311 research outputs found
Tunable Split-Ring Resonator Photonic Crystals for Left-Handed Electromagnetic Meta-Materials
Evidence for Braggoriton Excitations in Opal Photonic Crystals Infiltrated with Highly Polarizable Dyes
We studied angle-dependent reflectivity spectra of opal photonic crystals
infiltrated with cyanine dyes, which are highly polarizable media with very
large Rabi frequency. We show that when resonance conditions between the
exciton-polariton of the infiltrated dye and Bragg frequencies exist, then the
Bragg stop band decomposes into two reflectivity bands with a semi-transparent
spectral range in between that is due to light propagation inside the gap
caused by the existence of braggoriton excitations. These novel excitations
result from the interplay interaction between the Bragg gap with spatial
modulation origin and the polariton gap due to the excitons, and may lead to
optical communication traffic inside the gap of photonic crystals via channel
waveguiding.Comment: LaTex, 5 pages, 3 figures include
Anomalous Coherent Backscattering of Light from Opal Photonic Crystals
We studied coherent backscattering (CBS) of light from opal photonic crystals
in air at different incident inclination angles, wavelengths and along various
[hkl] directions inside the opals. Similar to previously obtained CBS cones
from various random media, we found that when Bragg condition with the incident
light beam is not met then the CBS cones from opals show a triangular line
shape in excellent agreement with light diffusion theory. At Bragg condition,
however, we observed a dramatic broadening of the opal CBS cones that depends
on the incident angle and [hkl] direction. This broadening is explained as due
to the light intensity decay in course of propagation along the Bragg direction
{\em before the first} and {\em after the last} scattering events. We modified
the CBS theory to incorporate the attenuation that results from the photonic
band structure of the medium. Using the modified theory we extract from our CBS
data the light mean free path and Bragg attenuation length at different [hkl].
Our study shows that CBS measurements are a unique experimental technique to
explore photonic crystals with disorder, when other spectroscopical methods
become ambiguous due to disorder-induced broadening.Comment: 10 pages, 5 figure
Cathodoluminescence from Organic Bilayer Induced by Field Electron Emission of Carbon Nanotubes
Magnetocrystalline anisotropic effect in GdCoFeAsO ()
From a systematic study of the electrical resistivity , magnetic
susceptibility , isothermal magnetization and the specific
heat , a temperature-magnetic field (-) phase diagram has been
established for GdCoFeAsO ( and ) polycrystalline
compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic
(FM) transition of Co electrons () and
antiferromagnetic (AFM) transition of Gd electrons
(). For the Fe-doped sample (), an extra
magnetic reorientation transition takes place below ,
which is likely associated with Co moments. The two magnetic species of Gd and
Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM)
behavior in the magnetic susceptibility. Upon decreasing the temperature (), the magnetocrystalline anisotropy breaks up the FM
order of Co by aligning the moments with the local easy axes of the various
grains, leading to a spin reorientation transition at
. By applying a magnetic field,
monotonically decreases to lower temperatures, while
the is relatively robust against the external field.
On the other hand, the applied magnetic field pulls the magnetization of grains
from the local easy direction to the field direction via a first-order
reorientation transition, with the transition field () increasing
upon cooling the temperature.Comment: accepted by physical Review B 6 figures and 7 page
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