Photonic Crystal Vertical Cavity Lasers

Photonic crystal confinement in vertical cavity surface-emitting lasers (VCSELs) is a robust and reliable technology for achieving operation in the fundamental lateral mode and is potentially applicable to a variety of materials systems and operating wavelengths. We demonstrate photonic crystal VCSELs operating in a single transverse mode with over 30 dB side mode suppression and over 1 mW of output power. These lasers have been subjected to a post-process technique to introduce the etched holes making up the photonic crystals that surround a centralized defect in which lasing occurs. We also show that coupling between adjacent defects in a photonic lattice is possible, further increasing the power available in the devices

Broadband spin-controlled focusing via logarithmic-spiral nanoslits of varying width

This work presents analytical, numerical and experimental demonstrations of light diffracted through a logarithmic spiral (LS) nanoslit, which forms a type of switchable and focus-tunable structure. Owing to a strong dependence on the incident photon spin, the proposed LS-nanoslit converges incoming light of opposite handedness (to that of the LS-nanoslit) into a confined subwavelength spot, while it shapes light with similar chirality into a donut-like intensity profile. Benefitting from the varying width of the LS-nanoslit, different incident wavelengths interfere constructively at different positions, i.e., the focal length shifts from 7.5 μm (at λ = 632.8 nm) to 10 μm (at λ = 488 nm), which opens up new opportunities for tuning and spatially separating broadband light at the micrometer scale

City Energy Demand Simulation (CEDS) Feasibility Study

CEDS - The City Energy Demand Simulation - provides cities and local authorities with the means to visualise future energy demand (including gas and electricity) for any geographic area ranging from a street, to a district, to the city itself, including both residential and industrial/commercial energy demand.CEDS allows planners and decision makers to easily visualise the impacts of alternative demand and supply side energy investment strategies on overall energy costs, emissions, and fuel poverty levels. This will be important for cities and planners because the innovative features of CEDS will enable planners to understand the impacts of new energy technology deployments on the energy demand of buildings. - CEDS can demonstrate the relative economic and environmental attractiveness of local energy supply schemes such as district heating combined with power versus importing electricity from the grid; - CEDS can illuminate the impacts of technology deployment by social geography within the city boundaries Therefore, CEDS will allow cities and local authorities to clearly identify how to deploy limited capital budgets to greatest effect when developing low carbon, cost effective local energy infrastructure. By modelling future demand, supply and cost scenarios, cities will be able to prioritise the development of local energy assets, such as district heating networks, energy from waste, retrofit and new build locations. CEDS will enable cities to understand how they can deliver on their priorities for ensuring there are cost effective locations for business and industry, with a secure supply, whilst also helping tackle fuel poverty and reducing carbon emissions and energy costs

HST Observations of SGR 0526-66: New Constraints on Accretion and Magnetar Models

Soft Gamma-ray Repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X- and Gamma-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the Anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk or decay of an enormous (~10^15 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models, and suggest that the spectral energy distribution may peak in the hard-UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~10^15-10^16 G field.Comment: 23 pages, 5 figures. Accepted by Ap

Universal scaling behavior of coupled chains of interacting fermions

The single-particle hopping between two chains is investigated by exact-diagonalizations techniques supplemented by finite-size scaling analysis. In the case of two coupled strongly-correlated chains of spinless fermions, the Taylor expansion of the expectation value of the single-particle interchain hopping operator of an electron at momentum k_F in powers of the interchain hopping t_perp is shown to become unstable in the thermodynamic limit. In the regime alpha<alpha_{tp} (alpha_{tp} simeq 0.41) where transverse two-particle hopping is less relevant than single-particle hopping, the finite-size effects can be described in terms of a universal scaling function. From this analysis it is found that the single-particle transverse hopping behaves as t_perp^{alpha/(1-alpha)} in agreement with a RPA-like treatment of the interchain coupling. For alpha>alpha_{tp}, the scaling law is proven to change its functional form, thus signaling, for the first time numerically, the onset of coherent transverse two-particle hopping.Comment: 12 pages, Late

Coherent vs incoherent interlayer transport in layered metals

The magnetic-field, temperature, and angular dependence of the interlayer magnetoresistance of two different quasi-two-dimensional (2D) organic superconductors is reported. For $\kappa$-(BEDT-TTF)$_2$I$_3$ we find a well-resolved peak in the angle-dependent magnetoresistance at $\Theta = 90^\circ$ (field parallel to the layers). This clear-cut proof for the coherent nature of the interlayer transport is absent for $\beta$''-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. This and the non-metallic behavior of the magnetoresistance suggest an incoherent quasiparticle motion for the latter 2D metal.Comment: 4 pages, 4 figures. Phys. Rev. B, in pres

Impact of animal strain on gene expression in a rat model of acute cardiac rejection

<p>Abstract</p> <p>Background</p> <p>The expression levels of many genes show wide natural variation among strains or populations. This study investigated the potential for animal strain-related genotypic differences to confound gene expression profiles in acute cellular rejection (ACR). Using a rat heart transplant model and 2 different rat strains (Dark Agouti, and Brown Norway), microarrays were performed on native hearts, transplanted hearts, and peripheral blood mononuclear cells (PBMC).</p> <p>Results</p> <p>In heart tissue, strain alone affected the expression of only 33 probesets while rejection affected the expression of 1368 probesets (FDR 10% and FC ≥ 3). Only 13 genes were affected by both strain and rejection, which was < 1% (13/1368) of all probesets differentially expressed in ACR. However, for PBMC, strain alone affected 265 probesets (FDR 10% and FC ≥ 3) and the addition of ACR had little further effect. Pathway analysis of these differentially expressed strain effect genes connected them with immune response, cell motility and cell death, functional themes that overlap with those related to ACR. After accounting for animal strain, additional analysis identified 30 PBMC candidate genes potentially associated with ACR.</p> <p>Conclusion</p> <p>In ACR, genetic background has a large impact on the transcriptome of immune cells, but not heart tissue. Gene expression studies of ACR should avoid study designs that require cross strain comparisons between leukocytes.</p

On-chain electrodynamics of metallic (TMTSF)_2 X salts: Observation of Tomonaga-Luttinger liquid response

We have measured the electrodynamic response in the metallic state of three highly anisotropic conductors, (TMTSF)_2 X, where X=PF_6, AsF_6, or ClO_4, and TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three cases we find dramatic deviations from a simple Drude response. The optical conductivity has two features: a narrow mode at zero frequency, with a small spectral weight, and a mode centered around 200 cm^{-1}, with nearly all of the spectral weight expected for the relevant number of carriers and single particle bandmass. We argue that these features are characteristic of a nearly one-dimensional half- or quarter-filled band with Coulomb correlations, and evaluate the finite energy mode in terms of a one-dimensional Mott insulator. At high frequencies (\hbar\omega > t_\perp, the transfer integral perpendicular to the chains), the frequency dependence of the optical conductivity \sigma_1(\omega) is in agreement with calculations based on an interacting Tomonaga-Luttinger liquid, and is different from what is expected for an uncorrelated one-dimensional semiconductor. The zero frequency mode shows deviations from a simple Drude response, and can be adequately described with a frequency dependent mass and relaxation rate.Comment: 12 pages, 7 figures, RevTeX; minor corrections to text and references; To be published in Phys. Rev. B, 15 July 199

Possible Triplet Electron Pairing and an Anisotropic Spin Susceptibility in Organic Superconductors (TMTSF)_2 X

We argue that (TMTSF)_2 PF_6 compound under pressure is likely a triplet superconductor with a vector order parameter d(k) \equiv (d_a(k) \neq 0, d_c(k) = ?, d_{b'}(k) = 0); |d_a(k)| > |d_c(k)|. It corresponds to an anisotropic spin susceptibility at T=0: \chi_{b'} = \chi_0, \chi_a \ll \chi_0, where \chi_0 is its value in a metallic phase. [The spin quantization axis, z, is parallel to a so-called b'-axis]. We show that the suggested order parameter explains why the upper critical field along the b'-axis exceeds all paramagnetic limiting fields, including that for a nonuniform superconducting state, whereas the upper critical field along the a-axis (a \perp b') is limited by the Pauli paramagnetic effects [I. J. Lee, M. J. Naughton, G. M. Danner and P. M. Chaikin, Phys. Rev. Lett. 78, 3555 (1997)]. The triplet order parameter is in agreement with the recent Knight shift measurements by I. J. Lee et al. as well as with the early results on a destruction of superconductivity by nonmagnetic impurities and on the absence of the Hebel-Slichter peak in the NMR relaxation rate.Comment: 4 pages, 1 eps figur