353 research outputs found
Topological Photonic Phase in Chiral Hyperbolic Metamaterials
Recently the possibility of achieving one-way backscatter immune
transportation of light by mimicking the topological order present within
certain solid state systems, such as topological insulators, has received much
attention. Thus far however, demonstrations of non-trivial topology in
photonics have relied on photonic crystals with precisely engineered lattice
structures, periodic on the scale of the operational wavelength and composed of
finely tuned, complex materials. Here we propose a novel effective medium
approach towards achieving topologically protected photonic surface states
robust against disorder on all length scales and for a wide range of material
parameters. Remarkably, the non-trivial topology of our metamaterial design
results from the Berry curvature arising from the transversality of
electromagnetic waves in a homogeneous medium. Our investigation therefore acts
to bridge the gap between the advancing field of topological band theory and
classical optical phenomena such as the Spin Hall effect of light. The
effective medium route to topological phases will pave the way for highly
compact one-way transportation of electromagnetic waves in integrated photonic
circuits.Comment: 11 pages, 3 figures. To appear in PR
Three dimensional photonic Dirac points in metamaterials
Topological semimetals, representing a new topological phase that lacks a
full bandgap in bulk states and exhibiting nontrivial topological orders,
recently have been extended to photonic systems, predominantly in photonic
crystals and to a lesser extent, metamaterials. Photonic crystal realizations
of Dirac degeneracies are protected by various space symmetries, where Bloch
modes span the spin and orbital subspaces. Here, we theoretically show that
Dirac points can also be realized in effective media through the intrinsic
degrees of freedom in electromagnetism under electromagnetic duality. A pair of
spin polarized Fermi arc like surface states is observed at the interface
between air and the Dirac metamaterials. These surface states show linear
k-space dispersion relation, resulting in nearly diffraction-less propagation.
Furthermore, eigen reflection fields show the decomposition from a Dirac point
to two Weyl points. We also find the topological correlation between a Dirac
point and vortex/vector beams in classic photonics. The theoretical proposal of
photonic Dirac point lays foundation for unveiling the connection between
intrinsic physics and global topology in electromagnetism.Comment: 15 pages, 5 figure
Second-order topological insulator and fragile topology in topological circuitry simulation
Second-order topological insulators (SOTIs) are the topological phases of
matter in d dimensions that manifest (d-2)-dimensional localized modes at the
intersection of the edges. We show that SOTIs can be designed via stacked Chern
insulators with opposite chiralities connected by interlayer coupling. To
characterize the bulk-corner correspondence, we establish a
Jacobian-transformed nested Wilson loop method and an edge theory that are
applicable to a wider class of higher-order topological systems. The
corresponding topological invariant admits a filling anomaly of the corner
modes with fractional charges. The system manifests a fragile topological phase
characterized by the absence of a Wannier gap in the Wilson loop spectrum.
Furthermore, we argue that the proposed approach can be generalized to
multilayers. Our work offers perspectives for exploring and understanding
higher-order topological phenomena.Comment: 5 pages, 4 figure
tSF: Transformer-based Semantic Filter for Few-Shot Learning
Few-Shot Learning (FSL) alleviates the data shortage challenge via embedding
discriminative target-aware features among plenty seen (base) and few unseen
(novel) labeled samples. Most feature embedding modules in recent FSL methods
are specially designed for corresponding learning tasks (e.g., classification,
segmentation, and object detection), which limits the utility of embedding
features. To this end, we propose a light and universal module named
transformer-based Semantic Filter (tSF), which can be applied for different FSL
tasks. The proposed tSF redesigns the inputs of a transformer-based structure
by a semantic filter, which not only embeds the knowledge from whole base set
to novel set but also filters semantic features for target category.
Furthermore, the parameters of tSF is equal to half of a standard transformer
block (less than 1M). In the experiments, our tSF is able to boost the
performances in different classic few-shot learning tasks (about 2%
improvement), especially outperforms the state-of-the-arts on multiple
benchmark datasets in few-shot classification task
Modeling and Analysis of Bifurcation in a Delayed Worm Propagation Model
A delayed worm propagation model with birth and death rates is formulated. The stability of the positive equilibrium is studied. Through theoretical analysis, a critical value Ď„0 of Hopf bifurcation is derived. The worm propagation system is locally asymptotically stable when time delay is less than Ď„0. However, Hopf bifurcation appears when time delay Ď„ passes the threshold Ď„0, which means that the worm propagation system is unstable and out of control. Consequently, time delay should be adjusted to be less than Ď„0 to ensure the stability of the system stable and better prediction of the scale and speed of Internet worm spreading. Finally, numerical and simulation experiments are presented to simulate the system, which fully support our analysis
Plasmon Weyl Degeneracies in Magnetized Plasma
In this letter, we report the presence of novel type of plasmon Weyl points
in a naturally existing material - magnetized plasma. In such a medium,
conventional, purely longitudinal bulk plasma oscillations exists only along
the direction of applied magnetic field (z direction). With strong enough
magnetic field, there exist helical propagating modes along z direction with
circular polarizations. The orthogonality between the longitudinal bulk plasmon
mode and the transverse helical propagating modes guarantees their crossing at
the bulk plasmon frequency. These crossing points, embedded in the bulk plasmon
dispersion line, serve as monopoles in the k space - the so called Weyl points.
These Weyl points lead to salient observable features. These include the highly
intriguing observation that, at a magnetized plasma surface which is parallel
to the applied magnetic field, reflection of an electromagnetic wave with
in-plane wave-vector close to the Weyl points exhibits chiral behavior only in
half of the k plane, which is bounded by the projection of the bulk plasmon
dispersion line. We also verify the presence of 'Fermi arcs' connecting the two
Weyl points with opposite chiralities when magnetized plasma interfaces with
trivial photonic materials. Our study introduces the concept of Weyl photonics
into homogeneous strongly dispersive photonic materials, which could pave way
for realizing new topological photonic devices.Comment: 13 pages, 5 figure
High Performance Metasurface Antennas
Recently, metasurfaces (MSs) have received tremendous attention because their electromagnetic properties can be controlled at will. Generally, metasurface with hyperbolic phase distributions, namely, focusing metasurface, can be used to design high-gain antennas. Besides, metasurface has the ability of controlling the polarization state of electromagnetic wave. In this chapter, we first propose a new ultrathin broadband reflected MS and take it into application for high-gain planar antenna. Then, we propose multilayer multifunctional transmitted MSs to simultaneously enhance the gain and transform the linear polarization to circular polarization of the patch antenna. This kind of high-gain antenna eliminates the feed-block effect of the reflected ones
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