305 research outputs found
All-Dielectric Meta-optics for High-Efficiency Independent Amplitude and Phase Manipulation
Metasurfaces, composed of subwavelength scattering elements, have
demonstrated remarkable control over the transmitted amplitude, phase, and
polarization of light. However, manipulating the amplitude upon transmission
has required loss if a single metasurface is used. Here, we describe
high-efficiency independent manipulation of the amplitude and phase of a beam
using two lossless phase-only metasurfaces separated by a distance. With this
configuration, we experimentally demonstrate optical components such as
combined beam-forming and splitting devices, as well as those for forming
complex-valued, three-dimensional holograms. The compound meta-optic platform
provides a promising approach for achieving high performance optical
holographic displays and compact optical components, while exhibiting a high
overall efficiency
Intelligent Multi-channel Meta-imagers for Accelerating Machine Vision
Rapid developments in machine vision have led to advances in a variety of
industries, from medical image analysis to autonomous systems. These
achievements, however, typically necessitate digital neural networks with heavy
computational requirements, which are limited by high energy consumption and
further hinder real-time decision-making when computation resources are not
accessible. Here, we demonstrate an intelligent meta-imager that is designed to
work in concert with a digital back-end to off-load computationally expensive
convolution operations into high-speed and low-power optics. In this
architecture, metasurfaces enable both angle and polarization multiplexing to
create multiple information channels that perform positive and negatively
valued convolution operations in a single shot. The meta-imager is employed for
object classification, experimentally achieving 98.6% accurate classification
of handwritten digits and 88.8% accuracy in classifying fashion images. With
compactness, high speed, and low power consumption, this approach could find a
wide range of applications in artificial intelligence and machine vision
applications.Comment: 15 pages, 5 figure
Large effective magnetic fields from chiral phonons in rare-earth halides
Time-reversal symmetry (TRS) is pivotal for materials optical, magnetic,
topological, and transport properties. Chiral phonons, characterized by atoms
rotating unidirectionally around their equilibrium positions, generate dynamic
lattice structures that break TRS. Here we report that coherent chiral phonons,
driven by circularly polarized terahertz light pulses, can polarize the
paramagnetic spins in CeF3 like a quasi-static magnetic field on the order of 1
Tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found
the transient magnetization is only excited by pulses resonant with phonons,
proportional to the angular momentum of the phonons, and growing with magnetic
susceptibility at cryogenic temperatures, as expected from the spin-phonon
coupling model. The time-dependent effective magnetic field quantitatively
agrees with that calculated from phonon dynamics. Our results may open a new
route to directly investigate mode-specific spin-phonon interaction in
ultrafast magnetism, energy-efficient spintronics, and non-equilibrium phases
of matter with broken TRS
Quasi-molecular and atomic phases of dense solid hydrogen
The high-pressure phases of solid hydrogen are of fundamental interest and
relevant to the interior of giant planets; however, knowledge of these phases
is far from complete. Particle swarm optimization (PSO) techniques were applied
to a structural search, yielding hitherto unexpected high-pressure phases of
solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24
structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of
intramolecular bonds was predicted, providing a deeper understanding of
molecular dissociation in solid hydrogen, which has been a mystery for decades.
We further predicted the existence of two atomic phases: (i) the oC12 structure
(space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and
(ii) the cI16 structure, previously observed in lithium and sodium, stable
above 3.5 TPa upon consideration of the zero-point energy. This work clearly
revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram
for solid hydrogen and has implications for the constituent structures of giant
planets.Comment: accepted in The Journal of Physical Chemistr
Interaction between maïŹc magma and lithospheric mantle: Evidence from the geochemistry of olivines and olivine-hosted melt inclusions in lavas from Kibblewhite Volcano, Kermadec arc
Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system.
The quantum nature of the proton can crucially affect the structural and physical properties of hydrogen compounds. For example, in the high-pressure phases of H2O, quantum proton fluctuations lead to symmetrization of the hydrogen bond and reduce the boundary between asymmetric and symmetric structures in the phase diagram by 30 gigapascals (ref. 3). Here we show that an analogous quantum symmetrization occurs in the recently discovered sulfur hydride superconductor with a superconducting transition temperature Tc of 203 kelvin at 155 gigapascals--the highest Tc reported for any superconductor so far. Superconductivity occurs via the formation of a compound with chemical formula H3S (sulfur trihydride) with sulfur atoms arranged on a body-centred cubic lattice. If the hydrogen atoms are treated as classical particles, then for pressures greater than about 175 gigapascals they are predicted to sit exactly halfway between two sulfur atoms in a structure with Im3m symmetry. At lower pressures, the hydrogen atoms move to an off-centre position, forming a short H-S covalent bond and a longer H···S hydrogen bond in a structure with R3m symmetry. X-ray diffraction experiments confirm the H3S stoichiometry and the sulfur lattice sites, but were unable to discriminate between the two phases. Ab initio density-functional-theory calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 gigapascals for H3S and by 60 gigapascals for D3S. Consequently, we predict that the Im3m phase dominates the pressure range within which the high Tc was measured. The observed pressure dependence of Tc is accurately reproduced in our calculations for the phase, but not for the R3m phase. Therefore, the quantum nature of the proton fundamentally changes the superconducting phase diagram of H3S.We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (FIS2013- 48286-C2-2-P), French Agence Nationale de la Recherche (Grant No. ANR-13-IS10-0003- 392 01), EPSRC (UK) (Grant No. EP/J017639/1), Cambridge Commonwealth Trust, National Natural Science Foundation of China (Grants No. 11204111, 11404148, and 11274136), and 2012 Changjiang Scholars Program of China. Work at Carnegie was supported by EFree, an Energy Frontier Research Center funded by the DOE, Office of Science, Basic Energy Sciences under Award No. DE-SC-0001057. Computer facilities were provided by the PRACE project AESFT and the Donostia International Physics Center (DIPC).This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nature1717
IODP Expedition 330: Drilling the Louisville Seamount Trail in the SW Pacific
Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth's rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to "fixed" deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved ~15° south between 80 and 50 million years ago. This raises a fundamental question about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48°S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesnât involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homogenous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (characteristically up to 95% in volume) and which exhibit a large variability in (isotope) geochemistry and their mantle source components
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