3,289 research outputs found
A phase-space approach to directional switching in semiconductor ring lasers
We show that a topological investigation of the phase space of a
Semiconductor Ring Laser can be used to devise switching schemes which are
alternative to optical pulse injection of counter-propagating light. To provide
physical insight in these switching mechanisms, a full bifurcation analysis and
an investigation of the topology is performed on a two-dimensional asymptotic
model. Numerical simulations confirm the topological predictions.Comment: 9 pages, 7 figure
Kinetics of the neutralizing antibody response to respiratory syncytial virus infections in a birth cohort
The kinetics of respiratory syncytial virus (RSV) neutralizing antibodies following birth, primary and secondary infections are poorly defined. The aims of the study were to measure and compare neutralizing antibody responses at different time points in a birth cohort followed-up over three RSV epidemics. Rural Kenyan children, recruited at birth between 2002 and 2003, were monitored for RSV infection over three epidemic seasons. Cord and 3-monthly sera, and acute and convalescent sera following RSV infection, were assayed in 28 children by plaque reduction neutralization test (PRNT). Relative to the neutralizing antibody titers of pre-exposure control sera (1.8 log10 PRNT), antibody titers following primary infection were (i) no different in sera collected between 0 and 0.4 months post-infection (1.9 log10 PRNT, P = 0.146), (ii) higher in sera collected between 0.5 and 0.9 (2.8 log10 PRNT, P < 0.0001), 1.0–1.9 (2.5 log10 PRNT, P < 0.0001), and 2.0–2.9 (2.3 log10 PRNT, P < 0.001) months post-infection, and (iii) no different in sera collected at between 3.0 and 3.9 months post-infection (2.0 log10 PRNT, P = 0.052). The early serum neutralizing response to secondary infection (3.02 log10 PRNT) was significantly greater than the early primary response (1.9 log10 PRNT, P < 0.0001). Variation in population-level virus transmission corresponded with changes in the mean cohort-level neutralizing titers. It is concluded that following primary RSV infection the neutralizing antibody response declines to pre-infection levels rapidly (∼3 months) which may facilitate repeat infection. The kinetics of the aggregate levels of acquired antibody reflect seasonal RSV occurrence, age, and infection history
Single voxel magnetic resonance spectroscopy in distinguishing focal neoplastic from non-neoplatic brain lesions
Objective: Assess diagnostic utility of combined magnetic resonance imaging and magnetic resonance spectroscopy (MRI, MRS) in differentiating focal neoplastic lesions from focal non- neoplastic (infective or degenerative) brain lesions.Design: Descriptive, analytical - prospective study.Setting: The Aga Khan University MRI department.Subject: Seventy four consecutive patients.Main outcome measures: Kappa measurement of agreement was used to determine the agreement between MRI and MRI, MRS with the final diagnosis. Sensitivity, specificity, positive predictive value, negative predictive value and accuracy of the two tests were calculated. The difference between the number of indeterminate lesions in the twotests was determined. Logistic regression demonstrated the role of confounding factors in the diagnostic use of MRS.Results: MRI, MRS had a higher agreement with the final diagnosis than MRI in isolation. The sensitivity of MRI, MRS was 4.82 times greater than that of MRI. MRI, MRS had a 1.7% increase in accuracy. MRI, MRS reduced the indeterminate MRI lesions by 5.4%. Logistic regression showed that for lesions which were enhancing, MRS yield was more helpful if the voxel position included the enhancing part.Conclusion: MRI,MRS is better than MRI alone in characterisation of neoplastic from non- neoplastic focal brain lesions
Excitability in semiconductor microring lasers: Experimental and theoretical pulse characterization
We characterize the operation of semiconductor microring lasers in an
excitable regime. Our experiments reveal a statistical distribution of the
characteristics of noise-triggered optical pulses that is not observed in other
excitable systems. In particular, an inverse correlation exists between the
pulse amplitude and duration. Numerical simulations and an interpretation in an
asymptotic phase space confirm and explain these experimentally observed pulse
characteristics.Comment: 9 pages, 10 figure
Exploring multi-stability in semiconductor ring lasers: theory and experiment
We report the first experimental observation of multi-stable states in a
single-longitudinal mode semiconductor ring laser. We show how the operation of
the device can be steered to either monostable, bistable or multi-stable
dynamical regimes in a controlled way. We observe that the dynamical regimes
are organized in well reproducible sequences that match the bifurcation
diagrams of a two-dimensional model. By analyzing the phase space in this
model, we predict how the stochastic transitions between multi-stable states
take place and confirm it experimentally.Comment: 4 pages, 5 figure
Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
Through a survey of (sub-)millimetre emission lines of various
sulphur-bearing molecules, we aim to determine which molecules are the primary
carriers of sulphur in different types of AGB stars. In this paper, the first
in a series, we investigate the occurrence of HS in AGB circumstellar
envelopes and determine its abundance, where possible. We have surveyed 20 AGB
stars with a range of mass-loss rates and of different chemical types using the
APEX telescope to search for rotational transition lines of five key
sulphur-bearing molecules: CS, SiS, SO, SO and HS. Here we present our
results for HS, including detections, non-detections and detailed radiative
transfer modelling of the detected lines. We compare results based on different
descriptions of the molecular excitation of HS and different abundance
distributions, including those derived from chemical modelling results. We
detected HS towards five AGB stars, all of which have high mass-loss rates
of yr and are oxygen-rich. HS
was not detected towards the carbon or S-type stars that fall in a similar
mass-loss range. For the stars in our sample with detections, we find peak
o-HS abundances relative to H between and . Overall, we conclude that HS can play a significant role in
oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a
key role in stars of other chemical types or the lower mass-loss rate
oxygen-rich stars. For two sources, V1300 Aql and GX Mon, HS is most likely
the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&
P02-05. All-trans retinoic acid during vaccination increases Ag-specific CD8 T cell homing to mucosal sites and improves recall responses to vaginal challenge
Impact of nonlocal interactions in dissipative systems: towards minimal-sized localized structures
In order to investigate the size limit on spatial localized structures in a
nonlinear system, we explore the impact of linear nonlocality on their domains
of existence and stability. Our system of choice is an optical microresonator
containing an additional metamaterial layer in the cavity, allowing the
nonlocal response of the material to become the dominating spatial process. In
that case, our bifurcation analysis shows that this nonlocality imposes a new
limit on the width of localized structures going beyond the traditional
diffraction limit.Comment: 4 pages, 4 figure
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