Enhanced localization of light in slow wave slot photonic crystal waveguides

A flexible design of slot photonic crystal waveguide with a wide comb is investigated. Introduction of a carefully designed comb within the photonic crystal waveguide allows an accurate dispersion engineering in order to achieve slow light and increase the optical confinement within the comb. The strong light confinement results in an extremely small nonlinear effective area around 0.015 μm². We report experimental realization of a comb photonic crystal waveguide with measured group indices higher than 100 in a Mach-Zehnder interferometer configuration and extract losses limited to 3.7?dB for a 100?μm device at n_g=37.     

Substituent effects on the thermochemistry of thiophenes. a theoretical (G3(MP2)//B3LYP and G3) study

Very good linear correlations between experimental and calculated enthalpies of formation in the gas phase (G3(MP2)//B3LYP and G3) for 48 thiophene derivatives have been obtained. These correlations permit a correction of the calculated enthalpies of formation in order to estimate more reliable "experimental" values for the enthalpies of formation of substituted thiophenes, check the reliability of experimental measurements, and also predict the enthalpies of formation of new thiophenes that are not available in the literature. Moreover, the difference between the enthalpies of formation of isomeric thiophenes with the same substituent in positions 2 and 3 of the ring has been analyzed. Likewise, a comparison of the substituent effect in the thiophene and benzene rings has been established.     

Sensitivity kernel for surface scattering in a waveguide

Using the Born approximation, a linearized sensitivity kernel is derived to describe the relationship between a local change at the free surface and its effect on the acoustic propagation in the water column. The structure of the surface scattering kernel is investigated numerically and experimentally for the case of a waveguide at the ultrasonic scale. To better demonstrate the sensitivity of the multipath propagation to the introduction of a localized perturbation at the air-water interface, the kernel is formulated both in terms of point-to-point and beam-to-beam representations. Agreement between theory and experiment suggests applications to sensitivity analysis of the wavefield for sea surface perturbations.     

Enhancing the emergence rate of coherent wavefronts from ocean ambient noise correlations using spatio-temporal filters

Extracting coherent wavefronts between passive receivers using cross-correlations of ambient noise (CAN) provides a means for monitoring the seismoacoustic environment without using active sources. However, using cross-correlations between single receivers can require a long recording time in order to extract stable coherent arrivals from CAN. This becomes an issue if the propagation medium fluctuates significantly during the recording period. To address this issue, this article presents a general spatio-temporal filtering procedure to enhance the emergence rate for coherent wavefronts extracted from time-averaged ambient noise correlations between two spatially separated arrays. The robustness of this array-based CAN technique is investigated using ambient shipping noise recorded over 24?h in the frequency band [250-850 Hz] on two vertical line arrays deployed 143?m apart in shallow water (depth 20?m). Experimental results confirm that the array-based CAN technique can significantly reduce the recording duration (e.g., from 22?h to 30?min) required for extracting coherent wavefronts of sufficient amplitude (e.g., 20?dB over residual temporal fluctations) when compared to conventional CAN implementations between single pairs of hydrophones. These improvements of the CAN technique could benefit the development of noise-based ocean monitoring applications such as passive acoustic tomography.     

Control of island morphology by dynamic coalescence of soft-landed clusters

The deposition of preformed clusters on surfaces has been established as a new way for growing nano-suctures on surfaces. It has been shown that supported island morphology relies on the dynamics of clusters, during the growth, giving rise to shapes from compact to ramified types. This paper identifies and discusses, in the case of antimony cluster deposits, several processes responsible for the non-equilibrium island shapes: limited kinetic cluster aggregation, size dependent coalescence, “wetting-like behavior” of antimony clusters on antimony islands. Using successive predetermined cluster sizes during the deposition process to synthesize polymorphic structure involves the interplay of those mechanisms. Received 1st December 2000     

Digital Image Mechanical Identification (DIMI)

A continuous pathway from digital images acquired during a mechanical test to quantitative identification of a constitutive law is presented herein based on displacement field analysis. From images, displacement fields are directly estimated within a finite element framework. From the latter, the application of the equilibrium gap method provides the means for rigidity field evaluation. In the present case, a reconditioned formulation is proposed for a better stability. Last, postulating a specific form of a damage law, a linear system is formed that gives a direct access to the (non-linear) damage growth law in one step. The two last procedures are presented, validated on an artificial case, and applied to the case of a biaxial tension of a composite sample driven up to failure. A quantitative estimate of the quality of the determination is proposed, and in the last application, it is shown that no more than 7% of the displacement field fluctuations are not accounted for by the determined damage law.     

Simulation of shallow flows over variable topographies using unstructured grids

Simulation of shallow flows over variable topographies is a challenging case for most available shock‐capturing schemes. This problem arises because the source terms and flux gradients are not balanced in the numerical computations. Treatments for this problem generally work well on structured grids, but they are usually too expensive, and most of them are not directly applicable to unstructured grids. In this paper we propose two efficient methods to treat the source terms without upwinding and to satisfy the compatibility condition on unstructured grids. In the first method, the calculation of the bed slope source term is performed by employing a compatible approximation of water depth at the cell interfaces. In the second one, different components of the bed slope term are considered separately and a compatible discretization of the components is proposed. The present treatments are applicable for most schemes including the Roe's method without changing the performance of the original scheme for smooth topographies. Copyright © 2006 John Wiley & Sons, Ltd.     

Flow regimes and frequency selection of a cylinder oscillating in an upstream cylinder wake

This paper describes flow around a pair of cylinders in tandem arrangement with a downstream cylinder being fixed or forced to oscillate transversely. A sinusoidal parietal velocity is applied to simulate cylinder oscillation. Time-dependent Navier-Stokes equations are solved using finite element method. It is shown that there exist two distinct flow regimes: ‘vortex suppression regime’ and ‘vortex formation regime’. Averaged vortex lengths between the two cylinders, pressure variations at back and front stagnant points as well as circumferential pressure profiles of the downstream cylinder are found completely different in the two regimes and, thus, can be used to identify the flow regimes. It is shown that frequency selection in the wake of the oscillating cylinder is a result of non-linear interaction among vortex wakes upstream and downstream of the second cylinder and its forced oscillation. Increasing cylinder spacing results in a stronger oscillatory incident flow upstream of the second cylinder and, thus, a smaller synchronization zone.     

Multifractality in elastic percolation

We present numerical results on the distribution of forces in the central-force percolation model at threshold in two dimensions. We conjecture a relation between the multifractal spectrum of scalar and vector percolation that we test for central-foce percolation. This relation is in excellent agreement with our numerical data.