5,561 research outputs found
Tunable plasmonic resonances in highly porous nano-bamboo Si-Au superlattice-type thin films
We report on fabrication of spatially-coherent columnar plasmonic
nanostructure superlattice-type thin films with high porosity and strong
optical anisotropy using glancing angle deposition. Subsequent and repeated
depositions of silicon and gold lead to nanometer-dimension subcolumns with
controlled lengths. The superlattice-type columns resemble bamboo structures
where smaller column sections of gold form junctions sandwiched between larger
silicon column sections ("nano-bamboo"). We perform generalized spectroscopic
ellipsometry measurements and finite element method computations to elucidate
the strongly anisotropic optical properties of the highly-porous nano-bamboo
structures. The occurrence of a strongly localized plasmonic mode with
displacement pattern reminiscent of a dark quadrupole mode is observed in the
vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like
mode frequency within the near-infrared spectral range by varying the geometry
of the nano-bamboo structure. In addition, coupled-plasmon-like and inter-band
transition-like modes occur in the visible and ultra-violet spectral regions,
respectively. We elucidate an example for the potential use of the nano-bamboo
structures as a highly porous plasmonic sensor with optical read out
sensitivity to few parts-per-million solvent levels in water
The Spine of the Cosmic Web
We present the SpineWeb framework for the topological analysis of the Cosmic
Web and the identification of its walls, filaments and cluster nodes. Based on
the watershed segmentation of the cosmic density field, the SpineWeb method
invokes the local adjacency properties of the boundaries between the watershed
basins to trace the critical points in the density field and the separatrices
defined by them. The separatrices are classified into walls and the spine, the
network of filaments and nodes in the matter distribution. Testing the method
with a heuristic Voronoi model yields outstanding results. Following the
discussion of the test results, we apply the SpineWeb method to a set of
cosmological N-body simulations. The latter illustrates the potential for
studying the structure and dynamics of the Cosmic Web.Comment: Accepted for publication HIGH-RES version:
http://skysrv.pha.jhu.edu/~miguel/SpineWeb
Finding faint HI structure in and around galaxies: scraping the barrel
Soon to be operational HI survey instruments such as APERTIF and ASKAP will
produce large datasets. These surveys will provide information about the HI in
and around hundreds of galaxies with a typical signal-to-noise ratio of
10 in the inner regions and 1 in the outer regions. In addition, such
surveys will make it possible to probe faint HI structures, typically located
in the vicinity of galaxies, such as extra-planar-gas, tails and filaments.
These structures are crucial for understanding galaxy evolution, particularly
when they are studied in relation to the local environment. Our aim is to find
optimized kernels for the discovery of faint and morphologically complex HI
structures. Therefore, using HI data from a variety of galaxies, we explore
state-of-the-art filtering algorithms. We show that the intensity-driven
gradient filter, due to its adaptive characteristics, is the optimal choice. In
fact, this filter requires only minimal tuning of the input parameters to
enhance the signal-to-noise ratio of faint components. In addition, it does not
degrade the resolution of the high signal-to-noise component of a source. The
filtering process must be fast and be embedded in an interactive visualization
tool in order to support fast inspection of a large number of sources. To
achieve such interactive exploration, we implemented a multi-core CPU (OpenMP)
and a GPU (OpenGL) version of this filter in a 3D visualization environment
().Comment: 17 pages, 9 figures, 4 tables. Astronomy and Computing, accepte
The Hierarchical Structure and Dynamics of Voids
Contrary to the common view voids have very complex internal structure and
dynamics. Here we show how the hierarchy of structures in the density field
inside voids is reflected by a similar hierarchy of structures in the velocity
field. Voids defined by dense filaments and clusters can de described as simple
expanding domains with coherent flows everywhere except at their boundaries. At
scales smaller that the void radius the velocity field breaks into expanding
sub-domains corresponding to sub- voids. These sub-domains break into even
smaller sub-sub domains at smaller scales resulting in a nesting hierarchy of
locally expanding domains. The ratio between the magnitude of the velocity
field responsible for the expansion of the void and the velocity field defining
the sub voids is approximately one order of magnitude. The small-scale
components of the velocity field play a minor role in the shaping of the voids
but they define the local dynamics directly affecting the processes of galaxy
formation and evolution. The super-Hubble expansion inside voids makes them
cosmic magnifiers by stretching their internal primordial density fluctuations
allowing us to probe the small scales in the primordial density field. Voids
also act like time machines by "freezing" the development of the medium-scale
density fluctuations responsible for the formation of the tenuous web of
structures seen connecting proto galaxies in computer simulations. As a result
of this freezing haloes in voids can remain "connected" to this tenuous web
until the present time. This may have an important effect in the formation and
evolution of galaxies in voids by providing an efficient gas accretion
mechanism via coherent low-velocity streams that can keep a steady inflow of
matter for extended periods of time.Comment: High-res version are related media here:
http://skysrv.pha.jhu.edu/~miguel/Papers/Hierarchy_voids/index.htm
Tunable plasmonic resonances in Si-Au slanted columnar heterostructure thin films
We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous Si-Au slanted columnar heterostructures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of Si-Au slanted columnar heterostructures. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of Si-Au slanted columnar heterostructures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water
Multi-dimensional models of circumstellar shells around evolved massive stars
Massive stars shape their surrounding medium through the force of their
stellar winds, which collide with the circumstellar medium. Because the
characteristics of these stellar winds vary over the course of the evolution of
the star, the circumstellar matter becomes a reflection of the stellar
evolution and can be used to determine the characteristics of the progenitor
star. In particular, whenever a fast wind phase follows a slow wind phase, the
fast wind sweeps up its predecessor in a shell, which is observed as a
circumstellar nebula. We make 2-D and 3-D numerical simulations of fast stellar
winds sweeping up their slow predecessors to investigate whether numerical
models of these shells have to be 3-D, or whether 2-D models are sufficient to
reproduce the shells correctly. We focus on those situations where a fast
Wolf-Rayet (WR) star wind sweeps up the slower wind emitted by its predecessor,
being either a red supergiant or a luminous blue variable. As the fast WR wind
expands, it creates a dense shell of swept up material that expands outward,
driven by the high pressure of the shocked WR wind. These shells are subject to
a fair variety of hydrodynamic-radiative instabilities. If the WR wind is
expanding into the wind of a luminous blue variable phase, the instabilities
will tend to form a fairly small-scale, regular filamentary lattice with thin
filaments connecting knotty features. If the WR wind is sweeping up a red
supergiant wind, the instabilities will form larger interconnected structures
with less regularity. Our results show that 3-D models, when translated to
observed morphologies, give realistic results that can be compared directly to
observations. The 3-D structure of the nebula will help to distinguish
different progenitor scenarios.Comment: Accepted for publication in A&A. All figures in low resolution. v2:
language corrections and addition of DOI numbe
Surface Reconstruction from Noisy and Sparse Data
We introduce a set of algorithms for registering, filtering and measuring the similarity of unorganized 3d point clouds, usually obtained from multiple views.
We contribute a method for computing the similarity between point clouds that represent closed surfaces, specifically segmented tumors from CT scans. We obtain watertight surfaces and utilize volumetric overlap to determine similarity in a volumetric way. This similarity measure is used to quantify treatment variability based on target volume segmentation both prior to and following radiotherapy planning stages.
We also contribute an algorithm for the drift-free registration of thin, non- rigid scans, where drift is the build-up of error caused by sequential pairwise registration, which is the alignment of each scan to its neighbor. We construct an average scan using mutual nearest neighbors, each scan is registered to this average scan, after which we update the average scan and continue this process until convergence. The use case herein is for merging scans of plants from multiple views and registering vascular scans together.
Our final contribution is a method for filtering noisy point clouds, specif- ically those constructed from merged depth maps as obtained from a range scanner or multiple view stereo (MVS), applying techniques that have been utilized in finding outliers in clustered data, but not in MVS. We utilize ker- nel density estimation to obtain a probability density function over the space of observed points, utilizing variable bandwidths based on the nature of the neighboring points, Mahalanobis and reachability distances that is more dis- criminative than a classical Mahalanobis distance-based metric
Tunable plasmonic resonances in highly porous nano-bamboo Si-Au superlattice-type thin films
We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. The superlattice-type columns resemble bamboo structures where smaller column sections of gold form junctions sandwiched between larger silicon column sections (“nano-bamboo”). We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous nano-bamboo structures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of the nano-bamboo structure. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of the nano-bamboo structures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water
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