69 research outputs found
High-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator
We report on a high-power mid-infrared frequency comb source based on a
femtosecond Er:fiber oscillator with a stabilized repetition rate at 250 MHz.
The mid-infrared frequency comb is produced through difference frequency
generation in a periodically poled MgO-doped lithium niobate crystal. The
output power is about 120 mW with a pulse duration of about 80 fs, and spectrum
coverage from 2.9 to 3.6 um. The coherence properties of the produced
high-power broadband mid-infrared frequency comb are maintained, which was
verified by heterodyne measurements. As the first application, the spectrum of
a ~200 ppm methane-air mixture in a short 20 cm glass cell at ambient
atmospheric pressure and temperature was measured.Comment: 3 pages, 5 figure
Transverse and longitudinal characterization of electron beams using interaction with optical near-fields
We demonstrate an experimental technique for both transverse and longitudinal
characterization of bunched femtosecond free electron beams. The operation
principle is based on monitoring of the current of electrons that obtained an
energy gain during the interaction with the synchronized optical near-field
wave excited by femtosecond laser pulses. The synchronous
accelerating/decelerating fields confined to the surface of a silicon
nanostructure are characterized using a highly focused sub-relativistic
electron beam. Here the transverse spatial resolution of 450 nm and femtosecond
temporal resolution achievable by this technique are demonstrated
Pulsed sputtering during homoepitaxial surface growth: layer-by-layer forever
The homoepitaxial growth of initially flat surfaces has so far always led to
surfaces which become rougher and rougher as the number of layers increases:
even in systems exhibiting ``layer by layer'' growth the registry of the layers
is gradually lost. We propose that pulsed glancing-angle sputtering, once per
monolayer, can in principle lead to layer-by-layer growth that continues
indefinitely, if one additional parameter is controlled. We illustrate our
suggestion with a fairly realistic simulation of the growth of a Pt (111)
surface, coupled with a simplified model for the sputtering process.Comment: Five pages, four figure
Biomarkers of human gastrointestinal tract regions
Dysregulation of intestinal epithelial cell performance is associated with an array of pathologies whose onset mechanisms are incompletely understood. While whole-genomics approaches have been valuable for studying the molecular basis of several intestinal diseases, a thorough analysis of gene expression along the healthy gastrointestinal tract is still lacking. The aim of this study was to map gene expression in gastrointestinal regions of healthy human adults and to implement a procedure for microarray data analysis that would allow its use as a reference when screening for pathological deviations. We analyzed the gene expression signature of antrum, duodenum, jejunum, ileum, and transverse colon biopsies using a biostatistical method based on a multivariate and univariate approach to identify region-selective genes. One hundred sixty-six genes were found responsible for distinguishing the five regions considered. Nineteen had never been described in the GI tract, including a semaphorin probably implicated in pathogen invasion and six novel genes. Moreover, by crossing these genes with those retrieved from an existing data set of gene expression in the intestine of ulcerative colitis and Crohn's disease patients, we identified genes that might be biomarkers of Crohn's and/or ulcerative colitis in ileum and/or colon. These include CLCA4 and SLC26A2, both implicated in ion transport. This study furnishes the first map of gene expression along the healthy human gastrointestinal tract. Furthermore, the approach implemented here, and validated by retrieving known gene profiles, allowed the identification of promising new leads in both healthy and disease state
Soliton quantization and internal symmetry
We apply the method of collective coordinate quantization to a model of
solitons in two spacetime dimensions with a global symmetry. In
particular we consider the dynamics of the charged states associated with
rotational excitations of the soliton in the internal space and their
interactions with the quanta of the background field (mesons). By solving a
system of coupled saddle-point equations we effectively sum all tree-graphs
contributing to the one-point Green's function of the meson field in the
background of a rotating soliton. We find that the resulting one-point function
evaluated between soliton states of definite charge exhibits a pole on
the meson mass shell and we extract the corresponding S-matrix element for the
decay of an excited state via the emission of a single meson using the standard
LSZ reduction formula. This S-matrix element has a natural interpretation in
terms of an effective Lagrangian for the charged soliton states with an
explicit Yukawa coupling to the meson field. We calculate the leading-order
semi-classical decay width of the excited soliton states discuss the
consequences of these results for the hadronic decay of the resonance
in the Skyrme model.Comment: 23 pages, LA-UR-93-299
Skyrmion Quantization and the Decay of the Delta
We present the complete solution to the so-called ``Yukawa problem'' of the
Skyrme model. This refers to the perceived difficulty of reproducing---purely
from soliton physics---the usual pseudovector pion-nucleon coupling, echoed by
pion coupling to the higher spin/isospin baryons in a manner fixed by large- group theory. The solution involves
surprisingly elegant interplay between the classical and quantum properties of
a new configuration, the ``new improved skyrmion''. This is the near-hedgehog
obtained by minimizing the usual skyrmion mass functional augmented by an
all-important isorotational kinetic term. The numerics are pleasing: a
decay width within a few MeV of its measured value, and furthermore, the
higher-spin baryons with widths so large ()
that these undesirable large- artifacts effectively drop out of the
spectrum, and pose no phenomenological problem. Beyond these specific results,
we ground the Skyrme model in the Feynman Path Integral, and set up a
transparent collective coordinate formalism that makes maximal use of the
expansion. This approach elucidates the connection between skyrmions on
the one hand, and Feynman diagrams in an effective field theory on the other.Comment: This TeX file inputs the macropackage harvmac.tex . Choose the ``b''
(big) option or equations will overrun
Nucleon Charge and Magnetization Densities from Sachs Form Factors
Relativistic prescriptions relating Sachs form factors to nucleon charge and
magnetization densities are used to fit recent data for both the proton and the
neutron. The analysis uses expansions in complete radial bases to minimize
model dependence and to estimate the uncertainties in radial densities due to
limitation of the range of momentum transfer. We find that the charge
distribution for the proton is significantly broad than its magnetization
density and that the magnetization density is slightly broader for the neutron
than the proton. The neutron charge form factor is consistent with the Galster
parametrization over the available range of Q^2, but relativistic inversion
produces a softer radial density. Discrete ambiguities in the inversion method
are analyzed in detail. The method of Mitra and Kumari ensures compatibility
with pQCD and is most useful for extrapolating form factors to large Q^2.Comment: To appear in Phys. Rev. C. Two new figures and accompanying text have
been added and several discussions have been clarified with no significant
changes to the conclusions. Now contains 47 pages including 21 figures and 2
table
Advancement into the Arctic Region for Bioactive Sponge Secondary Metabolites
Porifera have long been a reservoir for the discovery of bioactive compounds and drug discovery. Most research in the area has focused on sponges from tropical and temperate waters, but more recently the focus has shifted to the less accessible colder waters of the Antarctic and, to a lesser extent, the Arctic. The Antarctic region in particular has been a more popular location for natural products discovery and has provided promising candidates for drug development. This article reviews groups of bioactive compounds that have been isolated and reported from the southern reaches of the Arctic Circle, surveys the known sponge diversity present in the Arctic waters, and details a recent sponge collection by our group in the Aleutian Islands, Alaska. The collection has yielded previously undescribed sponge species along with primary activity against opportunistic infectious diseases, malaria, and HCV. The discovery of new sponge species and bioactive crude extracts gives optimism for the isolation of new bioactive compounds from a relatively unexplored source
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