Giant Enhancement of Stimulated Brillouin Scattering in the Subwavelength Limit

Stimulated Brillouin scattering (SBS) is traditionally viewed as a process whose strength is dictated by intrinsic material nonlinearities with little dependence on waveguide geometry. We show that this paradigm breaks down at the nanoscale, as tremendous radiation pressures produce new forms of SBS nonlinearities. A coherent combination of radiation pressure and electrostrictive forces is seen to enhance both forward and backward SBS processes by orders of magnitude, creating new geometric degrees of freedom through which photon-phonon coupling becomes highly tailorable. At nanoscales, the backward-SBS gain is seen to be 10(4) times greater than in conventional silica fibers with 100 times greater values than predicted by conventional SBS treatments. Furthermore, radically enhanced forward-SBS processes are 10(5) times larger than any known waveguide system. In addition, when nanoscale silicon waveguides are cooled to low temperatures, a further 10-100 times increase in SBS gain is seen as phonon losses are reduced. As a result, a 100-mu m segment of the waveguide has equivalent nonlinearity to a kilometer of fiber. Couplings of this magnitude would enable efficient chip-scale stimulated Brillouin scattering in silicon waveguides for the first time. More generally, we develop a new full-vectorial theoretical formulation of stimulated Brillouin scattering that accurately incorporates the effects of boundary-induced nonlinearities and radiation pressure, both of which are seen to have tremendous impact on photon-phonon coupling at subwavelength scales. This formalism, which treats both intermode and intramode coupling within periodic and translationally invariant waveguide systems, reveals a rich landscape of new stimulated Brillouin processes when applied to nanoscale systems.U.S. Department of Energy's NNSA DE-AC04-94AL85000DDRE under Air Force FA8721-05-C-000MesoDynamic Architectures program at DARPASandia's Laboratory Directed Research and Development programChemical Engineerin

On the gradient of the yield plateau in structural carbon steels

New design methodologies are being developed to allow stocky steel members to attain and exceed the full plastic condition. For theoretical validation, such methods require a characterisation of the uniaxial stress-strain behaviour of structural steel beyond an idealised elastic-plastic representation. However, the strain hardening properties of carbon steels are not currently guaranteed by the standards or by any steel manufacturer. Assumptions must thus be made on what values of these properties are appropriate, often based on limited information in the form of individual stress-strain curves. There is very little consistency in the choices made. This paper first illustrates, using an example elastic-plastic finite element calculation, that a stocky tubular structure can attain the full plastic condition at slendernesses comparable with those defined in current standards and supported by experiment when using only a very modest level of strain hardening, initiated at first yield. It is then hypothesised that the yield plateau in the stress-strain curve for structural carbon steels, classically treated as flat and with zero tangent modulus, actually has a small but statistically significant positive finite gradient. Finally, a robust set of linear regression analyses of yield plateau gradients extracted from 225 tensile tests appears to support this hypothesis, finding that the plateau gradient is of the order of 0.3% of the initial elastic modulus, consistent with what the finite element example suggests is sufficient to reproduce the full plastic condition at experimentally-supported slendernesses

Design of Peptide Inhibitors That Bind the bZIP Domain of Epstein–Barr Virus Protein BZLF1

Designing proteins or peptides that bind native protein targets can aid the development of novel reagents and/or therapeutics. Rational design also tests our understanding of the principles underlying protein recognition. This article describes several strategies used to design peptides that bind to the basic region leucine zipper (bZIP) domain of the viral transcription factor BZLF1, which is encoded by the Epstein–Barr virus. BZLF1 regulates the transition of the Epstein–Barr virus from a latent state to a lytic state. It shares some properties in common with the more studied human bZIP transcription factors, but also includes novel structural elements that pose interesting challenges to inhibitor design. In designing peptides that bind to BZLF1 by forming a coiled-coil structure, we considered both affinity for BZLF1 and undesired self-association, which can weaken the effectiveness of an inhibitor. Several designed peptides exhibited different degrees of target-binding affinity and self-association. Rationally engineered molecules were more potent inhibitors of DNA binding than a control peptide corresponding to the native BZLF1 dimerization region itself. The most potent inhibitors included both positive and negative design elements and exploited interaction with the coiled-coil and basic DNA-binding regions of BZLF1.David H. Koch Institute for Integrative Cancer Research at MIT (Graduate Fellowship)National Institutes of Health (U.S.) (Award GM067681)National Science Foundation (U.S.) (Award 0821391

Coupled opto-electronic simulation of organic bulk-heterojunction solar cells: parameter extraction and sensitivity analysis

A general problem arising in computer simulations is the number of material and device parameters, which have to be determined by dedicated experiments and simulation-based parameter extraction. In this study we analyze measurements of the short-circuit current dependence on the active layer thickness and current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells. We have identified a set of parameter values including dissociation parameters that describe the experimental data. The overall agreement of our model with experiment is good, however a discrepancy in the thickness dependence of the current-voltage curve questions the influence of the electric field in the dissociation process. In addition transient simulations are analyzed which show that a measurement of the turn-off photocurrent can be useful for estimating charge carrier mobilities.Comment: 10 pages, 12 figures, 2 tables, Accepted for publication in Journal of Applied Physic

Rain volume estimation over areas using satellite and radar data

The analysis of 18 convective clusters demonstrates that the extension of the Area-Time-Integral (ATI) technique to the use of satellite data is possible. The differences of the internal structures of the radar reflectivity features, and of the satellite features, give rise to differences in estimating rain volumes by delineating area; however, by focusing upon the area integrated over the lifetime of the storm, it is suggested that some of the errors produced by the differences in the cloud geometries as viewed by radar or satellite are minimized. The results are good and future developments should consider data from different climatic regions and should allow for implementation of the technique in a general circulation model

Impurity transport in Alcator C-Mod in the presence of poloidal density variation induced by ion cyclotron resonance heating

Impurity particle transport in an ion cyclotron resonance heated Alcator C-Mod discharge is studied with local gyrokinetic simulations and a theoretical model including the effect of poloidal asymmetries and elongation. In spite of the strong minority temperature anisotropy in the deep core region, the poloidal asymmetries are found to have a negligible effect on the turbulent impurity transport due to low magnetic shear in this region, in agreement with the experimental observations. According to the theoretical model, in outer core regions poloidal asymmetries may contribute to the reduction of the impurity peaking, but uncertainties in atomic physics processes prevent quantitative comparison with experiments.Comment: 32 pages, 12 figure

A Role for Actin, Cdc1p, and Myo2p in the Inheritance of Late Golgi Elements in \u3cem\u3eSaccharomyces cerevisiae\u3c/em\u3e

In Saccharomyces cerevisiae, Golgi elements are present in the bud very early in the cell cycle. We have analyzed this Golgi inheritance process using fluorescence microscopy and genetics. In rapidly growing cells, late Golgi elements show an actin-dependent concentration at sites of polarized growth. Late Golgi elements are apparently transported into the bud along actin cables and are also retained in the bud by a mechanism that may involve actin. A visual screen for mutants defective in the inheritance of late Golgi elements yielded multiple alleles of CDC1. Mutations in CDC1 severely depolarize the actin cytoskeleton, and these mutations prevent late Golgi elements from being retained in the bud. The efficient localization of late Golgi elements to the bud requires the type V myosin Myo2p, further suggesting that actin plays a role in Golgi inheritance. Surprisingly, early and late Golgi elements are inherited by different pathways, with early Golgi elements localizing to the bud in a Cdc1p- and Myo2p-independent manner. We propose that early Golgi elements arise from ER membranes that are present in the bud. These two pathways of Golgi inheritance in S. cerevisiae resemble Golgi inheritance pathways in vertebrate cells

Non-neoclassical up/down asymmetry of impurity emission on Alcator C-Mod

We demonstrate that existing theories are insufficient to explain up/down asymmetries of argon x-ray emission in Alcator C-Mod ohmic plasmas. Instead of the poloidal variation, ñ[subscript z]/〈n[subscript z]〉, being of order the inverse aspect ratio, ϵ, and scaling linearly with B[subscript t][superscript _ over n][subscript e]/I[2 over p], it is observed over 0.8 < r/a < 1.0 to be of order unity and exhibits a threshold behaviour between 3.5 <B[subscript t][superscript _ over n][subscript e]/I[subscript p] < 4.0 (T10[superscript 20] m[superscript −3] MA[superscript −1]). The transition from a poloidally symmetric to asymmetric impurity distribution is shown to occur at densities just below those that trigger a reversal of the core toroidal rotation direction, thought to be linked to the transition between the linear and saturated ohmic confinement regimes. A possible drive is discussed by which anomalous radial transport might sustain the impurity density asymmetry as the ratio of the perpendicular to parallel equilibration times, τ[subscript ⊥,z]/τ[subscript ∥,z], approaches unity. This explanation requires a strong up/down asymmetry in radial flux which, while not observable on C-Mod, has been measured in TEXT and Tore Supra ohmic plasmas.United States. Dept. of Energy (Contract DE-FC02-99ER54512)United States. Dept. of Energy (Fusion Research Postdoctoral Research Program