation theory in which
depolarization regions of nonzero volume and ellipsoidal geometry are
accommodated. Therein, both the size and spatial distribution of the component
phase particles are taken into account. The analysis is developed within the
most general linear setting of bianisotropic homogenized composite mediums
(HCMs). Numerical studies of the constitutive parameters are presented for
representative examples of HCM; both Lorentz-reciprocal and
Lorentz-nonreciprocal HCMs are considered. These studies reveal that estimates
of the HCM constitutive parameters in relation to volume fraction, particle
eccentricity, particle orientation and correlation length are all significantly
influenced by the size of the component phase particles
Theory of Dyakonov-Tamm waves at the planar interface of a sculptured nematic thin film and an isotropic dielectric material
In order to ascertain conditions for surface-wave propagation guided by the
planar interface of an isotropic dielectric material and a sculptured nematic
thin film (SNTF) with periodic nonhomogeneity, we formulated a boundary-value
problem, obtained a dispersion equation therefrom, and numerically solved it.
The surface waves obtained are Dyakonov-Tamm waves. The angular domain formed
by the directions of propagation of the Dyakonov--Tamm waves can be very wide
(even as wide as to allow propagation in every direction in the interface
plane), because of the periodic nonhomogeneity of the SNTF. A search for
Dyakonov-Tamm waves is, at the present time, the most promising route to take
for experimental verification of surface-wave propagation guided by the
interface of two dielectric materials, at least one of which is anisotropic.
That would also assist in realizing the potential of such surface waves for
optical sensing of various types of analytes infiltrating one or both of the
two dielectric materials.Comment: accepted for publication in J. Opt.
Enhanced diffraction by a rectangular grating made of a negative phase--velocity (or negative index) material
The diffraction of electromagnetic plane waves by a rectangular grating
formed by discrete steps in the interface of a homogeneous, isotropic, linear,
negative phase--velocity (negative index) material with free space is studied
using the semi--analytic C method. When a nonspecular diffracted order is of
the propagating type, coupling to that order is significantly larger for a
negative index material than for conventional material. The computed coupling
strengths reported here are in agreement with recent experiments, and
illustrate the role of evanescent fields localized at the grating interface in
producing this enhanced coupling.Comment: 12 pages, 4 figure
Quantification of optical pulsed-plane-wave-shaping by chiral sculptured thin films
The durations and average speeds of ultrashort optical pulses transmitted
through chiral sculptured thin films (STFs) were calculated using a
finite-difference time-domain algorithm. Chiral STFs are a class of
nanoengineered materials whose microstructure comprises parallel helicoidal
nanowires grown normal to a substrate. The nanowires are 10-300 nm in
diameter and m in length. Durations of transmitted pulses tend to
increase with decreasing (free-space) wavelength of the carrier plane wave,
while average speeds tend to increase with increasing wavelength. An increase
in nonlinearity, as manifested by an intensity-dependent refractive index in
the frequency domain, tends to increase durations of transmitted pulses and
decrease average speeds. The circular Bragg phenomenon exhibited by a chiral
STFs manifests itself in the frequency domain as high reflectivity for normally
incident carrier plane waves whose circular polarization state is matched to
the structural handedness of the film and whose wavelength falls in a range
known as the Bragg regime; films of the opposite structural handedness reflect
such plane waves little. This effect tends to distort the shapes of transmitted
pulses with respect to the incident pulses, and such shaping can cause sharp
changes in some measures of average speed with respect to carrier wavelength. A
local maximum in the variation of one measure of the pulse duration with
respect to wavelength is noted and attributed to the circular Bragg phenomenon.
Several of these effects are explained via frequency-domain arguments. The
presented results serve as a foundation for future theoretical and experimental
studies of optical pulse propagation through causal, nonlinear, nonhomogeneous,
and anisotropic materials.Comment: To appear in Journal of Modern Optic
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