9,825 research outputs found
Linearizing nonlinear optics
In the framework of linear optics, light fields do not interact with each
other in a medium. Yet, when their field amplitude becomes comparable to the
electron binding energies of matter, the nonlinear motion of these electrons
emits new dipole radiation whose amplitude, frequency and phase differ from the
incoming fields. Such high fields are typically achieved with ultra-short,
femtosecond (1fs = 10-15 sec.) laser pulses containing very broad frequency
spectra. Here, the matter not only couples incoming and outgoing fields but
also causes different spectral components to interact and mix through a
convolution process. In this contribution, we describe how frequency domain
nonlinear optics overcomes the shortcomings arising from this convolution in
conventional time domain nonlinear optics1. We generate light fields with
previously inaccessible properties because the uncontrolled coupling of
amplitudes and phases is turned off. For example, arbitrary phase functions are
transferred linearly to the second harmonic frequency while maintaining the
exact shape of the input power spectrum squared.
This nonlinear control over output amplitudes and phases opens up new avenues
for applications based on manipulation of coherent light fields. One could
investigate c.f. the effect of tailored nonlinear perturbations on the
evolution of discrete eigenmodes in Anderson localization2. Our approach might
also open a new chapter for controlling electronic and vibrational couplings in
2D-spectroscopy3 by the geometrical optical arrangement
Decoupling frequencies, amplitudes and phases in nonlinear optics
In linear optics, light fields do not mutually interact in a medium. However, they do mix when their field strength becomes comparable to electron binding energies in the so-called nonlinear optical regime. Such high fields are typically achieved with ultra-short laser pulses containing very broad frequency spectra where their amplitudes and phases are mutually coupled in a convolution process. Here, we describe a regime of nonlinear interactions without mixing of different frequencies. We demonstrate both in theory and experiment how frequency domain nonlinear optics overcomes the shortcomings arising from the convolution in conventional time domain interactions. We generate light fields with previously inaccessible properties by avoiding these uncontrolled couplings. Consequently, arbitrary phase functions are transferred linearly to other frequencies while preserving the general shape of the input spectrum. As a powerful application, we introduce deep UV phase control at 207 nm by using a conventional NIR pulse shaper
The Complement of the Djokovic-Winkler Relation
The Djokovi\'{c}-Winkler relation is a binary relation defined on
the edge set of a given graph that is based on the distances of certain
vertices and which plays a prominent role in graph theory. In this paper, we
explore the relatively uncharted ``reflexive complement'' of
, where if and only if or for edges and . We establish the relationship between
and the set , comprising the distances between
the vertices of and and shed some light on the intricacies of its
transitive closure . Notably, we demonstrate that
exhibits multiple equivalence classes only within a
restricted subclass of complete multipartite graphs. In addition, we
characterize non-trivial relations that coincide with as
those where the graph representation is disconnected, with each connected
component being the (join of) Cartesian product of complete graphs. The latter
results imply, somewhat surprisingly, that knowledge about the distances
between vertices is not required to determine . Moreover,
has either exactly one or three equivalence classes
Antiferromagnetic interlayer exchange coupling across an amorphous metallic spacer layer
By means of magneto-optical Kerr effect we observe for the first time
antiferromagnetic coupling between ferromagnetic layers across an amorphous
metallic spacer layer. Biquadratic coupling occurs at the transition from a
ferromagnetically to an antiferromagnetically coupled region. Scanning
tunneling microscopy images of all involved layers are used to extract
thickness fluctuations and to verify the amorphous state of the spacer. The
observed antiferromagnetic coupling behavior is explained by RKKY interaction
taking into account the amorphous structure of the spacer material.Comment: Typset using RevTex, 4 pages with 4 figures (.eps
Chemicals Possessing a Neurotrophin-Like Activity on Dopaminergic Neurons in Primary Culture
BACKGROUND:Neurotrophic factors have been shown to possess strong neuroprotective and neurorestaurative properties in Parkinson's disease patients. However the issues to control their delivery into the interest areas of the brain and their surgical administration linked to their unability to cross the blood brain barrier are many drawbacks responsible of undesirable side effects limiting their clinical use. A strategy implying the use of neurotrophic small molecules could provide an interesting alternative avoiding neurotrophin administration and side effects. In an attempt to develop drugs mimicking neurotrophic factors, we have designed and synthesized low molecular weight molecules that exhibit neuroprotective and neuritogenic potential for dopaminergic neurons. PRINCIPAL FINDINGS:A cell-based screening of an in-house quinoline-derived compound collection led to the characterization of compounds exhibiting both activities in the nanomolar range on mesencephalic dopaminergic neurons in spontaneous or 1-methyl-4-phenylpyridinium (MPP(+))-induced neurodegeneration. This study provides evidence that rescued neurons possess a functional dopamine transporter and underlines the involvement of the extracellular signal-regulated kinase 1/2 signaling pathway in these processes. CONCLUSION:Cell-based screening led to the discovery of a potent neurotrophic compound possessing expected physico-chemical properties for blood brain barrier penetration as a serious candidate for therapeutic use in Parkinson disease
Clinically early-stage CSPα mutation carrier exhibits remarkable terminal stage neuronal pathology with minimal evidence of synaptic loss
Autosomal dominant adult-onset neuronal ceroid lipofuscinosis (AD-ANCL) is a multisystem disease caused by mutations in the DNAJC5 gene. DNAJC5 encodes Cysteine String Protein-alpha (CSPα), a putative synaptic protein. AD-ANCL has been traditionally considered a lysosomal storage disease based on the intracellular accumulation of ceroid material. Here, we report for the first time the pathological findings of a patient in a clinically early stage of disease, which exhibits the typical neuronal intracellular ceroid accumulation and incipient neuroinflammation but no signs of brain atrophy, neurodegeneration or massive synaptic loss. Interestingly, we found minimal or no apparent reductions in CSPα or synaptophysin in the neuropil. In contrast, brain homogenates from terminal AD-ANCL patients exhibit significant reductions in SNARE-complex forming presynaptic protein levels, including a significant reduction in CSPα and SNAP-25. Frozen samples for the biochemical analyses of synaptic proteins were not available for the early stage AD-ANLC patient. These results suggest that the degeneration seen in the patients with AD-ANCL reported here might be a consequence of both the early effects of CSPα mutations at the cellular soma, most likely lysosome function, and subsequent neuronal loss and synaptic dysfunction
Preliminary Spectral Analysis of the Type II Supernova 1999em
We have calculated fast direct spectral model fits to two early-time spectra
of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The
first is an extremely early blue optical spectrum and the second a combined HST
and optical spectrum obtained one week later. Spectroscopically this supernova
appears to be a normal Type II and these fits are in excellent agreement with
the observed spectra. Our direct analysis suggests the presence of enhanced
nitrogen. We have further studied these spectra with the full NLTE general
model atmosphere code PHOENIX. While we do not find confirmation for enhanced
nitrogen (nor do we rule it out), we do require enhanced helium. An even more
intriguing possible line identification is complicated Balmer and He I lines,
which we show falls naturally out of the detailed calculations with a shallow
density gradient. We also show that very early spectra such as those presented
here combined with sophisticated spectral modeling allows an independent
estimate of the total reddening to the supernova, since when the spectrum is
very blue, dereddening leads to changes in the blue flux that cannot be
reproduced by altering the ``temperature'' of the emitted radiation. These
results are extremely encouraging since they imply that detailed modeling of
early spectra can shed light on both the abundances and total extinction of SNe
II, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54
Spin Reorientations Induced by Morphology Changes in Fe/Ag(001)
By means of magneto-optical Kerr effect we observe spin reorientations from
in-plane to out-of-plane and vice versa upon annealing thin Fe films on Ag(001)
at increasing temperatures. Scanning tunneling microscopy images of the
different Fe films are used to quantify the surface roughness. The observed
spin reorientations can be explained with the experimentally acquired roughness
parameters by taking into account the effect of roughness on both the magnetic
dipolar and the magnetocrystalline anisotropy.Comment: 4 pages with 3 EPS figure
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