214 research outputs found
Absorption losses in periodic arrays of thin metallic wires
We analyze the transmission and reflection of the electromagnetic wave
calculated from transfer matrix simulations of periodic arrangements of thin
metallic wires. The effective permittivity and the absorption is determined.
Their dependence on the wire thickness and the conductance of the metallic
wires is studied. The cutoff frequency or effective plasma frequency is
obtained and is compared with analytical predictions. It is shown that the
periodic arrangement of wires exhibits a frequency region in which the real
part of the permittivity is negative while its imaginary part is very small.
This behavior is seen for wires with thickness as small as 17 m with a
lattice constant of 3.33 mm
Gap deformation and classical wave localization in disordered two-dimensional photonic band gap materials
By using two ab initio numerical methods we study the effects that disorder
has on the spectral gaps and on wave localization in two-dimensional photonic
band gap materials. We find that there are basically two different responses
depending on the lattice realization (solid dielectric cylinders in air or vise
versa), the wave polarization, and the particular form under which disorder is
introduced. Two different pictures for the photonic states are employed, the
``nearly free'' photon and the ``strongly localized'' photon. These originate
from the two different mechanisms responsible for the formation of the spectral
gaps, ie. multiple scattering and single scatterer resonances, and they
qualitatively explain our results.Comment: Accepted for publication in Phys. Rev.
Tight-binding parameterization for photonic band gap materials
The ideas of the linear combination of atomic orbitals (LCAO) method, well
known from the study of electrons, is extended to the classical wave case. The
Mie resonances of the isolated scatterer in the classical wave case, are
analogous to the localized eigenstates in the electronic case. The matrix
elements of the two-dimensional tight-binding (TB) Hamiltonian are obtained by
fitting to ab initio results. The transferability of the TB model is tested by
reproducing accurately the band structure of different 2D lattices, with and
without defects, thus proving that the obtained TB parameters can be used to
study other properties of the photonic band gap materials.Comment: 4 pages, 3 postscript figures, sumbitted to Phys. rev. Let
Absolute negative refraction and imaging of unpolarized electromagnetic waves by two-dimensional photonic crystals
Absolute negative refraction regions for both polarizations of
electromagnetic wave in two-dimensional photonic crystal have been found
through both the analysis and the exact numerical simulation. Especially,
absolute all-angle negative refraction for both polarizations has also been
demonstrated. Thus, the focusing and image of unpolarized light can be realized
by a microsuperlens consisting of the two-dimensional photonic crystals. The
absorption and compensation for the losses by introducing optical gain in these
systems have also been discussed
Scattering of elastic waves by periodic arrays of spherical bodies
We develop a formalism for the calculation of the frequency band structure of
a phononic crystal consisting of non-overlapping elastic spheres, characterized
by Lam\'e coefficients which may be complex and frequency dependent, arranged
periodically in a host medium with different mass density and Lam\'e
coefficients. We view the crystal as a sequence of planes of spheres, parallel
to and having the two dimensional periodicity of a given crystallographic
plane, and obtain the complex band structure of the infinite crystal associated
with this plane. The method allows one to calculate, also, the transmission,
reflection, and absorption coefficients for an elastic wave (longitudinal or
transverse) incident, at any angle, on a slab of the crystal of finite
thickness. We demonstrate the efficiency of the method by applying it to a
specific example.Comment: 19 pages, 5 figures, Phys. Rev. B (in press
What is the right form of the probability distribution of the conductance at the mobility edge?
The probability distribution of the conductance Pc(g) at the Anderson
critical point is calculated. It is find that Pc(g) has a dip at small g in
agreement with epsilon expansion results. The Pc(g) for the 3d system is quite
different from the 2d quantum critical point of the integer quantum Hall
effect. The universality or not of these distributions is of central importance
to the field of disordered systems.Comment: 1 page, 1 figure submitted to Phys. Rev. Lett. (Comment
Impact of the inherent periodic structure on the effective medium description of left-handed and related meta-materials
We study the frequency dependence of the effective electromagnetic parameters
of left-handed and related meta-materials of the split ring resonator and wire
type. We show that the reduced translational symmetry (periodic structure)
inherent to these meta-materials influences their effective electromagnetic
response. To anticipate this periodicity, we formulate a periodic effective
medium model which enables us to distinguish the resonant behavior of
electromagnetic parameters from effects of the periodicity of the structure. We
use this model for the analysis of numerical data for the transmission and
reflection of periodic arrays of split ring resonators, thin metallic wires,
cut wires as well as the left-handed structures. The present method enables us
to identify the origin of the previously observed resonance/anti-resonance
coupling as well as the occurrence of negative imaginary parts in the effective
permittivities and permeabilities of those materials. Our analysis shows that
the periodicity of the structure can be neglected only for the wavelength of
the electromagnetic wave larger than 30 space periods of the investigated
structure.Comment: 23 pages, 14 figure
Laser-micromachined Millimeter-wave Photonic band gap cavity structures
Cataloged from PDF version of article.We have used laser-micromachined alumina substrates to build a three-dimensional photonic
band-gap crystal. The rod-based structure has a three-dimensional full photonic band gap between
90 and 100 GHz. The high resistivity of alumina results in a typical attenuation rate of 15 dB per
unit cell within the band gap. By removing material, we have built defects which can be used as
millimeter-wave cavity structures. The resulting quality ~Q! factors of the millimeter-wave cavity
structures were as high as 1000 with a peak transmission of 10 dB below the incident
signal. © 1995 American Institute of Physics
Transmission Studies of Left-handed Materials
Left-handed materials are studied numerically using an improved version of
the transfer-matrix method. The transmission, reflection, the phase of the
reflection and the absorption are calculated and compared with experiments for
both single split-ring resonators (SRR) with negative permeability and
left-handed materials (LHMs) which have both the permittivity and permeability
negative. Our results suggest ways of positively identifying materials that
have both permittivity and permeability negative, from materials that have
either permeability or permittivity negative
Nutritional and tissue-specific regulation of cytochrome P450 CYP711A MAX1 homologues and strigolactone biosynthesis in wheat
Strigolactones (SLs) are a class of phytohormones regulating branching/tillering, and their biosynthesis has been associated with nutritional signals and plant adaptation to nutrient-limiting conditions. The enzymes in the SL biosynthetic pathway downstream of carlactone are of interest as they are responsible for structural diversity in SLs, particularly cytochrome P450 CYP711A subfamily members, such as MORE AXILLARY GROWTH1 (MAX1) in Arabidopsis. We identified 13 MAX1 homologues in wheat, clustering in four clades and five homoeologous subgroups. The utilization of RNA-sequencing data revealed a distinct expression pattern of MAX1 homologues in above- and below-ground tissues, providing insights into the distinct roles of MAX1 homologues in wheat. In addition, a transcriptional analysis showed that SL biosynthetic genes were systematically regulated by nitrogen supply. Nitrogen limitation led to larger transcriptional changes in the basal nodes than phosphorus limitation, which was consistent with the observed tillering suppression, as wheat showed higher sensitivity to nitrogen. The opposite was observed in roots, with phosphorus limitation leading to stronger induction of most SL biosynthetic genes compared with nitrogen limitation. The observed tissue-specific regulation of SL biosynthetic genes in response to nutritional signals is likely to reflect the dual role of SLs as rhizosphere signals and branching inhibitors
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