3,563 research outputs found
Highly efficient singular surface plasmon generation by achiral apertures
We report a highly efficient generation of singular surface plasmon (SP)
field by an achiral plasmonic structure consisting of -shaped
apertures. Our quantitative analysis based on leakage radiation microscopy
(LRM) demonstrates that the induced spin-orbit coupling can be tuned by
adjusting the apex angle of the -shaped aperture. Specifically, the
array of -shaped apertures with the apex angle is shown to
give rise to the directional coupling efficiency. The ring of -shaped
apertures with the apex angle realized to generate the maximum
extinction ratio (ER=11) for the SP singularities between two different
polarization states. This result provides a more efficient way for developing
SP focusing and SP vortex in the field of nanophotonics such as optical
tweezers
Directional and singular surface plasmon generation in chiral and achiral nanostructures demonstrated by Leakage Radiation Microscopy
In this paper, we describe the implementation of leakage radiation microscopy
(LRM) to probe the chirality of plasmonic nanostructures. We demonstrate
experimentally spin-driven directional coupling as well as vortex generation of
surface plasmon polaritons (SPPs) by nanostructures built with T-shaped and
- shaped apertures. Using this far-field method, quantitative
inspections, including directivity and extinction ratio measurements, are
achieved via polarization analysis in both image and Fourier planes. To support
our experimental findings, we develop an analytical model based on a
multidipolar representation of - and T-shaped aperture plasmonic
coupler allowing a theoretical explanation of both directionality and singular
SPP formation. Furthermore, the roles of symmetry breaking and phases are
emphasized in this work. This quantitative characterization of spin-orbit
interactions paves the way for developing new directional couplers for
subwavelength routing
Image Extraction by Wide Angle Foveated Lens for Overt-Attention
This paper defines Wide Angle Foveated (WAF)
imaging. A proposed model combines Cartesian coordinate
system, a log-polar coordinate system, and a unique camera
model composed of planar projection and spherical projection
for all-purpose use of a single imaging device. The central field-of-view (FOV) and intermediate FOV are given translation-invariance
and, rotation and scale-invariance for pattern
recognition, respectively. Further, the peripheral FOV is more
useful for camera’s view direction control, because its image
height is linear to an incident angle to the camera model’s optical
center point. Thus, this imaging model improves its usability
especially when a camera is dynamically moved, that is, overt-attention.
Moreover, simulation results of image extraction show
advantages of the proposed model, in view of its magnification
factor of the central FOV, accuracy of scale-invariance and
flexibility to describe other WAF vision sensors
The Global Star-Formation Law by Supernova Feedback
We address a simple model where the Kennicutt-Schmidt (KS) relation between
the macroscopic densities of star-formation rate (SFR, ) and
gas () in galactic discs emerges from self-regulation of the SFR via
supernova feedback. It arises from the physics of supernova bubbles,
insensitive to the microscopic SFR recipe and not explicitly dependent on
gravity. The key is that the filling factor of SFR-suppressed supernova bubbles
self-regulates to a constant, . Expressing the bubble fading radius
and time in terms of , the filling factor is with
, where is the supernova rate density. A constant thus
refers to , with a density-independent SFR
efficiency per free-fall time . The self-regulation to
and the convergence to a KS relation independent of the local SFR recipe are
demonstrated in cosmological and isolated-galaxy simulations using different
codes and recipes. In parallel, the spherical analysis of bubble evolution is
generalized to clustered supernovae, analytically and via simulations, yielding
. An analysis of photo-ionized bubbles about
pre-supernova stars yields a range of KS slopes but the KS relation is
dominated by the supernova bubbles. Superbubble blowouts may lead to an
alternative self-regulation by outflows and recycling. While the model is
over-simplified, its simplicity and validity in the simulations may argue that
it captures the origin of the KS relation
Molecular gas toward supernova remnant Cassiopeia A
We mapped 12CO J=1-0, 12CO J=2-1, 13CO J=1-0, and 13CO J=2-1 lines toward
supernova remnant (SNR) Cassiopeia A with the IRAM 30m telescope. The molecular
clouds (MCs) along the line of sight of Cas A do not show optically thin,
shock-broadened 12CO lines ( km s toward Cas A), or
high-temperature features from shock heating ( K toward Cas A).
Therefore, we suggest that there is no physical evidence to support that the
SNR is impacting the molecular gas. All the detected MCs are likely in front of
Cas A, as implied by the HCO+ absorption line detected in the same velocity
ranges. These MCs contribute H column densities of
cm, cm, and cm in the
west, south, and center of the SNR, respectively. The 20 K warm gas at
km s is distributed along a large-scale molecular
ridge in the south of Cas A. Part of the gas is projected onto Cas A, providing
a foreground H mass of Msun, consistent with the
mass of cold dust (15--20 K; 2--4 Msun) found in front of the SNR. We suggest
that the 20 K warm gas is heated by background cosmic rays with an ionization
rate of s. The cosmic rays or
X-ray emission from Cas A are excluded as the heating sources of the clouds.Comment: 18 pages, 8 figures, 2 tables; Accepted to Ap
uShuffle: A useful tool for shuffling biological sequences while preserving the k-let counts
Abstract We present a bioinformatics tool (named uShuffle) for generating uniform random permutations of biological sequences (such as DNAs, RNAs, and proteins) that preserve the exact k-let counts. The uShuffle program is based on the Euler algorithm and uses Wilson’s algorithm in the crucial step of arborescence generation. Our implementation of these algorithms is carefully engineered; it is shown by our experiments to be both extremely efficient and very scalable. By allowing arbitrary alphabet size and let size, the uShuffle program is also far superior to the existing tools in terms of versatility. For the convenience of the users, we provide the uShuffle program in a variety of programming languages: C, Java, C#, Perl, and Python. The websit
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