Second mode transition in microstructured optical fibers: determination of the critical geometrical parameter and study of the matrix refractive index and effects of cladding size

We carried out a numerical study of the second mode transition in finite-sized, microstructured optical fibers (MOFs) for several values of the matrix refractive index. We determined a unique critical geometrical parameter for the second mode cutoff that is valid for all the matrix refractive indices studied. Finite size effects and extrapolated results for infinite structures are described. Using scaling laws, we provide a generalized phase diagram for solid-core MOFs that is valid for all refractive indices, including those of the promising chalcogenide MOFs.     

A physico-chemical investigation of poly(ethylene oxide)-block-poly(L-lysine) copolymer adsorption onto silica nanoparticles

The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface.     

Ferrocene and Porphyrin Monolayers on Si(100) Surfaces: Preparation and Effect of Linker Length on Electron Transfer

The missing link : Ferrocene and porphyrin monolayers are tethered on silicon surfaces with short (see picture, left) or long (right) linkers. Electron transfer to the silicon substrate is faster for monolayers with a short linker.

     

A model for the oxidation of Mg(0001) based upon LEED,AES, ELS and work function measurements

The Mg(0001) face is subjected to oxygen adsorption from 0 to 10³ L. Three characteristic stages of oxygen adsorption are detected from 0 to 10 L. The AES signal of clean Mg decays exponentially against exposure with slopes α _ai such that α_A2 (0.75 → 3 L) >α_A1 (0 → 0.75 L)>α_A3 (3 → 10 L). For increasing exposures, they correspond to: (1) a clear (1 × 1)-Mg(0001), (2) a diffuse (1 × 1)-Mg(0001) and (3) a (1 × 1) with a weaker (1 × 1)-R30°-MgO(111) LEED patterns, respectively. At the end of the third stage, a supplementary ($\text{7}\text{×}\text{7}\text{2}\text{)?R19°?MgO(111)}$ pattern is observed. In ELS, a very fast intensity decrease of energy loss peaks due to surface and bulk plasmon excitations of the clean metal is recorded during the first stage. The energy loss peak due to the oxidized surface plasmon excitation reaches a maximum intensity at the end of the second stage. Energy loss peaks to be attributed to excitations in bulk MgO appear during the third stage. The work function of the sample decreases and shows a minimum around 6 L, and then slowly increases. Beyond 10 L, a logarithmic relation between oxide thickness and exposure seems to exist. These results are interpreted by the following sequential processes: stage 1: random oxygen chemisorption followed by oxygen incorporation (α_A1); stage 2: assembling into islands and lateral island growth (α_A2); stage 3: oxide formation (α_A3) and stage 4: oxide thickening. Lattice models describing these processes are proposed and discussed. The influence of surface roughness on the results is emphasized.     

Synthesis of cubic-BN using new flux-precursors: The fluoronitrides

Abstract

Instead of alkaline or alkaline-earth nitrides, fluoro-nitrides with mixed composition have been developed as flux-precursors for the elaboration process of c-BN. If the pressure and temperature conditions for the h-BN → c-BN conversion are sensibly reduced, the conversion rates are largely increased. On the other hand, such compounds are appropriate for growth of c-BN single crystal.     

A [Cyclentetrakis(methylene)]tetrakis[2‐hydroxybenzamide] Ligand That Complexes and Sensitizes Lanthanide(III) Ions

The synthesis of the cyclen derivative H₄ L ¹ ?2 HBr containing four 2‐hydroxybenzamide groups is described. The spectroscopic properties of the Ln^III conplexes of L ¹ (Ln=Gd, Tb, Yb, and Eu) reveal changes of the UV/VIS‐absorption, circular‐dichroism‐absorption, luminescence, and circularly polarized luminescence spectra. It is shown that at least two metal‐complex species are present in solution, whose relative amounts are pH dependent. At pH>8.0, an intense long‐lived emission is observed (for [Tb L ¹ ] and [Yb L ¹ ]), while at pH<8.0, a weaker, shorter‐lived species predominates. Unconventional Ln^III emitters (Pr, Nd, Sm, Dy, and Tm) were sensitized in basic solution, both in the VIS and in the near‐IR, to measure the emission of these ions.     

A combinatorial approach to jumping particles: The parallel TASEP

In this article, we continue the combinatorial study of models of particles jumping on a row of cells which we initiated with the standard totally asymmetric simple exclusion process or TASEP (Duchi and Schaeffer, Journal of Combinatorial Theory, Series A, 110(2005), 1–29). We consider here the parallel TASEP, in which particles can jump simultaneously. On the one hand, the interest in this process comes from highway traffic modeling: it is the only solvable special case of the Nagel‐Schreckenberg automaton, the most popular model in that context. On the other hand, the parallel TASEP is of some theoretical interest because the derivation of its stationary distribution, as appearing in the physics literature, is harder than that of the standard TASEP. We offer here an elementary derivation that extends the combinatorial approach we developed for the standard TASEP. In particular, we show that this stationary distribution can be expressed in terms of refinements of Catalan numbers. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008     

Solid‐State Gas Sensors Developed from Functional Difluoroboradiazaindacene Dyes

This article describes the synthesis and characterization of several new difluoroboradiazaindacene (BODIPY?) dyes functionalized at the central 8‐position by a phenyliodo, phenylheptynoate or phenylheptynoic fragment and at the 3‐ or 3/5‐position(s) by 4‐dimethylaminophenylstyryl residue(s). Single‐crystal structural determinations confirm the planarity of the dyes, while the absorption and fluorescence spectroscopic properties are highly sensitive to the state of protonation (or alkylation) of the terminal anilino donor group(s). Reversible color tuning from green to blue for absorption and from colorless (i.e., near‐IR region) to red for fluorescence is obtained on successive addition of acid and base. The difunctionalized derivative is especially interesting in this respect and shows two well‐resolved pK_a values of 5.10 and 3.04 in acetonitrile. Addition of the first proton causes only small spectral changes and deactivates the molecule towards addition of the second proton. It is this latter step that accommodates the large change in absorption and emission properties, due to the reversible extinction of the intramolecular charge‐transfer character inherent to this type of dye. The main focus of the work is the covalent anchoring of the dyes to inert, porous polyacrylate beads so as to form a solid‐state sensor suitable for analysis of gases or flowing liquids. The final material is highly stable—its performance is undiminished after more than one year—and fully reversible over many cycles. The sensitivity is such that reactions can be followed by the naked eye and the detection limit is about 600 ppb for HCl and about 80 ppb for ammonia. Trace amounts of diphosgene can be detected, as can alkylating agents. The sensing action is indiscriminate and also operates when the beads are dispersed in aqueous media.