Semianalytical approach to short-wavelength dispersion and modal properties of photonic crystal fibers

Photonic crystal fibers made from arbitrary base materials are considered, and a unified semianalytical approach for the dispersion and modal properties is derived that applies to the short-wavelength regime. In particular, the dispersion and the effective index are calculated and compared with fully vectorial plane-wave simulations, and excellent agreement is found. Asymptotic results for the mode-field diameter and the V parameter are also calculated, and from the latter it is predicted that the fibers are endlessly single mode for a normalized airhole diameter smaller than 0.42, independently of the base material.     

Performance of nonlinear photonic crystal devices at high bit rates

I investigate theoretically the performance limits for all-optical switching devices incorporating photonic crystals made of nonlinear materials operating at high bit rates. I compare two switching techniques--shifting of the photonic bandgap and the more traditional interferometric method enhanced by slow wave propagation in photonic crystals-and show that the latter always has an advantage. I also demonstrate that the benefits provided by the photonic crystal in the interferometer are still severely limited by the dispersion and become significant only in materials combining high nonlinearity, high index contrast, and high damage threshold.     

Phase-constrained parallel MR image reconstruction

A generalized method for phase-constrained parallel MR image reconstruction is presented that combines and extends the concepts of partial-Fourier reconstruction and parallel imaging. It provides a framework for reconstructing images employing either or both techniques and for comparing image quality achieved by varying k-space sampling schemes. The method can be used as a parallel image reconstruction with a partial-Fourier reconstruction built in. It can also be used with trajectories not readily handled by straightforward combinations of partial-Fourier and SENSE-like parallel reconstructions, including variable-density, and non-Cartesian trajectories. The phase constraint specifies a better-conditioned inverse problem compared to unconstrained parallel MR reconstruction alone. This phase-constrained parallel MRI reconstruction offers a one-step alternative to the standard combination of homodyne and SENSE reconstructions with the added benefit of flexibility of sampling trajectory. The theory of the phase-constrained approach is outlined, and its calibration requirements and limitations are discussed. Simulations, phantom experiments, and in vivo experiments are presented.     

Surface-directed capillary system; theory, experiments and applications

We present a capillary flow system for liquid transport in microsystems. Our simple microfluidic system consists of two planar parallel surfaces, separated by spacers. One of the surfaces is entirely hydrophobic, the other mainly hydrophobic, but with hydrophilic pathways defined on it by photolithographic means. By controlling the wetting properties of the surfaces in this manner, the liquid can be confined to certain areas defined by the hydrophilic pathways. This technique eliminates the need for alignment of the two surfaces. Patterned plasma-polymerized hexafluoropropene constitutes the hydrophobic areas, whereas the untreated glass surface constitutes the hydrophilic pathways. We developed a theoretical model of the capillary flow and obtained analytical solutions which are in good agreement with the experimental results. The capillarity-driven microflow system was also used to pattern and immobilize biological material on planar substrates: well-defined 200 microm wide strips of human cells (HeLa) and fluorescence labelled proteins (fluorescein isothiocyanate-labelled bovine serum albumin, i.e., FITC-BSA) were fabricated using the capillary flow system presented here.     

A new two-chip concept for continuous measurements on PMMA-microchips

A new concept for continuous measurements on microchips is presented. A PMMA (polymethylmethacrylate) based capillary electrophoresis chip with integrated conductivity detection is combined with a second chip, which undertakes the task of fluid handling and electrical connections. The combination of electrokinetic and hydrodynamic flows allows long-term continuous stable analyses with good reproducibilities of migration time and peak heights of analytes. The two-chip system is characterized in terms of stability and reproducibility of separation and detection of small ions. Relative standard deviations of <1% and 3% respectively for retention times and peak heights during long-term measurements can be achieved. The new system combines simple handling and automated analysis without the need for refilling, cleaning or removal of the separation chip after one or several measurements.     

A comparison of mesogenic properties for one- and two-ring dipentyl derivatives of p-carboranes, bicyclo[2.2.2]octane, and benzene

A series of nonpolar single- and two-ring dipentyl derivatives of p-carboranes, bicyclo[2.2.2]-octane and benzene was investigated in the pure state and in binary mixtures with a nematic host. The resulting virtual nematic-isotropic transition temperatures [T_NI] for single ring compounds were compared with those for two ring compounds. All [T_NI] were compared with the molecular aspect ratios X and filling fractions F obtained from MNDO calculations. The highest effectiveness in promotion of the nematic phase was found for bicyclo[2.2.2]octane and 12-vertex p-carborane and ascribed to exceptional molecular rigidity and electronic properties, respectively. Results show that a high filling fraction F and molecular stiffness are the necessary factors for a highly stable nematic phase.     

Applications of ESCA to industrial chemistry

Electron Spectroscopy for Chemical Analysis (ESCA) is a new spectrotechnique which is assuming significant importance as a research tool in industrial The unique value of ESCA is its potential for providing insight into problems which are quite diverse in nature. This presentation includes direct indu of ESCA to research in catalysis, surface coatings, organic structure, qualitative and quantitative measurements of materials present at low levels and in solids amongst others.     

Absorption spectrum of VO2+ in Na2Zn(SO4)2 · 4H2O single crystals

Optical absorption spectrum of VO²⁺ ion in sodium zinc sulphate tetrahydrate single crystal has been reported at room and liquid nitrogen temperatures. From the nature and position of the observed bands, they have been attributed to VO²⁺ ion in C_4v symmetry. The crystal field and molecular orbital coefficients have been evaluated.