350 research outputs found
Transparent Perfect Mirror
A mirror that reflects light fully and yet is transparent appears
paradoxical. Current so-called transparent or "one-way" mirrors are not
perfectly reflective and thus can be distinguished from a standard mirror.
Constructing a transparent "perfect" mirror has profound implications for
security, privacy, and camouflage. However, such a hypothetical device cannot
be implemented in a passive structure. We demonstrate here a transparent
perfect mirror in a non-Hermitian configuration: an active optical cavity where
a certain prelasing gain extinguishes Poynting's vector at the device entrance.
At this threshold, all vestiges of the cavity's structural resonances are
eliminated and the device presents spectrally flat unity-reflectivity, thus,
becoming indistinguishable from a perfect mirror when probed optically across
the gain bandwidth. Nevertheless, the device is rendered transparent by virtue
of persisting amplified transmission resonances. We confirm these predictions
in two photonic realizations: a compact integrated active waveguide and a
macroscopic all-optical-fiber system.Comment: The paper is highlighted in Nature Photonics:
http://www.nature.com/nphoton/journal/v11/n6/full/nphoton.2017.90.html The
supplementary data is available in:
http://pubs.acs.org/doi/suppl/10.1021/acsphotonics.7b0005
Multimaterial disc-to-fiber approach to efficiently produce robust infrared fibers
A critical challenge in the fabrication of chalcogenide-glass infrared optical fibers is the need for first producing large volumes of high-purity glass-a formidable task, particularly in the case of multicomponent glasses. We describe here a procedure based on multimaterial coextrusion of a hybrid glass-polymer preform from which extended lengths of robust infrared fibers are readily drawn. Only similar to 2 g of glass is required to produce 46 m of step-index fiber with core diameters in the range 10-18 mu m. This process enables rapid prototyping of a variety of glasses for applications in the delivery of quantum cascade laser light, spectroscopy, sensing, and astronomy
The Non-BPS Black Hole Attractor Equation
We study the attractor mechanism for extremal non-BPS black holes with an
infinite throat near horizon geometry, developing, as we do so, a physical
argument as to why such a mechanism does not exist in non-extremal cases. We
present a detailed derivation of the non-supersymmetric attractor equation.
This equation defines the stabilization of moduli near the black hole horizon:
the fixed moduli take values specified by electric and magnetic charges
corresponding to the fluxes in a Calabi Yau compactification of string theory.
They also define the so-called double-extremal solutions. In some examples,
studied previously by Tripathy and Trivedi, we solve the equation and show that
the moduli are fixed at values which may also be derived from the critical
points of the black hole potential.Comment: 32 Pages, 2 Figures, LaTeX; v2: typos corrected, references adde
Molecular Dynamics Insights into the Structural and Water Transport Properties of a Forward Osmosis Polyamide Thin-Film Nanocomposite Membrane Modified with Graphene Quantum Dots
An approach combining molecular dynamics (MD) simulations and laboratory experiments was applied to provide new theoretical insights into the chemical structure of polyamide (PA) thin-film composite (TFC) membranes modified with graphene quantum dots (GQDs). Interaction energies, fractional free volumes, mean-square displacements, densities, and water diffusion coefficients were computed for PA and four likely chemical structures of the GQD-embedded PA membranes. These theoretical results aided with experimentally measured water fluxes allowed for determining the most likely structure of the GQD-PA membrane. The compatibility of the GQDs and PA chains was found to be due to the formation of hydrogen and covalent bonds to m-phenylenediamine units. The modified membrane has a higher water diffusivity but a lower overall free volume, compared to the pristine PA membrane. MD simulations in concert with laboratory experiments were found to provide a good understanding of the relationship between the microscopic characteristics and macroscopic transport properties of TFC membranes
Phenotypic and genotypic characteristics of tetracycline resistant Acinetobacter baumannii isolates from nosocomial infections at Tehran hospitals
Objective(s): To date, the most important genes responsible for tetracycline resistance among Acinetobacter baumannii isolates have been identified as tet A and tet B. This study was carried out to determine the rate of resistance to tetracycline and related antibiotics, and mechanisms of resistance. Materials and Methods: During the years 2010 and 2011, a total of 100 A. baumannii isolates were recovered from patients in different hospitals of Tehran, Iran. Antimicrobial susceptibility to tetracycline, minocycline, doxicycline and tigecycline was evaluated by E-test. Polymerase chain reaction (PCR) of the tet A and tet B genes was performed using specific primers, after which the isolates were subjected to Repetitive Extragenic Palindromic-PCR (PCR) to identify the major genotypes. Results: Of all isolates, 89 were resistant to tetracycline (MIC50 = 32 mu g/ml, MIC90 = 512 mu g/ml). Minocycline with the resistant rate of 35 (MIC50 = 16 mu g/ml, MIC90 = 32 mu g/ml) and doxicycline with the resistant rate of 25 (MIC50 = 16 mu g/ml, MIC90= 32 mu g/ml) have a good activity against A. baumannii isolates. All isolates were sensitive to tigecycline. Frequencies of tet B and tet A genes and coexistence of tet A and tet B among the isolates resistant to tetracycline, were 87.6, 2.2 and 1.1, respectively. Distribution of REP-types among A. baumannii isolates was types A (40), B (30), C (10), D (5) and E (5). Conclusion: It seems that tet A and tet B genes play an important role in the induction of resistance towards tetracyclines used in this study. It is suggested that further studies focus on other antimicrobial drugs and combinations in order to achieve a successful therapy against multi drug resistance (MDR) A. baumannii strains in Iran
BRST Quantization of Noncommutative Gauge Theories
In this paper, the BRST symmetry transformation is presented for the
noncommutative U(N) gauge theory. The nilpotency of the charge associated to
this symmetry is then proved. As a consequence for the space-like
non-commutativity parameter, the Hilbert space of physical states is determined
by the cohomology space of the BRST operator as in the commutative case.
Further, the unitarity of the S-matrix elements projected onto the subspace of
physical states is deduced.Comment: 20 pages, LaTeX, no figures, one reference added, to appear in Phys.
Rev.
Deciphering the porcine intestinal microRNA transcriptome
<p>Abstract</p> <p>Background</p> <p>While more than 700 microRNAs (miRNAs) are known in human, a comparably low number has been identified in swine. Because of the close phylogenetic distance to humans, pigs serve as a suitable model for studying e.g. intestinal development or disease. Recent studies indicate that miRNAs are key regulators of intestinal development and their aberrant expression leads to intestinal malignancy.</p> <p>Results</p> <p>Here, we present the identification of hundreds of apparently novel miRNAs in the porcine intestine. MiRNAs were first identified by means of deep sequencing followed by miRNA precursor prediction using the miRDeep algorithm as well as searching for conserved miRNAs. Second, the porcine miRNAome along the entire intestine (duodenum, proximal and distal jejunum, ileum, ascending and transverse colon) was unraveled using customized miRNA microarrays based on the identified sequences as well as known porcine and human ones. In total, the expression of 332 intestinal miRNAs was discovered, of which 201 represented assumed novel porcine miRNAs. The identified hairpin forming precursors were in part organized in genomic clusters, and most of the precursors were located on chromosomes 3 and 1, respectively. Hierarchical clustering of the expression data revealed subsets of miRNAs that are specific to distinct parts of the intestine pointing to their impact on cellular signaling networks.</p> <p>Conclusions</p> <p>In this study, we have applied a straight forward approach to decipher the porcine intestinal miRNAome for the first time in mammals using a piglet model. The high number of identified novel miRNAs in the porcine intestine points out their crucial role in intestinal function as shown by pathway analysis. On the other hand, the reported miRNAs may share orthologs in other mammals such as human still to be discovered.</p
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