Achieving centimetre-scale supercollimation in a large-area two-dimensional photonic crystal

Diffraction, a fundamental process in wave physics, leads to spreading of the optical beams as they propagate. However, new photonic crystal (PhC) meta-materials can be nano-engineered to generate extreme anisotropy, resulting in apparent propagation of light without diffraction. This surprising phenomenon, called supercollimation, effectively freezes the spatial width of a light beam inside a PhC, observed over a few isotropic diffraction-lengths. However, using such experiments to predict the behaviour for longer propagation lengths is difficult, as a tiny error in a measured width can extrapolate to order unity uncertainty in the width at distances over hundreds of diffraction-lengths. Here, supercollimation is demonstrated in a macroscopic PhC system over centimetre-scale distances, retaining spatial width confinement without the need for waveguides or nonlinearities. Through quantitative studies of the beam evolution in a two-dimensional PhC, we find that supercollimation possesses unexpected but inherent robustness with respect to short-scale disorder such as fabrication roughness, enabling supercollimation over 600 isotropic diffraction-lengths. The effects of disorder are identified through experiments and understood through rigorous simulations. In addition, a supercollimation steering capability is proposed.     

Crystallinity development in cellular poly(lactic acid) in the presence of supercritical carbon dioxide

This article investigates the crystallinity development in cellular poly(lactic acid) (PLA) and the effect of the achieved crystalline content on its properties and microstructure. Carbon dioxide (CO₂) in its supercritical state was used as the expansion agent for three different grades of PLA that differed in terms of L‐lactic acid content. Cellular PLA was produced on a twin‐screw extrusion line using capillary dies of various diameters. The obtained crystalline contents were measured by differential scanning calorimetry and X‐ray diffraction techniques. The morphology of the cellular structures was examined using scanning electron microscopy. The crystallinity developed on expansion depended on L‐lactic acid content, on supercritical CO₂ concentration, polymer flow rate, and die diameter. Cellular PLA, with densities as low as 30 kg/m³, was obtained under the most favorable conditions. It was shown that the crystallinity development in PLA enhances its cellular structure formation and enables the fabrication of quality cellular materials at lower CO₂ concentration. The presence of PLA crystallites within expanded cell walls leads to a peculiar 2D‐cavitation phenomena observed only in the cell walls of semicrystalline foams. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nonlinear multiscale wavelet diffusion for speckle suppression and edge enhancement in ultrasound images

This paper introduces a novel nonlinear multiscale wavelet diffusion method for ultrasound speckle suppression and edge enhancement. This method is designed to utilize the favorable denoising properties of two frequently used techniques: the sparsity and multiresolution properties of the wavelet, and the iterative edge enhancement feature of nonlinear diffusion. With fully exploited knowledge of speckle image models, the edges of images are detected using normalized wavelet modulus. Relying on this feature, both the envelope-detected speckle image and the log-compressed ultrasonic image can be directly processed by the algorithm without need for additional preprocessing. Speckle is suppressed by employing the iterative multiscale diffusion on the wavelet coefficients. With a tuning diffusion threshold strategy, the proposed method can improve the image quality for both visualization and auto-segmentation applications. We validate our method using synthetic speckle images and real ultrasonic images. Performance improvement over other despeckling filters is quantified in terms of noise suppression and edge preservation indices.     

Model checking with bounded context switching

We discuss the implementation of a bounded context switching algorithm in the Spin model checker. The algorithm allows us to find counter-examples that are often simpler to understand, and that may be more likely to occur in practice. We discuss extensions of the algorithm that allow us to use this new algorithm in combination with most other search modes supported in Spin, including partial order reduction and bitstate hashing. We show that, other than often assumed, the enforcement of a bounded context switching discipline does not decrease but increases the complexity of the model checking procedure. We discuss the performance of the algorithm on a range of applications.     

Inhibition effect of some aromatic amines on copper electrodeposition from acidic baths

To improve the quality requirements for copper deposits, the influence of some inhibition agents added to the acidic copper bath has been studied. Several aromatic nitrogen compounds have been tested as inhibition agents, such as aniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline (DMA), and N,N-diethylaniline (DEA). The electrochemical behavior of these organic additives and the most relevant aspects of the electrochemical behavior of copper in acid solutions have been analyzed by cyclic voltammetry. At the same time, a correlation between voltammetric data and molecular properties of protonated amines obtained by molecular modeling has been performed. The morphology of the copper deposits obtained in the absence and presence of organic compounds has been studied by scanning electron microscopy. An improvement of the roughness degree and crystallite size upon addition of aromatic amine has been found. Best results were obtained for DMA and DEA working at room temperature (25 ± 0.1 °C) and 200 A m⁻² current density, in 1 mol L⁻¹ sulfuric acid solution with 50 g L⁻¹ Cu²⁺.     

Remote calibration using mobile, multiagent technology

Instrument calibration, though unavoidable, is extensively time and resource consuming. It often involves a distinct layer of data management and security. Since many of the available digital instruments are provided with communication interfaces, one can build a remote calibration system from the actual hardware and a computing unit with Internet connection capabilities. This paper, after showing a simple client-server architecture, discusses how the use of mobile, multiagent techniques is expected to solve most of the security issues, working as well and effectively as a traditional, agent-free client-server architecture.     

Use of fine aggregate matrix for computational modeling of low temperature fracture of asphalt concrete

In this study, a discrete element computational model is applied to simulate the fracture behavior of asphalt mixtures at low temperatures. In this model, coarse aggregates are explicitly represented by rigid spherical particles. The bonds that connect these particles represent the fine aggregate matrix (FAM), which is defined as the combination of asphalt binder and fine aggregates. The bending beam rheometer (BBR) tests are performed to determine the strength and Young’s modulus of FAM at low temperatures. The model is then used to simulate the semi-circular bend (SCB) tests on the mixtures. The model is verified by a series of BBR and SCB tests on both conventional and graphite nano-platelet modified asphalt materials. The comparison between the experimental and simulated results indicates that the peak load capacity of the SCB specimens is primarily governed by the tensile strength of the FAM. However, in order to capture the entire load–displacement curve of the SCB specimens, one needs to employ a softening constitutive model of the FAM, which requires the information on its fracture energy. Several experimental methods for measuring the fracture energy of FAM are discussed for future prediction of the complete load–displacement response of asphalt mixtures at low temperatures.     

Order Statistics in the Farey Sequences in Sublinear Time and Counting Primitive Lattice Points in Polygons

We present the first sublinear-time algorithms for computing order statistics in the Farey sequence and for the related problem of ranking. Our algorithms achieve a running times of nearly O(n ^2/3), which is a significant improvement over the previous algorithms taking time O(n). We also initiate the study of a more general problem: counting primitive lattice points inside planar shapes. For rational polygons containing the origin, we obtain a running time proportional to D ^6/7, where D is the diameter of the polygon. This work represents a merging of 19 and 21, with additional extensions.