2,256 research outputs found
Ray and wave chaos in asymmetric resonant optical cavities
Optical resonators are essential components of lasers and other
wavelength-sensitive optical devices. A resonator is characterized by a set of
modes, each with a resonant frequency omega and resonance width Delta
omega=1/tau, where tau is the lifetime of a photon in the mode. In a
cylindrical or spherical dielectric resonator, extremely long-lived resonances
are due to `whispering gallery' modes in which light circulates around the
perimeter trapped by total internal reflection. These resonators emit light
isotropically. Recently, a new category of asymmetric resonant cavities (ARCs)
has been proposed in which substantial shape deformation leads to partially
chaotic ray dynamics. This has been predicted to give rise to a universal,
frequency-independent broadening of the whispering-gallery resonances, and
highly anisotropic emission. Here we present solutions of the wave equation for
ARCs which confirm many aspects of the earlier ray-optics model, but also
reveal interesting frequency-dependent effects characteristic of quantum chaos.
For small deformations the lifetime is controlled by evanescent leakage, the
optical analogue of quantum tunneling. We find that the lifetime is much
shortened by a process known as `chaos-assisted tunneling'. In contrast, for
large deformations (~10%) some resonances are found to have longer lifetimes
than predicted by the ray chaos model due to `dynamical localization'.Comment: 4 pages RevTeX with 7 Postscript figure
Applications of Laboratory Technology in the Evaluation of the Risk of Rabies Transmissions by Biting Dogs and Cats
While rabies is not a common disease in domestic animal species of the United States, potential exposures to rabies in the form of bites are very common and increasing. A nationwide study conducted among general hospitals shows that 1 percent of emergency room visits are for animal bites, of which 80-90 percent are inflicted by the dog (Callaham 1980). This figure is conservative, as the study did not include pediatric hospitals, the bite of victims that progress only to a physician\u27s office, or those that receive no medical care at all. In Missouri alone, this study would infer about 1500 dog bites per year reaching only the general hospital. The number of dog and other animal bites across the country is unknown but may safely be assumed to be staggering in magnitude
Emergence of Spatial Structure in Cell Groups and the Evolution of Cooperation
On its own, a single cell cannot exert more than a microscopic influence on its immediate surroundings. However, via strength in numbers and the expression of cooperative phenotypes, such cells can enormously impact their environments. Simple cooperative phenotypes appear to abound in the microbial world, but explaining their evolution is challenging because they are often subject to exploitation by rapidly growing, non-cooperative cell lines. Population spatial structure may be critical for this problem because it influences the extent of interaction between cooperative and non-cooperative individuals. It is difficult for cooperative cells to succeed in competition if they become mixed with non-cooperative cells, which can exploit the public good without themselves paying a cost. However, if cooperative cells are segregated in space and preferentially interact with each other, they may prevail. Here we use a multi-agent computational model to study the origin of spatial structure within growing cell groups. Our simulations reveal that the spatial distribution of genetic lineages within these groups is linked to a small number of physical and biological parameters, including cell growth rate, nutrient availability, and nutrient diffusivity. Realistic changes in these parameters qualitatively alter the emergent structure of cell groups, and thereby determine whether cells with cooperative phenotypes can locally and globally outcompete exploitative cells. We argue that cooperative and exploitative cell lineages will spontaneously segregate in space under a wide range of conditions and, therefore, that cellular cooperation may evolve more readily than naively expected
Manipulating infrared photons using plasmons in transparent graphene superlattices
Superlattices are artificial periodic nanostructures which can control the
flow of electrons. Their operation typically relies on the periodic modulation
of the electric potential in the direction of electron wave propagation. Here
we demonstrate transparent graphene superlattices which can manipulate infrared
photons utilizing the collective oscillations of carriers, i.e., plasmons of
the ensemble of multiple graphene layers. The superlattice is formed by
depositing alternating wafer-scale graphene sheets and thin insulating layers,
followed by patterning them all together into 3-dimensional
photonic-crystal-like structures. We demonstrate experimentally that the
collective oscillation of Dirac fermions in such graphene superlattices is
unambiguously nonclassical: compared to doping single layer graphene,
distributing carriers into multiple graphene layers strongly enhances the
plasmonic resonance frequency and magnitude, which is fundamentally different
from that in a conventional semiconductor superlattice. This property allows us
to construct widely tunable far-infrared notch filters with 8.2 dB rejection
ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a
superlattice with merely five graphene atomic layers. Moreover, an unpatterned
superlattice shields up to 97.5% of the electromagnetic radiations below 1.2
terahertz. This demonstration also opens an avenue for the realization of other
transparent mid- and far-infrared photonic devices such as detectors,
modulators, and 3-dimensional meta-material systems.Comment: under revie
Lectin-like bacteriocins from pseudomonas spp. utilise D-rhamnose containing lipopolysaccharide as a cellular receptor
Lectin-like bacteriocins consist of tandem monocot mannose-binding domains and display a genus-specific killing activity. Here we show that pyocin L1, a novel member of this family from Pseudomonas aeruginosa, targets susceptible strains of this species through recognition of the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide that is predominantly a homopolymer of d-rhamnose. Structural and biophysical analyses show that recognition of CPA occurs through the C-terminal carbohydrate-binding domain of pyocin L1 and that this interaction is a prerequisite for bactericidal activity. Further to this, we show that the previously described lectin-like bacteriocin putidacin L1 shows a similar carbohydrate-binding specificity, indicating that oligosaccharides containing d-rhamnose and not d-mannose, as was previously thought, are the physiologically relevant ligands for this group of bacteriocins. The widespread inclusion of d-rhamnose in the lipopolysaccharide of members of the genus Pseudomonas explains the unusual genus-specific activity of the lectin-like bacteriocins
Purification and bioactivity of exendin-4, a peptide analogue of GLP-1, expressed in Pichia pastoris
Exendin-4, a peptide analogue of glucagon-like peptide-1 (GLP-1), has been developed for treatment of type 2 diabetes. Herein, the secretive exendin-4 fusion protein, expressed by methanol induction in Pichia pastoris system, was purified to homogeneity by chromatography followed by enterokinase cleavage of the fusion protein and subsequent purification of the recombinant exendin-4. Purity of the recombinant exendin-4 was 95.6%. Bioactivity assay revealed that it had glucose-lowering and insulin-releasing action in vivo
The path towards high-contrast imaging with the VLTI: the Hi-5 project
This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this recordThe development of high-contrast capabilities has long been recognized as one of the top priorities for the VLTI. As of today, the VLTI routinely achieves contrasts of a few 10− 3in the near-infrared with PIONIER (H band) and GRAVITY (K band). Nulling interferometers in the northern hemisphere and non-redundant aperture masking experiments have, however, demonstrated that contrasts of at least a few 10− 4are within reach using specific beam combination and data acquisition techniques. In this paper, we explore the possibility to reach similar or higher contrasts on the VLTI. After reviewing the state-of-the-art in high-contrast infrared interferometry, we discuss key features that made the success of other high-contrast interferometric instruments (e.g., integrated optics, nulling, closure phase, and statistical data reduction) and address possible avenues to improve the contrast of the VLTI by at least one order of magnitude. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 (Formula presented.)m), a sweet spot to image and characterize young extra-solar planetary systems. Finally, we address the science cases of a high-contrast VLTI imaging instrument and focus particularly on exoplanet science (young exoplanets, planet formation, and exozodiacal disks), stellar physics (fundamental parameters and multiplicity), and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed. This project is called Hi-5 for High-contrast Interferometry up to 5 μm.The authors acknowledge the support from the H2020 OPTICON Joint Research Network. DD and OA thank the Belgian national funds for scientific research (FNRS). SK acknowledges support from an ERC Starting Grant (Grant Agreement No. 639889) and STFC Rutherford Fellowship (ST/J004030/1)
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