128 research outputs found
Electromagnetic equivalent model for phase conjugate mirror based on the utilization of left-handed material
An electromagnetic equivalent model for the phase conjugate mirror (PCM) is proposed in this paper. The model is based on the unique property of the isotropic left-handed material (LHM) - the ability of LHM to reverse the phase factors of propagative waves. We show that a PCM interface can be substituted with a LHM-RHM (right-handed material) interface and associated image sources and objects in the LHM. This equivalent model is fully equivalent in the treatment of propagative wave components. However, we note that the presence of evanescent wave components can lead to undesirably surface resonance at the LHM-RHM interface. This artefact can be kept well bounded by introducing a small refractive index mismatch between the LHM and RHM. We demonstrate the usefulness of this model by modelling several representative scenarios of light patterns interacting with a PCM. The simulations were performed by applying the equivalent model to a commercial finite element method (FEM) software. This equivalent model also points to the intriguing possibility of realizing some unique LHM based systems in the optical domain by substituting a PCM in place of a LHM-RHM interface
Achieving Anisotropy in Metamaterials made of Dielectric Cylindrical Rods
We show that anisotropic negative effective dispersion relation can be
achieved in pure dielectric rod-type metamaterials by turning from the symmetry
of a square lattice to that of a rectangular one, i.e. by breaking the rotation
symmetry of effective homogeneous medium. Theoretical predictions and
conclusions are verified by both numerical calculations and computer based
simulations. The proposed anisotropic metamaterial, is used to construct a
refocusing slab-lens and a subdiffraction hyperlens. The all-dielectric origin
makes it more straightforward to address loss and scaling, two major issues of
metallic structures, thus facilitating future applications in both the
terahertz and optical range.Comment: Accepted for AP
Experimental observation of Weyl points
In 1929, Hermann Weyl derived the massless solutions from the Dirac equation
- the relativistic wave equation for electrons. Neutrinos were thought, for
decades, to be Weyl fermions until the discovery of the neutrino mass.
Moreover, it has been suggested that low energy excitations in condensed matter
can be the solutions to the Weyl Hamiltonian. Recently, photons have also been
proposed to emerge as Weyl particles inside photonic crystals. In all cases,
two linear dispersion bands in the three-dimensional (3D) momentum space
intersect at a single degenerate point - the Weyl point. Remarkably, these Weyl
points are monopoles of Berry flux with topological charges defined by the
Chern numbers. These topological invariants enable materials containing Weyl
points to exhibit a wide variety of novel phenomena including surface Fermi
arcs, chiral anomaly, negative magnetoresistance, nonlocal transport, quantum
anomalous Hall effect, unconventional superconductivity[15] and others [16,
17]. Nevertheless, Weyl points are yet to be experimentally observed in nature.
In this work, we report on precisely such an observation in an
inversion-breaking 3D double-gyroid photonic crystal without breaking
time-reversal symmetry.Comment: 4 pages, 3 figure
Second-Harmonic Generation and Spectrum Modulation by Active Nonlinear Metamaterial
The nonlinear properties of a metamaterial sample composed of double-layer
metallic patterns and voltage controllable diodes are experimentally
investigated. Second harmonics and spectrum modulations are clearly observed in
a wide band of microwave frequencies, showing that this kind of metamaterial is
not only tunable by low DC bias voltage, but also behaves strong nonlinear
property under a small power incidence. These properties are difficult to be
found in normal, naturally occurring materials.Comment: 14 pages, 4 figure
Metamaterial Broadband Angular Selectivity
We demonstrate how broadband angular selectivity can be achieved with stacks
of one-dimensionally periodic photonic crystals, each consisting of alternating
isotropic layers and effective anisotropic layers, where each effective
anisotropic layer is constructed from a multilayered metamaterial. We show that
by simply changing the structure of the metamaterials, the selective angle can
be tuned to a broad range of angles; and, by increasing the number of stacks,
the angular transmission window can be made as narrow as desired. As a proof of
principle, we realize the idea experimentally in the microwave regime. The
angular selectivity and tunability we report here can have various applications
such as in directional control of electromagnetic emitters and detectors.Comment: 5 pages, 5 figure
Skeletal camera network embedded structure-from-motion for 3D scene reconstruction from UAV images
Structure-from-Motion (SfM) techniques have been widely used for 3D scene reconstruction from multi-view images. However, due to the large computational costs of SfM methods there is a major challenge in processing highly overlapping images, e.g. images from unmanned aerial vehicles (UAV). This paper embeds a novel skeletal camera network (SCN) into SfM to enable efficient 3D scene reconstruction from a large set of UAV images. First, the flight control data are used within a weighted graph to construct a topologically connected camera network (TCN) to determine the spatial connections between UAV images. Second, the TCN is refined using a novel hierarchical degree bounded maximum spanning tree to generate a SCN, which contains a subset of edges from the TCN and ensures that each image is involved in at least a 3-view configuration. Third, the SCN is embedded into the SfM to produce a novel SCN-SfM method, which allows performing tie-point matching only for the actually connected image pairs. The proposed method was applied in three experiments with images from two fixed-wing UAVs and an octocopter UAV, respectively. In addition, the SCN-SfM method was compared to three other methods for image connectivity determination. The comparison shows a significant reduction in the number of matched images if our method is used, which leads to less computational costs. At the same time the achieved scene completeness and geometric accuracy are comparable
An Invisible Metallic Mesh
We introduce a solid material that is itself invisible, possessing identical
electromagnetic properties as air (i.e. not a cloak) at a desired frequency.
Such a material could provide improved mechanical stability, electrical
conduction and heat dissipation to a system, without disturbing incident
electromagnetic radiation. One immediate application would be towards perfect
antenna radomes. Unlike cloaks, such a transparent and self-invisible material
has yet to be demonstrated. Previous research has shown that a single sphere or
cylinder coated with plasmonic or dielectric layers can have a dark-state with
considerably suppressed scattering cross-section, due to the destructive
interference between two resonances in one of its scattering channels.
Nevertheless, a massive collection of these objects will have an accumulated
and detectable disturbance to the original field distribution. Here we overcome
this bottleneck by lining up the dark-state frequencies in different channels.
Specifically, we derive analytically, verify numerically and demonstrate
experimentally that deliberately designed corrugated metallic wires can have
record-low scattering amplitudes, achieved by aligning the nodal frequencies of
the first two scattering channels. This enables an arbitrary assembly of these
wires to be omnidirectionally invisible and the effective constitutive
parameters nearly identical to air. Measured transmission spectra at microwave
frequencies reveal indistinguishable results for all the arrangements of the
3D-printed samples studied.Comment: 11 pages, 3 figure
Controlling the Emission of Electromagnetic Sources by Coordinate transformation
The coordinate transformation on the space that contains electromagnetic
sources is studied. We find that, not only the permittivity and permeability
tensors of the media, but also the sources inside the media will take another
form in order to behave equivalently as the original case. It is demonstrated
that, a source of arbitrary shape and position in the free space can be
replaced by an appropriately designed metamaterial coating with current
distributed on the inner surface and would not be detected by outer observers,
because the emission of the source can be controlled at will in this way. As
examples, we show how to design conformal antennas by covering the sources with
transformation media. The method proposed in this letter provides a completely
new approach to develop novel active EM devices
Design and analytically full-wave validation of the invisibility cloaks, concentrators, and field rotators created with a general class of transformations
We investigate a general class of electromagnetic devices created with any
continuous transformation functions by rigorously calculating the analytical
expressions of the electromagnetic field in the whole space. Some interesting
phenomena associated with these transformation devices, including the
invisibility cloaks, concentrators, and field rotators, are discussed. By
carefully choosing the transformation function, we can realize cloaks which are
insensitive to perturbations at both the inner and outer boundaries.
Furthermore, we find that when the coating layer of the concentrator is
realized with left-handed materials, energy will circulate between the coating
and the core, and the energy transmits through the core of the concentrator can
be much bigger than that transmits through the concentrator. Therefore, such
concentrator is also a power flux amplifier. Finally, we propose a spherical
field rotator, which functions as not only a wave vector rotator, but also a
polarization rotator, depending on the orientations of the spherical rotator
with respect to the incident wave direction. The functionality of these novel
transformation devices are all successfully confirmed by our analytical full
wave method, which also provides an alternate computational efficient
validation method in contrast to numerical validation methods.Comment: 22 pages, 3 figure
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