14,000 research outputs found
Group Theory of Circular-Polarization Effects in Chiral Photonic Crystals with Four-Fold Rotation Axes, Applied to the Eight-Fold Intergrowth of Gyroid Nets
We use group or representation theory and scattering matrix calculations to
derive analytical results for the band structure topology and the scattering
parameters, applicable to any chiral photonic crystal with body-centered cubic
symmetry I432 for circularly-polarised incident light. We demonstrate in
particular that all bands along the cubic [100] direction can be identified
with the irreducible representations E+/-,A and B of the C4 point group. E+ and
E- modes represent the only transmission channels for plane waves with wave
vector along the ? line, and can be identified as non-interacting transmission
channels for right- (E-) and left-circularly polarised light (E+),
respectively. Scattering matrix calculations provide explicit relationships for
the transmission and reflectance amplitudes through a finite slab which
guarantee equal transmission rates for both polarisations and vanishing
ellipticity below a critical frequency, yet allowing for finite rotation of the
polarisation plane. All results are verified numerically for the so-called
8-srs geometry, consisting of eight interwoven equal-handed dielectric Gyroid
networks embedded in air. The combination of vanishing losses, vanishing
ellipticity, near-perfect transmission and optical activity comparable to that
of metallic meta-materials makes this geometry an attractive design for
nanofabricated photonic materials
Non-collinear long-range magnetic ordering in HgCr2S4
The low-temperature magnetic structure of \HG has been studied by
high-resolution powder neutron diffraction. Long-range incommensurate magnetic
order sets in at T22K with propagation vector
\textbf{k}=(0,0,0.18). On cooling below T, the propagation vector
increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The
magnetic structure below T consists of ferromagnetic layers in the
\textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis.
Symmetry analysis using corepresentations theory reveals a point group symmetry
in the ordered magnetic phase of 422 (D), which is incompatible with
macroscopic ferroelectricity. This finding indicates that the spontaneous
electric polarization observed experimentally cannot be coupled to the magnetic
order parameter
Magnetic structure of Ba(TiO)Cu(PO) probed using spherical neutron polarimetry
The antiferromagnetic compound Ba(TiO)Cu(PO) contains square
cupola of corner-sharing CuO plaquettes, which were proposed to form
effective quadrupolar order. To identify the magnetic structure, we have
performed spherical neutron polarimetry measurements. Based on symmetry
analysis and careful measurements we conclude that the orientation of the
Cu spins form a non-collinear in-out structure with spins approximately
perpendicular to the CuO motif. Strong Dzyaloshinskii-Moriya interaction
naturally lends itself to explain this phenomenon. The identification of the
ground state magnetic structure should serve well for future theoretical and
experimental studies into this and closely related compounds.Comment: 9 pages, 4 figure
Plasmon Modes of Axisymmetric Metallic Nanoparticles: A Group Theory Analysis
We report a thorough and rigorous analysis of the plasmon modes of axisymmetric metallic nanoparticles,
based on group theory techniques and block diagonalization of the scattering T matrix. In particular, we
discuss plasmonic excitations under plane-wave illumination of a silver nanorod and a nanodisk, and present
a detailed comparative study of elongated silver nanoparticles of different shape, but with the same length
and thickness. Our methodology allows for an unambiguous classification of the eigenmodes of nonspherical
particles, according to the irreducible representations of the appropriate point symmetry group, and provides
a consistent explanation of relevant extinction spectra elucidating aspects of the problem to a degree that
goes beyond usual interpretation
Optical chirality from dark-field illumination of planar plasmonic nanostructures
Dark-field illumination is shown to make planar chiral nanoparticle
arrangements exhibit circular dichroism in extinction analogous to true chiral
scatterers. Circular dichrosim is experimentally observed at the maximum
scattering of single oligomers consisting rotationally symmetric arrangements
of gold nanorods, with strong agreement to numerical simulation. A dipole model
is developed to show that this effect is caused by a difference in the
geometric projection of a nanorod onto the handed orientation of electric
fields created by a circularly polarized dark-field that is normally incident
on a glass substrate. Owing to this geometric origin, the wavelength of the
peak chiral response is also experimentally shown to shift depending on the
separation between nanoparticles. All presented oligomers have physical
dimensions less than the operating wavelength, and the applicable extension to
closely packed planar arrays of oligomers is demonstrated to amplify the
magnitude of circular dichroism. The realization of strong chirality in these
oligomers demonstrates a new path to engineer optical chirality from planar
devices using dark-field illumination
Generalized Debye Sources Based EFIE Solver on Subdivision Surfaces
The electric field integral equation is a well known workhorse for obtaining
fields scattered by a perfect electric conducting (PEC) object. As a result,
the nuances and challenges of solving this equation have been examined for a
while. Two recent papers motivate the effort presented in this paper. Unlike
traditional work that uses equivalent currents defined on surfaces, recent
research proposes a technique that results in well conditioned systems by
employing generalized Debye sources (GDS) as unknowns. In a complementary
effort, some of us developed a method that exploits the same representation for
both the geometry (subdivision surface representations) and functions defined
on the geometry, also known as isogeometric analysis (IGA). The challenge in
generalizing GDS method to a discretized geometry is the complexity of the
intermediate operators. However, thanks to our earlier work on subdivision
surfaces, the additional smoothness of geometric representation permits
discretizing these intermediate operations. In this paper, we employ both ideas
to present a well conditioned GDS-EFIE. Here, the intermediate surface
Laplacian is well discretized by using subdivision basis. Likewise, using
subdivision basis to represent the sources, results in an efficient and
accurate IGA framework. Numerous results are presented to demonstrate the
efficacy of the approach
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