1,177 research outputs found
Directional supercontinuum generation: the role of the soliton
In this paper we numerically study supercontinuum generation by pumping a
silicon nitride waveguide, with two zero-dispersion wavelengths, with
femtosecond pulses. The waveguide dispersion is designed so that the pump pulse
is in the normal-dispersion regime. We show that because of self-phase
modulation, the initial pulse broadens into the anomalous-dispersion regime,
which is sandwiched between the two normal-dispersion regimes, and here a
soliton is formed. The interaction of the soliton and the broadened pulse in
the normal-dispersion regime causes additional spectral broadening through
formation of dispersive waves by non-degenerate four-wave mixing and
cross-phase modulation. This broadening occurs mainly towards the second
normal-dispersion regime. We show that pumping in either normal-dispersion
regime allows broadening towards the other normal-dispersion regime. This
ability to steer the continuum extension towards the direction of the other
normal-dispersion regime beyond the sandwiched anomalous-dispersion regime
underlies the directional supercontinuum notation. We numerically confirm the
approach in a standard silica microstructured fiber geometry with two
zero-dispersion wavelengths
Ultra-low-noise supercontinuum generation with a flat near-zero normal dispersion fiber
A pure silica photonic crystal fiber with a group velocity dispersion
() of 4 ps/km at 1.55 m and less than 7 ps/km from 1.32
m to the zero dispersion wavelength (ZDW) 1.80 m was designed and
fabricated. The dispersion of the fiber was measured experimentally and found
to agree with the fiber design, which also provides low loss below 1.83 m
due to eight outer rings with increased hole diameter. The fiber was pumped
with a 1.55 m, 125 fs laser and, at the maximum in-coupled peak power
(P) of 9 kW, a 1.341.82 m low-noise spectrum with a relative
intensity noise below 2.2\% was measured. The numerical modeling agreed very
well with the experiments and showed that P could be increased to 26 kW
before noise from solitons above the ZDW started to influence the spectrum by
pushing high-noise dispersive waves through the spectrum
Circulating Factor Seven Activating Protease (FSAP) in the Hyperacute Phase of Stroke
Background. Factor VII activating protease (FSAP) is a circulating serine protease that could be involved in the pathophysiology of stroke. We analyzed the temporal changes in FSAP antigen and FSAP activity after acute cerebral ischemia (ACI) and tested if FSAP could be used to differentiate between stroke subtypes in the hyperacute phase (<4.5 hours after symptom onset). Methods. Of the 118 suspected stroke patients enrolled, 76 had ACI; of which 20 suffered from large vessel occlusion (LVO), 19 had intracerebral hemorrhage (ICH), and 23 had stroke mimics. Median time from symptom onset to the two plasma sample collections, <4.5 hours, were 66 and 107 minutes for the entire study population. Additional samples were collected up to 90 days post stroke in a subset of ACI patients (). FSAP antigen, FSAP activity, FSAP-α2-antiplasmin-complex (FSAP-AP complex), and nucleosomes were measured by activity assays or ELISA. Results. ACI patients treated with tissue plasminogen activator (tPA) had elevated FSAP hours () that subsequently normalized after 6 hours. FSAP-AP complex levels decreased significantly from <4.5 hours () to 6 hours after symptom onset. tPA did not increase FSAP activity significantly in plasma in vitro. FSAP antigen significantly hours after symptom onset in LVO () and ICH () patients. FSAP could not differentiate ACI from ICH or strokes (ACI and ICH) from stroke mimics. FSAP did not correlate with stroke severity. Conclusion. LVO and ICH seem to influence FSAP levels in the hyperacute phase of stroke, but FSAP does not differentiate between stroke subtypes in a hyperacute setting.publishedVersio
Detection of sub-shot-noise spatial correlation in high-gain parametric down-conversion
Using a 1GW-1ps pump laser pulse in high gain parametric down-conversion
allows us to detect sub-shot-noise spatial quantum correlation with up to one
hundred photoelectrons per mode, by means of a high efficiency CCD. The
statistics is performed in single-shot over independent spatial replica of the
system. The paper highlights the evidence of quantum correlation between
symmetrical signal and idler spatial areas in the far field, in the high gain
regime. In accordance with the predictions of numerical calculations the
observed transition from the quantum to the classical regime is interpreted as
a consequence of the narrowing of the down-converted beams in the very high
gain regime.Comment: 4,2 pages, 4 figure
Nonclassical statistics of intracavity coupled waveguides: the quantum optical dimer
A model is proposed where two nonlinear waveguides are contained
in a cavity suited for second-harmonic generation. The evanescent wave coupling
between the waveguides is considered as weak, and the interplay between this
coupling and the nonlinear interaction within the waveguides gives rise to
quantum violations of the classical limit. These violations are particularly
strong when two instabilities are competing, where twin-beam behavior is found
as almost complete noise suppression in the difference of the fundamental
intensities. Moreover, close to bistable transitions perfect twin-beam
correlations are seen in the sum of the fundamental intensities, and also the
self-pulsing instability as well as the transition from symmetric to asymmetric
states display nonclassical twin-beam correlations of both fundamental and
second-harmonic intensities. The results are based on the full quantum Langevin
equations derived from the Hamiltonian and including cavity damping effects.
The intensity correlations of the output fields are calculated
semi-analytically using a linearized version of the Langevin equations derived
through the positive-P representation. Confirmation of the analytical results
are obtained by numerical simulations of the nonlinear Langevin equations
derived using the truncated Wigner representation.Comment: 15 pages, 8 figures, submitted to Phys. Rev.
Modulation Instability of Ultrashort Pulses in Quadratic Nonlinear Media beyond the Slowly Varying Envelope Approximation
We report a modulational instability (MI) analysis of a mathematical model
appropriate for ultrashort pulses in cascaded quadratic-cubic nonlinear media
beyond the so-called slowly varying envelope approximation. Theoretically
predicted MI properties are found to be in good agreement with numerical
simulation. The study shows the possibility of controlling the generation of MI
and formation of solitons in a cascaded quadratic-cubic media in the few cycle
regimes. We also find that stable propagation of soliton-like few-cycle pulses
in the medium is subject to the fulfilment of the modulation instability
criteria
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Traps of multi-level governance. Lessons from the implementation of the Water Framework Directive in Italy
During recent decades, different patterns of multi-level governance (MLG) have spread across Europe as a consequence of Europeanisation of public policies, which have increasingly adopted decentralized and participatory procedures conceived as a tool of more effective and accountable policy-making. It appears, however, that the implementation of operational designs based on MLG may be rather problematic and it does not necessarily bring to the expected performance improvements. Referring to the case of the EU Water Framework Directive (2000/60/EC), which conceives the creation of new multi-level institutional settings as a key tool for enacting a new holistic approach to water management and protection, this article explores the difficulties that the implementation of such settings has brought in Italy, despite some favorable pre-conditions existing in the country. Evidence is provided that along with institutional and agency variables, the implementation effectiveness of MLG arrangements promoted by the EU can be challenged by their inherent characteristics
Does Health & Her app use improve menopausal symptoms? A longitudinal cohort study
Objectives: The Health & Her app provides menopausal women with a means of monitoring their symptoms, symptom triggers and menstrual periods, and enables them to engage in a variety of digital activities designed to promote well-being. This study aimed to examine whether sustained weekly engagement with the app is associated with improvements in menopausal symptoms. Design: A pre–post longitudinal cohort study. Setting: Analysed data collected from Health & Her app users. Participants: 1900 women who provided symptom data via the app across a 2-month period. Primary and secondary outcome measures: Symptom changes from baseline to 2 months was the outcome measure. A linear mixed effects model explored whether levels of weekly app engagement influenced symptom changes. Secondary analyses explored whether app-usage factors such as total number of days spent logging symptoms, reporting triggers, reporting menstrual periods and using in-app activities were independently predictive of symptom changes from baseline. Covariates included hormone replacement therapy use, hormonal contraceptive use, present comorbidities, age and dietary supplement use. Results: Findings demonstrated that greater engagement with the Health & Her app for 2 months was associated with greater reductions in symptoms over time. Daily use of in-app activities and logging symptoms and menstrual periods were each independently associated with symptom reductions. Conclusions: This study demonstrated that greater weekly engagement with the app was associated with greater reductions in symptoms. It is recommended that women be made aware of menopause-specific apps, such as that provided by Health & Her, to support them to manage their symptoms
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