Casimir effect across a layered medium

Using nonstandard recursion relations for Fresnel coefficients involving successive stacks of layers, we extend the Lifshitz formula to configurations with an inhomogeneous, n-layered, medium separating two planar objects. The force on each object is the sum of a Lifshitz like force and a force arising from the inhomogeneity of the medium. The theory correctly reproduces very recently obtained results for the Casimir force/energy in some simple systems of this kind. As a by product, we obtain a formula for the force on an (unspecified) stack of layers between two planar objects which generalizes our previous result for the force on a slab in a planar cavity.Comment: 5 pages, 1 figure, presented at QFEXT1

The scattering of a cylindrical invisibility cloak: reduced parameters and optimization

We investigate the scattering of 2D cylindrical invisibility cloaks with simplified constitutive parameters with the assistance of scattering coefficients. We show that the scattering of the cloaks originates not only from the boundary conditions but also from the spatial variation of the component of permittivity/permeability. According to our formulation, we propose some restrictions to the invisibility cloak in order to minimize its scattering after the simplification has taken place. With our theoretical analysis, it is possible to design a simplified cloak by using some peculiar composites like photonic crystals (PCs) which mimic an effective refractive index landscape rather than offering effective constitutives, meanwhile canceling the scattering from the inner and outer boundaries.Comment: Accepted for J. Phys.

Theoretical framework of entangled-photon generation from biexcitons in nano-to-bulk crossover regime with planar geometry

We have constructed a theoretical framework of the biexciton-resonant hyperparametric scattering for the pursuit of high-power and high-quality generation of entangled photon pairs. Our framework is applicable to nano-to-bulk crossover regime where the center-of-mass motion of excitons and biexcitons is confined. Material surroundings and the polarization correlation of generated photons can be considered. We have analyzed the entangled-photon generation from CuCl film, by which ultraviolet entangled-photon pairs are generated, and from dielectric microcavity embedding a CuCl layer. We have revealed that in the nano-to-bulk crossover regime we generally get a high performance from the viewpoint of statistical accuracy, and the generation efficiency can be enhanced by the optical cavity with maintaining the high performance. The nano-to-bulk crossover regime has a variety of degrees of freedom to tune the entangled-photon generation, and the scattering spectra explicitly reflect quantized exciton-photon coupled modes in the finite structure.Comment: 18 pages, 10 figure

Dissipation in intercluster plasma

We discuss dissipative processes in strongly gyrotropic, nearly collisionless plasma in clusters of galaxies (ICM). First, we point out that Braginsky theory, which assumes that collisions are more frequent that the system's dynamical time scale, is inapplicable to fast, sub-viscous ICM motion. Most importantly, the electron contribution to collisional magneto-viscosity dominates over that of ions for short-scale Alfvenic motions. Thus, if a turbulent cascade develops in the ICM and propagates down to scales $\leq 1$ kpc, it is damped collisionally not on ions, but on electrons. Second, in high beta plasma of ICM, small variations of the magnetic field strength, of relative value $\sim 1/\beta$, lead to development of anisotropic pressure instabilities (firehose, mirror and cyclotron). Unstable wave modes may provide additional resonant scattering of particles, effectively keeping the plasma in a state of marginal stability. We show that in this case the dissipation rate of a laminar, subsonic, incompressible flows scales as inverse of plasma beta parameter. We discuss application to the problem of ICM heating.Comment: 4 pages, accepted by ApJ Let

Shearing Box Simulations of the MRI in a Collisionless Plasma

We describe local shearing box simulations of turbulence driven by the magnetorotational instability (MRI) in a collisionless plasma. Collisionless effects may be important in radiatively inefficient accretion flows, such as near the black hole in the Galactic Center. The MHD version of ZEUS is modified to evolve an anisotropic pressure tensor. A fluid closure approximation is used to calculate heat conduction along magnetic field lines. The anisotropic pressure tensor provides a qualitatively new mechanism for transporting angular momentum in accretion flows (in addition to the Maxwell and Reynolds stresses). We estimate limits on the pressure anisotropy due to pitch angle scattering by kinetic instabilities. Such instabilities provide an effective ``collision'' rate in a collisionless plasma and lead to more MHD-like dynamics. We find that the MRI leads to efficient growth of the magnetic field in a collisionless plasma, with saturation amplitudes comparable to those in MHD. In the saturated state, the anisotropic stress is comparable to the Maxwell stress, implying that the rate of angular momentum transport may be moderately enhanced in a collisionless plasma.Comment: 20 pages, 9 figures, submitted to Ap

Computation and visualization of Casimir forces in arbitrary geometries: non-monotonic lateral forces and failure of proximity-force approximations

We present a method of computing Casimir forces for arbitrary geometries, with any desired accuracy, that can directly exploit the efficiency of standard numerical-electromagnetism techniques. Using the simplest possible finite-difference implementation of this approach, we obtain both agreement with past results for cylinder-plate geometries, and also present results for new geometries. In particular, we examine a piston-like problem involving two dielectric and metallic squares sliding between two metallic walls, in two and three dimensions, respectively, and demonstrate non-additive and non-monotonic changes in the force due to these lateral walls.Comment: Accepted for publication in Physical Review Letters. (Expected publication: Vol. 99 (8) 2007

Low prevalence of hepatitis C co-infection in recently HIV-infected minority men who have sex with men in Los Angeles: a cross-sectional study.

BackgroundGeographic and sociodemographic characterization of hepatitis C virus (HCV) transmission amongst men who have sex with men (MSM) has been limited. Our aim was to characterize HCV prevalence, risk factors for HCV co-infection, and patterns of HIV and HCV co-transmission and transmitted drug resistance mutations (DRMs) in newly HIV-diagnosed Los Angeles MSM.MethodsViral RNA was extracted from stored plasma samples from a Los Angeles cohort of newly diagnosed HIV-infected MSM with well-characterized substance use and sexual behavioral characteristics via computer-assisted self-interviewing surveys. Samples were screened for HCV by qPCR. HCV E1, E2, core, NS3 protease and NS5B polymerase and HIV-1 protease and reverse transcriptase regions were amplified and sequenced. Phylogenetic analysis was used to determine relatedness of HCV and HIV-1 isolates within the cohort and viral sequences were examined for DRMs.ResultsOf 185 newly HIV-diagnosed MSM, the majority (65%) were of minority race/ethnicity and recently infected (57.8%), with median age of 28.3 years. A minority (6.6%) reported injection drug use (IDU), whereas 96 (52.8%) reported recent substance use, primarily cannabis or stimulant use. High risk sexual behaviors included 132 (74.6%) with unprotected receptive anal intercourse, 60 (33.3%) with group sex, and 10 (5.7%) with fisting. Forty-five (24.3%) had acute gonorrhea or chlamydia infection. Only 3 (1.6%) subjects had detectable HCV RNA. Amongst these subjects, HIV and HCV isolates were unrelated by phylogenetic analysis and none possessed clinically relevant NS3 or NS5B HCV DRMs.ConclusionsPrevalence of HCV co-infection was low and there was no evidence of HIV-HCV co-transmission in this cohort of relatively young, predominantly minority, newly HIV-diagnosed MSM, most with early HIV infection, with high rates of high risk sexual behaviors, STI, and non-IDU. The low HCV prevalence in a group with high-risk behaviors for non-IDU HCV acquisition suggests an opportune time for targeted HCV prevention measures

Rigorous formulation of oblique incidence scattering from dispersive media

We formulate a finite-difference time-domain (FDTD) approach to simulate electromagnetic wave scattering from scatterers embedded in layered dielectric or dispersive media. At the heart of our approach is a derivation of an equivalent one-dimensional wave propagation equation for dispersive media characterized by a linear sum of Debye-, Drude- and Lorentz-type poles. The derivation is followed by a detailed discussion of the simulation setup and numerical issues. The developed methodology is tested by comparison with analytical reflection and transmission coefficients for scattering from a slab, illustrating good convergence behavior. The case of scattering from a sub-wavelength slit in a dispersive thin film is explored to demonstrate the applicability of our formulation to time- and incident angle-dependent analysis of surface waves generated by an obliquely incident plane wave.Comment: 35 pages, 8 figures, 4 table