4,890 research outputs found
Tailoring and enhancing spontaneous two-photon emission processes using resonant plasmonic nanostructures
The rate of spontaneous emission is known to depend on the environment of a
light source, and the enhancement of one-photon emission in a resonant cavity
is known as the Purcell effect. Here we develop a theory of spontaneous
two-photon emission for a general electromagnetic environment including
inhomogeneous dispersive and absorptive media. This theory is used to evaluate
the two-photon Purcell enhancement in the vicinity of metallic nanoparticles
and it is demonstrated that the surface plasmon resonances supported by these
particles can enhance the emission rate by more than two orders of magnitude.
The control over two-photon Purcell enhancement given by tailored
nanostructured environments could provide an emitter with any desired spectral
response and may serve as an ultimate route for designing light sources with
novel properties
Microscopic model of Purcell enhancement in hyperbolic metamaterials
We study theoretically a dramatic enhancement of spontaneous emission in
metamaterials with the hyperbolic dispersion modeled as a cubic lattice of
anisotropic resonant dipoles. We analyze the dependence of the Purcell factor
on the source position in the lattice unit cell and demonstrate that the
optimal emitter position to achieve large Purcell factors and Lamb shifts are
in the local field maxima. We show that the calculated Green function has a
characteristic cross-like shape, spatially modulated due to structure
discreteness. Our basic microscopic theory provides fundamental insights into
the rapidly developing field of hyperbolic metamaterials.Comment: 9 pages, 11 figure
Purcell effect in Hyperbolic Metamaterial Resonators
The radiation dynamics of optical emitters can be manipulated by properly
designed material structures providing high local density of photonic states, a
phenomenon often referred to as the Purcell effect. Plasmonic nanorod
metamaterials with hyperbolic dispersion of electromagnetic modes are believed
to deliver a significant Purcell enhancement with both broadband and
non-resonant nature. Here, we have investigated finite-size cavities formed by
nanorod metamaterials and shown that the main mechanism of the Purcell effect
in these hyperbolic resonators originates from the cavity hyperbolic modes,
which in a microscopic description stem from the interacting cylindrical
surface plasmon modes of the finite number of nanorods forming the cavity. It
is found that emitters polarized perpendicular to the nanorods exhibit strong
decay rate enhancement, which is predominantly influenced by the rod length. We
demonstrate that this enhancement originates from Fabry-Perot modes of the
metamaterial cavity. The Purcell factors, delivered by those cavity modes,
reach several hundred, which is 4-5 times larger than those emerging at the
epsilon near zero transition frequencies. The effect of enhancement is less
pronounced for dipoles, polarized along the rods. Furthermore, it was shown
that the Purcell factor delivered by Fabry-Perot modes follows the dimension
parameters of the array, while the decay rate in the epsilon near-zero regime
is almost insensitive to geometry. The presented analysis shows a possibility
to engineer emitter properties in the structured metamaterials, addressing
their microscopic structure
Brain charts for the human lifespan
Over the past few decades, neuroimaging has become a ubiquitous tool in basic
research and clinical studies of the human brain. However, no reference standards
currently exist to quantify individual diferences in neuroimaging metrics over time,
in contrast to growth charts for anthropometric traits such as height and weight1
.
Here we assemble an interactive open resource to benchmark brain morphology
derived from any current or future sample of MRI data (http://www.brainchart.io/).
With the goal of basing these reference charts on the largest and most inclusive
dataset available, acknowledging limitations due to known biases of MRI studies
relative to the diversity of the global population, we aggregated 123,984 MRI scans,
across more than 100 primary studies, from 101,457 human participants between 115
days post-conception to 100 years of age. MRI metrics were quantifed by centile
scores, relative to non-linear trajectories2
of brain structural changes, and rates of
change, over the lifespan. Brain charts identifed previously unreported neurodevelo pmental milestones3
, showed high stability of individuals across longitudinal
assessments, and demonstrated robustness to technical and methodological
diferences between primary studies. Centile scores showed increased heritability
compared with non-centiled MRI phenotypes, and provided a standardized measure
of atypical brain structure that revealed patterns of neuroanatomical variation across
neurological and psychiatric disorders. In summary, brain charts are an essential step
towards robust quantifcation of individual variation benchmarked to normative
trajectories in multiple, commonly used neuroimaging phenotypes
Frustration - how it can be measured
A misfit parameter is used to characterize the degree of frustration of
ordered and disordered systems. It measures the increase of the ground-state
energy due to frustration in comparison with that of a relevant reference
state. The misfit parameter is calculated for various spin-glass models. It
allows one to compare these models with each other. The extension of this
concept to other combinatorial optimization problems with frustration, e.g.
p-state Potts glasses, graph-partitioning problems and coloring problems is
given.Comment: 10 pages, 1 table, no figures, uses revtex.st
ETL-0254, Terrain analysis procedural guide for soil, February 1981
This report is one in a series of terrain analysis procedural guides being developed in support of the Topographic Support System (TSS). It was written specifically for a U.S. Army terrain analyst and presents the step-by- step methods needed for extraction, reducing, and recording soil information on a factor overlay and supporting data table. It is a contribution to the Department of Defense terrain intelligence effort.
The report contains a detailed bibliography and a lengthy glossary
ETL-0254, Terrain analysis procedural guide for soil, February 1981
This report is one in a series of terrain analysis procedural guides being developed in support of the Topographic Support System (TSS). It was written specifically for a U.S. Army terrain analyst and presents the step-by- step methods needed for extraction, reducing, and recording soil information on a factor overlay and supporting data table. It is a contribution to the Department of Defense terrain intelligence effort.
The report contains a detailed bibliography and a lengthy glossary
Vaccination Against Amyloidogenic Aggregates in Pancreatic Islets Prevents Development of Type 2 Diabetes Mellitus
Type 2 diabetes mellitus (T2DM) is a chronic progressive disease characterized by insulin resistance and insufficient insulin secretion to maintain normoglycemia. The majority of T2DM patients bear amyloid deposits mainly composed of islet amyloid polypeptide (IAPP) in their pancreatic islets. These-originally β-cell secretory products-extracellular aggregates are cytotoxic for insulin-producing β-cells and are associated with β-cell loss and inflammation in T2DM advanced stages. Due to the absence of T2DM preventive medicaments and the presence of only symptomatic drugs acting towards increasing hormone secretion and action, we aimed at establishing a novel disease-modifying therapy targeting the cytotoxic IAPP deposits in order to prevent the development of T2DM. We generated a vaccine based on virus-like particles (VLPs), devoid of genomic material, coupled to IAPP peptides inducing specific antibodies against aggregated, but not monomeric IAPP. Using a mouse model of islet amyloidosis, we demonstrate in vivo that our vaccine induced a potent antibody response against aggregated, but not soluble IAPP, strikingly preventing IAPP depositions, delaying onset of hyperglycemia and the induction of the associated pro-inflammatory cytokine Interleukin 1β (IL-1β). We offer the first cost-effective and safe disease-modifying approach targeting islet dysfunction in T2DM, preventing pathogenic aggregates without disturbing physiological IAPP function.These studies were funded by a project grant from the Swiss National Foundation (SNF). We acknowledge the technical assistance of Sydney W. Pence and Faith Slubowski at the Institute of Veterinary Physiology, University of Zürich. We appreciate the kind possibility given by Nanolive (Lausanne, Switzerland) for the opportunity and the collaborative acquisition of tomographic pictures.S
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