708 research outputs found
Experimental and numerical studies of terahertz surface waves on a thin metamaterial film
We present experimental and numerical studies of localized terahertz surface
waves on a subwavelength-thick metamaterial film consisting of in-plane
split-ring resonators. A simple and intuitive model is derived that describes
the propagation of surface waves as guided modes in a waveguide filled with a
Lorentz-like medium. The effective medium model allows to deduce the dispersion
relation of the surface waves in excellent agreement with the numerical data
obtained from 3-D full-wave calculations. Both the accuracy of the analytical
model and the numerical calculations are confirmed by spectroscopic terahertz
time domain measurements.Comment: 3 pages, 3 figure
Gradient Index Metamaterial Based on Slot Elements
We present a gradient-index (GRIN) metamaterial based on an array of annular
slots. The structure allows a large variation of the effective refractive index
under normal-to-plane incidence and thus enables the construction of GRIN
devices consisting of only a small number of functional layers. Using full-wave
simulations, we demonstrate the annular slot concept by means of a 3-unit-cell
thin GRIN lens for the terahertz (THz) range. In the presented realizations, we
achieved an index contrast of Delta n = 1.5 resulting in a highly refractive
lens suitable for focusing THz radiation to a spot size smaller than the
wavelength.Comment: 4 pages, 5 figure
In-Plane Focusing of Terahertz Surface Waves on a Gradient Index Metamaterial Film
We designed and implemented a gradient index metasurface for the in-plane
focusing of confined terahertz surface waves. We measured the spatial
propagation of the surface waves by two-dimensional mapping of the complex
electric field using a terahertz near-field spectroscope. The surface waves
were focused to a diameter of 500 \micro m after a focal length of approx. 2
mm. In the focus, we measured a field amplitude enhancement of a factor of 3.Comment: 6 pages, 4 figure
Metamaterial near-field sensor for deep-subwavelength thickness measurements and sensitive refractometry in the terahertz frequency range
We present a metamaterial-based terahertz (THz) sensor for thickness
measurements of subwavelength-thin materials and refractometry of liquids and
liquid mixtures. The sensor operates in reflection geometry and exploits the
frequency shift of a sharp Fano resonance minimum in the presence of dielectric
materials. We obtained a minimum thickness resolution of 12.5 nm (1/16000 times
the wavelength of the THz radiation) and a refractive index sensitivity of 0.43
THz per refractive index unit. We support the experimental results by an
analytical model that describes the dependence of the resonance frequency on
the sample material thickness and the refractive index.Comment: 10 pages, 5 figure
Collectively canalizing Boolean functions
This paper studies the mathematical properties of collectively canalizing
Boolean functions, a class of functions that has arisen from applications in
systems biology. Boolean networks are an increasingly popular modeling
framework for regulatory networks, and the class of functions studied here
captures a key feature of biological network dynamics, namely that a subset of
one or more variables, under certain conditions, can dominate the value of a
Boolean function, to the exclusion of all others. These functions have rich
mathematical properties to be explored. The paper shows how the number and type
of such sets influence a function's behavior and define a new measure for the
canalizing strength of any Boolean function. We further connect the concept of
collective canalization with the well-studied concept of the average
sensitivity of a Boolean function. The relationship between Boolean functions
and the dynamics of the networks they form is important in a wide range of
applications beyond biology, such as computer science, and has been studied
with statistical and simulation-based methods. But the rich relationship
between structure and dynamics remains largely unexplored, and this paper is
intended as a contribution to its mathematical foundation.Comment: 15 pages, 2 figure
Immobilized stemâloop structured probes as conformational switches for enzymatic detection of microbial 16S rRNA
We have designed and evaluated novel DNA stemâloop structured probes for enzymatic detection of nucleic acid targets. These probes constitute a novel class of conformational switches for enzymatic activity, which in the absence of a target sterically shield an affinity label and upon hybridization of the target to the recognition sequence that forms the loop of the probe restore accessibility of the label for the binding of a reporter enzyme. Analysis of probe characteristics revealed stem stability as the most important parameter governing detection functionality, while other factors such as the length of linker molecules attaching the label to the stemâloop structure and the nature of the solid support proved to be less critical. Apparently, the bulky nature of the reporter enzyme facilitates shielding of the label in the absence of the target, thereby conferring considerable structural tolerance to the conformational switch system. The stemâloop structured probes allow sensitive detection of unlabeled nucleic acid targets. Employing a microtiter assay format, 4 ng of bacterial 16S ribosomal RNA corresponding to 8 fmol could be detected, which can be compared favorably with current immobilized molecular beacon concepts based on fluorescence detection
Inference in receiver operating characteristic surface analysis via a trinormal modelâbased testing approach
Receiver operating characteristic (ROC) analysis is the methodological framework of choice for the assessment of diagnostic markers and classification procedures in general, in both twoâclass and multipleâclass classification problems. We focus on the threeâclass problem for which inference usually involves formal hypothesis testing using a proxy metric such as the volume under the ROC surface (VUS). In this article, we develop an existing approach from the twoâclass ROC framework. We define a hypothesisâtesting procedure that directly compares two ROC surfaces under the assumption of the trinormal model. In the case of the assessment of a single marker, the corresponding ROC surface is compared with the chance plane, that is, to an uninformative marker. A simulation study investigating the proposed tests with existing ones on the basis of the VUS metric follows. Finally, the proposed methodology is applied to a dataset of a panel of pancreatic cancer diagnostic markers. The described testing procedures along with related graphical tools are supported in the corresponding Râpackage trinROC, which we have developed for this purpose
Inference in receiver operating characteristic surface analysis via a trinormal modelâbased testing approach
Receiver operating characteristic (ROC) analysis is the methodological framework of choice for the assessment of diagnostic markers and classification procedures in general, in both twoâclass and multipleâclass classification problems. We focus on the threeâclass problem for which inference usually involves formal hypothesis testing using a proxy metric such as the volume under the ROC surface (VUS). In this article, we develop an existing approach from the twoâclass ROC framework. We define a hypothesisâtesting procedure that directly compares two ROC surfaces under the assumption of the trinormal model. In the case of the assessment of a single marker, the corresponding ROC surface is compared with the chance plane, that is, to an uninformative marker. A simulation study investigating the proposed tests with existing ones on the basis of the VUS metric follows. Finally, the proposed methodology is applied to a dataset of a panel of pancreatic cancer diagnostic markers. The described testing procedures along with related graphical tools are supported in the corresponding Râpackage trinROC, which we have developed for this purpose
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