20,950 research outputs found
Maxwell-Hydrodynamic Model for Simulating Nonlinear Terahertz Generation from Plasmonic Metasurfaces
The interaction between the electromagnetic field and plasmonic
nanostructures leads to both the strong linear response and inherent nonlinear
behavior. In this paper, a time-domain hydrodynamic model for describing the
motion of electrons in plasmonic nanostructures is presented, in which both
surface and bulk contributions of nonlinearity are considered. A coupled
Maxwell-hydrodynamic system capturing full-wave physics and free electron
dynamics is numerically solved with the parallel finite-difference time-domain
(FDTD) method. The validation of the proposed method is presented to simulate
linear and nonlinear responses from a plasmonic metasurface. The linear
response is compared with the Drude dispersion model and the nonlinear
terahertz emission from a difference-frequency generation process is validated
with theoretical analyses. The proposed scheme is fundamentally important to
design nonlinear plasmonic nanodevices, especially for efficient and broadband
THz emitters.Comment: 8 pages, 7 figures, IEEE Journal on Multiscale and Multiphysics
Computational Techniques, 201
Theoretical Study of Corundum as an Ideal Gate Dielectric Material for Graphene Transistors
Using physical insights and advanced first-principles calculations, we
suggest that corundum is an ideal gate dielectric material for graphene
transistors. Clean interface exists between graphene and Al-terminated (or
hydroxylated) Al2O3 and the valence band offsets for these systems are large
enough to create injection barrier. Remarkably, a band gap of {\guillemotright}
180 meV can be induced in graphene layer adsorbed on Al-terminated surface,
which could realize large ON/OFF ratio and high carrier mobility in graphene
transistors without additional band gap engineering and significant reduction
of transport properties. Moreover, the band gaps of graphene/Al2O3 system could
be tuned by an external electric field for practical applications
Full Hydrodynamic Model of Nonlinear Electromagnetic Response in Metallic Metamaterials
Applications of metallic metamaterials have generated significant interest in
recent years. Electromagnetic behavior of metamaterials in the optical range is
usually characterized by a local-linear response. In this article, we develop a
finite-difference time-domain (FDTD) solution of the hydrodynamic model that
describes a free electron gas in metals. Extending beyond the local-linear
response, the hydrodynamic model enables numerical investigation of nonlocal
and nonlinear interactions between electromagnetic waves and metallic
metamaterials. By explicitly imposing the current continuity constraint, the
proposed model is solved in a self-consistent manner. Charge, energy and
angular momentum conservation laws of high-order harmonic generation have been
demonstrated for the first time by the Maxwell-hydrodynamic FDTD model. The
model yields nonlinear optical responses for complex metallic metamaterials
irradiated by a variety of waveforms. Consequently, the multiphysics model
opens up unique opportunities for characterizing and designing nonlinear
nanodevices.Comment: 11 pages, 14 figure
Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2
We report the result of searching for globular clusters (GCs) around 55 Milky
Way satellite dwarf galaxies within the distance of 450 kpc from the Galactic
Center except for the Large and Small Magellanic Clouds and the Sagittarius
dwarf. For each dwarf, we analyze the stellar distribution of sources in Gaia
DR2, selected by magnitude, proper motion, and source morphology. Using the
kernel density estimation of stellar number counts, we identify eleven possible
GC candidates. Crossed-matched with existing imaging data, all eleven objects
are known either GCs or galaxies and only Fornax GC 1-6 among them are
associated with the targeted dwarf galaxy. Using simulated GCs, we calculate
the GC detection limit that spans the range from for distant dwarfs to for
nearby systems. Assuming a Gaussian GC luminosity function, we compute that the
completeness of the GC search is above 90 percent for most dwarf galaxies. We
construct the 90 percent credible intervals/upper limits on the GC specific
frequency of the MW dwarf galaxies: for
Fornax, for the dwarfs with , for the dwarfs with , and for
the dwarfs with . Based on , we derive the
probability of galaxies hosting GCs given their luminosity, finding that the
probability of galaxies fainter than to host GCs is lower than
0.1
Identifying RR Lyrae in the ZTF DR3 dataset
We present a RR Lyrae (RRL) catalogue based on the combination of the third
data release of the Zwicky Transient Facility (ZTF DR3) and \textit{Gaia} EDR3.
We use a multi-step classification pipeline relying on the Fourier
decomposition fitting to the multi-band ZTF light curves and random forest
classification. The resulting catalogue contains 71,755 RRLs with period and
light curve parameter measurements and has completeness of 0.92 and purity of
0.92 with respect to the SOS \textit{Gaia} DR2 RRLs. The catalogue covers the
Northern sky with declination , its completeness is for heliocentric distance ~kpc, and the most distant RRL at
132~kpc. Compared with several other RRL catalogues covering the Northern sky,
our catalogue has more RRLs around the Galactic halo and is more complete at
low Galactic latitude areas. Analysing the spatial distribution of RRL in the
catalogue reveals the previously known major over-densities of the Galactic
halo, such as the Virgo over-density and the Hercules-Aquila Cloud, with some
evidence of an association between the two. We also analyse the Oosterhoff
fraction differences throughout the halo, comparing it with the density
distribution, finding increasing Oosterhoff I fraction at the elliptical radii
between 16 and 32 kpc and some evidence of different Oosterhoff fractions
across various halo substructures
Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency
Plasmonic metamaterials and metasurfaces offer new opportunities in
developing high performance terahertz emitters and detectors beyond the
limitations of conventional nonlinear materials. However, simple meta-atoms for
second-order nonlinear applications encounter fundamental trade-offs in the
necessary symmetry breaking and local-field enhancement due to radiation
damping that is inherent to the operating resonant mode and cannot be
controlled separately. Here we present a novel concept that eliminates this
restriction obstructing the improvement of terahertz generation efficiency in
nonlinear metasurfaces based on metallic nanoresonators. This is achieved by
combining a resonant dark-state metasurface, which locally drives nonlinear
nanoresonators in the near field, with a specific spatial symmetry that enables
destructive interference of the radiating linear moments of the nanoresonators,
and perfect absorption via simultaneous electric and magnetic critical coupling
of the pump radiation to the dark mode. Our proposal allows eliminating linear
radiation damping, while maintaining constructive interference and effective
radiation of the nonlinear components. We numerically demonstrate a giant
second-order nonlinear susceptibility around Hundred-Billionth m/V, a one order
improvement compared with the previously reported split-ring-resonator
metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz
energy extraction should be expected with our configuration under the same
conditions. Our study offers a paradigm of high efficiency tunable nonlinear
metadevices and paves the way to revolutionary terahertz technologies and
optoelectronic nanocircuitry.Comment: 6 pages, 4 figure
Engineering analysis of biological variables: An example of blood pressure over 1 day
Almost all variables in biology are nonstationarily stochastic. For these variables, the conventional tools leave us a feeling that some valuable information is thrown away and that a complex phenomenon is presented imprecisely. Here, we apply recent advances initially made in the study of ocean waves to study the blood pressure waves in the lung. We note first that, in a long wave train, the handling of the local mean is of predominant importance. It is shown that a signal can be described by a sum of a series of intrinsic mode functions, each of which has zero local mean at all times. The process of deriving this series is called the “empirical mode decomposition method.” Conventionally, Fourier analysis represents the data by sine and cosine functions, but no instantaneous frequency can be defined. In the new way, the data are represented by intrinsic mode functions, to which Hilbert transform can be used. Titchmarsh [Titchmarsh, E. C. (1948) Introduction to the Theory of Fourier Integrals (Oxford Univ. Press, Oxford)] has shown that a signal and i times its Hilbert transform together define a complex variable. From that complex variable, the instantaneous frequency, instantaneous amplitude, Hilbert spectrum, and marginal Hilbert spectrum have been defined. In addition, the Gumbel extreme-value statistics are applied. We present all of these features of the blood pressure records here for the reader to see how they look. In the future, we have to learn how these features change with disease or interventions
- …