4,065 research outputs found
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
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
Weak Signal Detection Based on Adaptive Cascaded Bistable Stochastic Resonance System
AbstractStochastic resonance system is an effective method to extract weak signal, however, system output is directly influenced by system parameters. Aiming to this, a method about weak periodic signal extraction was developed based on adaptive stochastic resonance. Firstly cascaded stochastic resonance system was established in order to achieve better low-pass filtering effect. And then, variance of zero point distance was chosen as measurement index of cascade system. It's able to overcome the shortage that traditional adaptive stochastic resonance system needs to know the signal frequency beforehand. Also, it could obtain optimum system parameters adaptively. Basing on these parameters, input signal will be handled, and optimum output could be obtained. Furthermore, different periodic signal have been recognized, and finally the validity of the method is verified through simulation experiments
Negative Group Velocity in the Absence of Absorption Resonance
Scientific community has well recognized that a Lorentzian medium exhibits anomalous dispersion behavior in its resonance absorption region. To satisfy the Krammers-Kronig relation, such an anomalous region has to be accompanied with significant loss, and thus, experimental observations of negative group velocity in this region generally require a gain-assisted approach. In this letter, we demonstrate that the negative group velocity can also be observed in the absence of absorption resonance. We show that the k-surface of a passive uniaxial Lorentzian medium undergoes a distortion near the plasma frequency. This process yields an anomalous dispersion bandwidth that is far away from the absorption resonance region, and enables the observation of negative group velocity at the plasma frequency band. Introducing anomalous dispersion in a well-controlled manner would greatly benefit the research of ultrafast photonics and find potential applications in optical delay lines, optical data storage and devices for quantum information processing
DPSUR: Accelerating Differentially Private Stochastic Gradient Descent Using Selective Update and Release
Machine learning models are known to memorize private data to reduce their
training loss, which can be inadvertently exploited by privacy attacks such as
model inversion and membership inference. To protect against these attacks,
differential privacy (DP) has become the de facto standard for
privacy-preserving machine learning, particularly those popular training
algorithms using stochastic gradient descent, such as DPSGD. Nonetheless, DPSGD
still suffers from severe utility loss due to its slow convergence. This is
partially caused by the random sampling, which brings bias and variance to the
gradient, and partially by the Gaussian noise, which leads to fluctuation of
gradient updates.
Our key idea to address these issues is to apply selective updates to the
model training, while discarding those useless or even harmful updates.
Motivated by this, this paper proposes DPSUR, a Differentially Private training
framework based on Selective Updates and Release, where the gradient from each
iteration is evaluated based on a validation test, and only those updates
leading to convergence are applied to the model. As such, DPSUR ensures the
training in the right direction and thus can achieve faster convergence than
DPSGD. The main challenges lie in two aspects -- privacy concerns arising from
gradient evaluation, and gradient selection strategy for model update. To
address the challenges, DPSUR introduces a clipping strategy for update
randomization and a threshold mechanism for gradient selection. Experiments
conducted on MNIST, FMNIST, CIFAR-10, and IMDB datasets show that DPSUR
significantly outperforms previous works in terms of convergence speed and
model utility.Comment: This paper has been accepted by VLDB 202
1-Allyl-3-amino-1H-pyrazole-4-carboxylic acid
The title compound, C7H9N3O2, was prepared by alkaline hydrolysis of ethyl 1-allyl-3-amino-1H-pyrazole-4-carboxylate. The crystal structure is stabilized by three types of intermolecular hydrogen bond (N—H⋯O, N—H⋯N and O—H⋯N)
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