Low phonon energy, Nd:LaF3 channel waveguide lasersfabricated by molecular beam epitaxy

We report the first fabrication and laser operation of channel waveguides based on LaF₃ planar thin films grown by molecular beam epitaxy. To our knowledge, this is the lowest phonon energy dielectric material to have shown guided-wave laser operation to date. A full characterization, in terms of spectroscopy, laser results, and propagation losses, is given for the planar thin films upon which the channel waveguides are based. Two channel-fabrication methods are then described, the first involves ion milling and the second takes the novel approach of using a photo-definable polymer overlay. Laser operation in Nd-doped samples is demonstrated at 1.06, 1.05, and 1.3 μm, and the potential for mid-infrared laser sources based on such guides is discussed     

Reaction zones in highly unstable detonations

Experimental images of detonation fronts are made for several fuel-oxidizer mixtures, including hydrocarbon–air systems. Schlieren and planar laser induced fluorescence techniques are used to image both the shock configurations and the OH reaction front structure in a single experiment. The experiments are carried out in a narrow rectangular channel. The degree of instability of detonation fronts in different mixtures is evaluated by comparing calculated mixture parameters with the longitudinal neutral stability curve. The images reveal that the structure of the front increases dramatically in complexity as the mixture parameters move away from the neutral stability curve into the unstable region. Of the mixtures studied, nitrogen-diluted hydrocarbon mixtures are predicted to be the most unstable, and these show the greatest degree of wrinkling in the shock and OH fronts, with distortion occurring over a wide range of spatial scales. In the most unstable cases, separation of the shock and OH front occurs, and localized explosions in these regions are observed in a high-speed schlieren movie. This is in dramatic contrast to the weakly unstable waves that have smooth reaction fronts and quasi-steady reaction zones with no evidence of localized explosions. A key feature of highly unstable waves is very fine scale wrinkling of the OH and shock fronts, which is absent in the low-activation energy cases. This may be due to the superposition of cellular structures with a wide range of cell sizes. In contrast to soot foils, images of the OH front have a more stochastic appearance, and organized cellular structure is not as apparent.     

Asymptotic limits in peak envelope power reduction by redundantcoding in orthogonal frequency-division multiplex modulation

This paper uses some computational results to show that the peak envelope power (PEP) of orthogonal frequency-division multiplex (OFDM) waveforms can be reduced very substantially by coding to select only those messages with low peak factor as valid codewords in the scheme. It is shown that the asymptotic limit of the PEP is between two and three, and that only four bits of redundancy are necessary to limit the PEP to within 10% of this optimum value as the number of carriers becomes large     

An Iodine‐Vapor‐Induced Cyclization in a Crystalline Molecular Flask

A vapor‐induced cyclization has been observed in the host environment of a crystalline molecular flask (CMF), within which 1,8‐bis(2‐phenylethynyl)naphthalene (bpen), a diarenynyl system primed for cyclization, was exposed to iodine vapor to yield the corresponding indeno[2,1‐α]phenalene species. The cyclization process, unique in its vapor‐induced, solvent‐free nature, was followed spectroscopically, and found to occur concurrently with the displacement of lattice solvent for molecular iodine in CMF?0.75 bpen?2.25 CHCl₃?H₂O. The cyclization occurred under mild conditions and without the need to suspend the crystals in solvent. The ability of CMFs to host purely gas‐induced reactions is further highlighted by the subsequent sequential oxidation reaction of cyclized 7‐iodo‐12‐phenylindeno[2,1‐α]phenalene (ipp) with molecular oxygen derived from air, yielding 12‐hydroxy‐7‐iodo‐2‐phenylindeno[2,1‐α]phenalen‐1(12H)‐one (hipp).     

Alkyl and aryl dicationic derivatives of cyclic triphosphenium ions

Direct alkylation of cyclic triphosphenium ions by triflates to give di-ium dications is only possible for small organic substituents on the attacking reagent. The dicationic products are not intrinsically unstable, however, and in many instances they may be synthesised by an alternative route, pioneered by Schmidpeter and co-workers. These species may be readily identified in solution by (31)P NMR spectroscopy. The crystal and molecular structures of five such derivatives have been ascertained for the first time by single crystal X-ray diffraction at 120 K. The results confirm that normal single P-P bond lengths are present in the dications, in contrast with the monocationic parent cyclic triphosphenium ions, where structural determinations have shown that the P-P bond lengths are intermediate between single and double bonds.     

Isochoric heating of solids by laser-accelerated protons: Experimental characterization and self-consistent hydrodynamic modeling

A study of isochoric heating of Al foil by laser-accelerated proton beam is presented, coupling self-consistent hydrodynamic simulations (including proton stopping) with experimental measurements. The proton source that induces the heating has been characterized experimentally and the induced heating has been inferred through critical density expansion velocity measurement. The low-energy part of the proton spectrum that plays the dominant part in the heating process has been studied in detail. The experimental results are compared with the results of 1-dimensional hydrodynamic simulations that use as input the measured proton source and good agreement between the two is found using the SESAME EOS.     

Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50 ns) spark discharges were measured for undiluted and nitrogen-diluted H₂-N₂O mixtures of equivalence ratios ? = 0.15 and 0.2, dilution of 0% and 20% N₂, and initial pressures of 15–25 kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P = 15 kPa) to a narrow (P = 25 kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N₂ dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H₂-N₂O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H₂-O₂ mixtures.     

Microfluidic assembly of homogeneous and Janus colloid-filled hydrogel granules

The microfluidic assembly of colloid-filled hydrogel granules of varying shape and composition is described. First, drops are formed by shearing a concentrated colloidal microsphere-acrylamide suspension in a continuous oil phase using a sheath-flow device. Both homogeneous and Janus (hemispherically distinct) spheres and disks are produced by confining the assembled drops in microchannels of varying geometry. Next, photopolymerization is carried out shortly after drop breakup to preserve their morphology. Representative wet and dried granules are characterized using fluorescence and scanning electron microscopy, respectively. Our approach offers a facile route for assembling colloid-filled hydrogel granules with controlled shape and composition.     

Harvesting a logistic population in a slowly varying environment

The classic problem for a logistically evolving single species population being harvested involves three parameters: rate constant, carrying capacity and harvesting rate, which are taken to be positive constants. However, in real world situations, these parameters may vary with time. This paper considers the situation where these vary on a time scale much longer than that intrinsic to the population evolution itself. Application of a multiple time scale approach gives approximate explicit closed form expressions for the changing population, that compare favorably with those generated from numerical solutions.