526 research outputs found
Point-by-point inscription of apodized fiber Bragg gratings
We demonstrate apodized fiber Bragg gratings inscribed with a point-by-point
technique. We tailor the grating phase and coupling amplitude through precise
control over the longitudinal and transverse position of each laser-inscribed
modification. This method of apodization is facilitated by the
highly-localized, high-contrast modifications generated by focussed IR
femtosecond laser inscription. Our technique provides a simple method for the
design and implementation of point-by-point fiber Bragg gratings with complex
apodization profiles.Comment: 6 pages, 4 figures, article in revie
Virtual Shaping on NACA 0015 by Means of a High Momentum Coefficient Synthetic Jet
Results concerning flow control on a NACA 0015 airfoil using high power synthetic jets are presented for low incidences and for Reynolds numbers ranging from 132000 to 425000. The forcing was operated through a spanwise slit positioned near the leading edge at x/c=1.25% or at x/c=10% on the upper surface. Static pressure distribution measurements around the airfoil, wake surveys and smoke flow visualizations were performed. Pressure distributions were significantly modified around the injection location, showing an area of intense suction which increased the lift and strongly affected the drag. Flow visualizations highlighted that the intense suction was due to a virtual shaping effect caused by the formation of a recirculation bubble capable of displacing the streamlines. Low momentum deficits in the wake velocity distributions and, in certain conditions, jet-like flow was observed for the forced cases. Finally, a scaling law relating the bubble size to the forcing intensity is propose
A pattern matching technique for measuring sediment displacement levels
This paper describes a novel technique for obtaining accurate, high (spatial) resolution measurements of sediment redeposition levels. A sequence of different random patterns are projected onto a sediment layer and captured using a high-resolution camera, producing a set of reference images. The same patterns are used to obtain a corresponding sequence of deformed images after a region of the sediment layer has been displaced and redeposited, allowing the use of a high-accuracy pattern matching algorithm to quantify the distribution of the redeposited sediment. A set of experiments using the impact of a vortex ring with a glass ballotini particle layer as the resuspension mechanism are described to test and illustrate the technique. The accuracy of the procedure is assessed using a known crater profile, manufactured to simulate the features of the craters observed in the experiments
The Sunyaev-Zel'dovich Infrared Experiment: A Millimeter-wave Receiver for Cluster Cosmology
Measurements of the Sunyaev-Zel'dovich (S-Z) effect towards distant clusters
of galaxies can be used to determine the Hubble constant and the radial
component of cluster peculiar velocities. Determination of the cluster peculiar
velocity requires the separation of the two components of the S-Z effect, which
are due to the thermal and bulk velocities of the intracluster plasma. The two
components can be separated practically only at millimeter (mm) wavelengths.
Measurements of the S-Z effect at mm wavelengths are subject to minimal
astrophysical confusion and, therefore, provide an important test of results
obtained at longer wavelengths. We describe the instrument used to make the
first significant detections of the S-Z effect at millimeter wavelengths. This
instrument employs new filter, detector, and readout technologies to produce
sensitive measurements of differential sky brightness stable on long time
scales. These advances allow drift scan observations which achieve high
sensitivity while minimizing common sources of systematic error.Comment: 19 pages, 15 postscript figures, LaTeX(aaspptwo.sty), ApJ(in press
Confinement of fractional excitations in a triangular lattice antiferromagnet
High-resolution neutron and THz spectroscopies are used to study the magnetic
excitation spectrum of CsCoBr, a distorted-triangular-lattice
antiferromagnet with nearly XY-type anisotropy. What was previously thought of
as a broad excitation continuum [Phys. Rev. Lett. 129, 087201 (2022)] is shown
to be a series of dispersive bound states reminiscent of "Zeeman ladders" in
quasi-one-dimensional Ising systems. At wave vectors where inter-chain
interactions cancel at the Mean Field level, they can indeed be interpreted as
bound finite-width kinks in individual chains. Elsewhere in the Brillouin zone
their true two-dimensional structure and propagation are revealed.Comment: 11 pages, 8 figure
Three-dimensional femtosecond laser nanolithography of crystals
Nanostructuring hard optical crystals has so far been exclusively feasible at
their surface, as stress induced crack formation and propagation has rendered
high precision volume processes ineffective. We show that the inner chemical
etching reactivity of a crystal can be enhanced at the nanoscale by more than
five orders of magnitude by means of direct laser writing. The process allows
to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm
feature sizes inside large crystals in absence of brittle fracture. To showcase
the unique potential of the technique, we fabricate photonic structures such as
sub-wavelength diffraction gratings and nanostructured optical waveguides
capable of sustaining sub-wavelength propagating modes inside yttrium aluminum
garnet crystals. This technique could enable the transfer of concepts from
nanophotonics to the fields of solid state lasers and crystal optics.Comment: Submitted Manuscript and Supplementary Informatio
Attenuation technique for measuring sediment displacement levels
A technique for obtaining accurate, high (spatial) resolution measurements of sediment redeposition levels is described. In certain regimes, the method may also be employed to provide measurements of sediment layer thickness as a function of time. The method uses a uniform light source placed beneath the layer, consisting of transparent particles, so that the intensity of light at a point on the surface of the layer can be related to the depth of particles at that point. A set of experiments, using the impact of a vortex ring with a glass ballotini particle layer as the resuspension mechanism, are described to test and illustrate the technique
Genes Involved in the Balance between Neuronal Survival and Death during Inflammation
Glucocorticoids are potent regulators of the innate immune response, and alteration in this inhibitory feedback has detrimental consequences for the neural tissue. This study profiled and investigated functionally candidate genes mediating this switch between cell survival and death during an acute inflammatory reaction subsequent to the absence of glucocorticoid signaling. Oligonucleotide microarray analysis revealed that following lipopolysaccharide (LPS) intracerebral administration at striatum level, more modulated genes presented transcription impairment than exacerbation upon glucocorticoid receptor blockage. Among impaired genes we identified ceruloplasmin (Cp), which plays a key role in iron metabolism and is implicated in a neurodegenative disease. Microglial and endothelial induction of Cp is a natural neuroprotective mechanism during inflammation, because Cp-deficient mice exhibited increased iron accumulation and demyelination when exposed to LPS and neurovascular reactivity to pneumococcal meningitis. This study has identified genes that can play a critical role in programming the innate immune response, helping to clarify the mechanisms leading to protection or damage during inflammatory conditions in the CNS
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