1,485 research outputs found
Active biopolymer networks generate scale-free but euclidean clusters
We report analytical and numerical modelling of active elastic networks,
motivated by experiments on crosslinked actin networks contracted by myosin
motors. Within a broad range of parameters, the motor-driven collapse of active
elastic networks leads to a critical state. We show that this state is
qualitatively different from that of the random percolation model.
Intriguingly, it possesses both euclidean and scale-free structure with Fisher
exponent smaller than . Remarkably, an indistinguishable Fisher exponent and
the same euclidean structure is obtained at the critical point of the random
percolation model after absorbing all enclaves into their surrounding clusters.
We propose that in the experiment the enclaves are absorbed due to steric
interactions of network elements. We model the network collapse, taking into
account the steric interactions. The model shows how the system robustly drives
itself towards the critical point of the random percolation model with absorbed
enclaves, in agreement with the experiment.Comment: 6 pages, 7 figure
Pre-Symmetry Sets of 3D shapes
The investigation of 3D euclidean symmetry sets (SS) and medial axis is an
important area, due in particular to their various important applications.
The pre-symmetry set of a surface M in 3-space (resp. smooth closed curve in
2D) is the set of pairs of points which contribute to the symmetry set, that
is, the closure of the set of pairs of distinct points p and q in M, for which
there exists a sphere (resp. a circle) tangent to M at p and at q. The aim of
this paper is to address problems related to the smoothness and the
singularities of the pre-symmetry sets of 3D shapes.
We show that the pre-symmetry set of a smooth surface in 3-space has locally
the structure of the graph of a function from R^2 to R^2, in many cases of
interest.Comment: ACM-class: I.2; I.5; I.4; J.2. Latex, 3 grouped figures. The final
version will appear in the proceedings of the First International Workshop on
Deep Structure, Singularities and Computer Vision, Maastricht June 200
Trajectory Mapping ("TM''): A New Non-Metric Scaling Technique
Trajectory Mapping "TM'' is a new scaling technique designed to recover the parameterizations, axes, and paths used to traverse a feature space. Unlike Multidimensional Scaling (MDS), there is no assumption that the space is homogenous or metric. Although some metric ordering information is obtained with TM, the main output is the feature parameterizations that partition the given domain of object samples into different categories. Following an introductory example, the technique is further illustrated using first a set of colors and then a collection of textures taken from Brodatz (1966)
Optimum flux-detection in the absence of a priori knowledge about the signal
Detection systems based on photon counting have to discriminate between two types of fluctuations in the photon count: those resulting from statistical fluctuations (=noise) and those caused by changes in the radiance set by the source (=signal). In contrast with earlier studies on ways of discriminating noise from signal changes, no specific assumptions are made about the source. An optimal discrimination-method has been developed for a detector that has no prior information about the mean of the Poisson distribution that describes its input signal. Because the detector has no prior information at its disposal it has to assume an a priori probability for the mean in a unique and objective way and it has to estimate the actual mean using Bayes rule of inference. This new discrimination-method is discussed in the context of signal processing in the visual system, but is generally applicable in all systems where photon-noise is important
Local density of states, spectrum, and far-field interference of surface plasmon polaritons probed by cathodoluminescence
The surface plasmon polariton (SPP) field intensity in the vicinity of gratings patterned in an otherwise planar gold surface is spatially resolved using cathodoluminescence (CL). A detailed theoretical analysis is presented that successfully explains the measured CL signal based upon interference of transition radiation directly generated by electron impact and SPPs launched by the electron and outcoupled by the grating. The measured spectral dependence of the SPP yield per incoming electron is in excellent agreement with rigorous electromagnetic calculations. The CL emission is shown to be similar to that of a dipole oriented perpendicular to the surface and situated at the point of electron impact, which allows us to establish a solid connection between the CL signal and the photonic local density of states associated to the SPPs
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In our study, for a small number of antonyms, we investigate whether they are cross-modally or ideaesthetically related to the space of colors. We analyze the affinities of seven antonyms (cold-hot, dull-radiant, dead-vivid, soft-hard, transparent-chalky, dry-wet, and acid-treacly) and their intermediate connotations (cool-warm, matt-shiny, numb-lively, mellow-firm, semi-transparent-opaque, semi-dry-moist, and sour-sweet) as a function of color. We find that some antonyms relate to chromatic dimensions, others to achromatic ones. The cold-hot antonym proves to be the most salient dimension. The dry-wet dimension coincides with the cold-hot dimension, with dry corresponding to hot and wet to cold. The acid-treacly dimension proves to be transversal to the cold-hot dimension; hence, the pairs mutually span the chromatic domain. The cold-hot and acid-treacly antonyms perhaps recall Hering's opponent color system. The dull-radiant, transparent-chalky, and dead-vivid pairs depend little upon chromaticity. Of all seven antonyms, only the soft-hard one turns out to be independent of the chromatic structure
Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals
We present ultrafast optical switching experiments on 3D photonic band gap
crystals. Switching the Si inverse opal is achieved by optically exciting free
carriers by a two-photon process. We probe reflectivity in the frequency range
of second order Bragg diffraction where the photonic band gap is predicted. We
find good experimental switching conditions for free-carrier plasma frequencies
between 0.3 and 0.7 times the optical frequency: we thus observe a large
frequency shift of up to D omega/omega= 1.5% of all spectral features including
the peak that corresponds to the photonic band gap. We deduce a corresponding
large refractive index change of Dn'_Si/n'_Si= 2.0% and an induced absorption
length that is longer than the sample thickness. We observe a fast decay time
of 21 ps, which implies that switching could potentially be repeated at GHz
rates. Such a high switching rate is relevant to future switching and
modulation applications
From weak to strong coupling of localized surface plasmons to guided modes in a luminescent slab
We investigate a periodic array of aluminum nanoantennas embedded in a
light-emitting slab waveguide. By varying the waveguide thickness we
demonstrate the transition from weak to strong coupling between localized
surface plasmons in the nanoantennas and refractive index guided modes in the
waveguide. We experimentally observe a non-trivial relationship between
extinction and emission dispersion diagrams across the weak to strong coupling
transition. These results have implications for a broad class of photonic
structures where sources are embedded within coupled resonators. For
nanoantenna arrays, strong vs. weak coupling leads to drastic modifications of
radiation patterns without modifying the nanoantennas themselves, thereby
representing an unprecedented design strategy for nanoscale light sources
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