237 research outputs found
Nonreciprocal Metasurface with Space-Time Phase Modulation
Creating materials with time-variant properties is critical for breaking
reciprocity that imposes fundamental limitations to wave propagation. However,
it is challenging to realize efficient and ultrafast temporal modulation in a
photonic system. Here, leveraging both spatial and temporal phase manipulation
offered by an ultrathin nonlinear metasurface, we experimentally demonstrated
nonreciprocal light reflection at wavelengths around 860 nm. The metasurface,
with traveling-wave modulation upon nonlinear Kerr building blocks, creates
spatial phase gradient and multi-terahertz temporal phase wobbling, which leads
to unidirectional photonic transitions in both momentum and energy spaces. We
observed completely asymmetric reflections in forward and backward light
propagations within a sub-wavelength interaction length of 150 nm. Our approach
pointed out a potential means for creating miniaturized and integratable
nonreciprocal optical components.Comment: 25 pages, 5 figure
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Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.
Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes
Molding Free-Space Light with Guided-Wave-Driven Metasurfaces
Metasurfaces with unparalleled controllability of light have shown great
potential to revolutionize conventional optics. However, they mainly work with
free-space light input, which makes it difficult for full on-chip integration.
On the other hand, integrated photonics enables densely packed devices but has
limited free-space light controllability. Here, we show that judiciously
designed guided-wave-driven metasurfaces can mold guided waves into arbitrary
free-space modes to achieve complex free-space functions, such as beam steering
and focusing, with ultrasmall footprints and potentially no diffraction loss.
Based on the same concept together with broken inversion symmetry induced by
metasurfaces, we also realized direct orbital angular momentum (OAM) lasing
from a micro-ring resonator. Our study works towards complete control of light
across integrated photonics and free-space platforms, and paves new exciting
ways for creating multifunctional photonic integrated devices with agile access
to free space which could enable a plethora of applications in communications,
remote sensing, displays, and etc.Comment: 37 pages, 5 figure
Investigation of nanoelectrodes by Transmission Electron Microscopy
International audienceElectrodes for making connections to single molecules and clusters must have separations smaller than 10 nm. They are therefore difficult or impossible to image with atomic force microscopes (AFM) or Scanning Electron Microscopes (SEM). We have fabricated nanoelelectrodes by different methods to contacts nanoclusters and conjugated molecules and investigated their properties in transmission electron microscope (TEM) and their electrical characteristics at room temperature and at 4.2K. The electrodes are made on SiN4 membranes, which is transparent to high energy electrons and which make it possible to image features of a few nanometers in TEM
EFMVFL: An Efficient and Flexible Multi-party Vertical Federated Learning without a Third Party
Federated learning allows multiple participants to conduct joint modeling
without disclosing their local data. Vertical federated learning (VFL) handles
the situation where participants share the same ID space and different feature
spaces. In most VFL frameworks, to protect the security and privacy of the
participants' local data, a third party is needed to generate homomorphic
encryption key pairs and perform decryption operations. In this way, the third
party is granted the right to decrypt information related to model parameters.
However, it isn't easy to find such a credible entity in the real world.
Existing methods for solving this problem are either communication-intensive or
unsuitable for multi-party scenarios. By combining secret sharing and
homomorphic encryption, we propose a novel VFL framework without a third party
called EFMVFL, which supports flexible expansion to multiple participants with
low communication overhead and is applicable to generalized linear models. We
give instantiations of our framework under logistic regression and Poisson
regression. Theoretical analysis and experiments show that our framework is
secure, more efficient, and easy to be extended to multiple participants.Comment: 9pages,2 figure
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