6,185 research outputs found
Finite-Difference Time-Domain Simulation for Three-dimensional Polarized Light Imaging
Three-dimensional Polarized Light Imaging (3D-PLI) is a promising technique
to reconstruct the nerve fiber architecture of human post-mortem brains from
birefringence measurements of histological brain sections with micrometer
resolution. To better understand how the reconstructed fiber orientations are
related to the underlying fiber structure, numerical simulations are employed.
Here, we present two complementary simulation approaches that reproduce the
entire 3D-PLI analysis: First, we give a short review on a simulation approach
that uses the Jones matrix calculus to model the birefringent myelin sheaths.
Afterwards, we introduce a more sophisticated simulation tool: a 3D Maxwell
solver based on a Finite-Difference Time-Domain algorithm that simulates the
propagation of the electromagnetic light wave through the brain tissue. We
demonstrate that the Maxwell solver is a valuable tool to better understand the
interaction of polarized light with brain tissue and to enhance the accuracy of
the fiber orientations extracted by 3D-PLI.Comment: 13 pages, 5 figure
Generation and subwavelength focusing of longitudinal magnetic fields in a metallized fiber tip
We demonstrate experimentally and numerically that in fiber tips as they are
used in NSOMs azimuthally polarized electrical fields
(|E|/|E| 55% 5% for
1.4\mu m tip aperture diameter and \lambda = 1550nm), respectively
subwavelength confined (FWHM 450nm \lambda/3.5)
magnetic fields, are generated for a certain tip aperture diameter (d = 1.4\mu
m). We attribute the generation of this field distribution in metal-coated
fiber tips to symmetry breaking in the bend and subsequent plasmonic mode
filtering in the truncated conical taper.Comment: 11 pages, 6 figure
A Jones matrix formalism for simulating three-dimensional polarized light imaging of brain tissue
The neuroimaging technique three-dimensional polarized light imaging (3D-PLI)
provides a high-resolution reconstruction of nerve fibres in human post-mortem
brains. The orientations of the fibres are derived from birefringence
measurements of histological brain sections assuming that the nerve fibres -
consisting of an axon and a surrounding myelin sheath - are uniaxial
birefringent and that the measured optic axis is oriented in direction of the
nerve fibres (macroscopic model). Although experimental studies support this
assumption, the molecular structure of the myelin sheath suggests that the
birefringence of a nerve fibre can be described more precisely by multiple
optic axes oriented radially around the fibre axis (microscopic model). In this
paper, we compare the use of the macroscopic and the microscopic model for
simulating 3D-PLI by means of the Jones matrix formalism. The simulations show
that the macroscopic model ensures a reliable estimation of the fibre
orientations as long as the polarimeter does not resolve structures smaller
than the diameter of single fibres. In the case of fibre bundles, polarimeters
with even higher resolutions can be used without losing reliability. When
taking the myelin density into account, the derived fibre orientations are
considerably improved.Comment: 20 pages, 8 figure
The conformational evolution of elongated polymer solutions tailors the polarization of light-emission from organic nanofibers
Polymer fibers are currently exploited in tremendously important
technologies. Their innovative properties are mainly determined by the behavior
of the polymer macromolecules under the elongation induced by external
mechanical or electrostatic forces, characterizing the fiber drawing process.
Although enhanced physical properties were observed in polymer fibers produced
under strong stretching conditions, studies of the process-induced nanoscale
organization of the polymer molecules are not available, and most of fiber
properties are still obtained on an empirical basis. Here we reveal the
orientational properties of semiflexible polymers in electrospun nanofibers,
which allow the polarization properties of active fibers to be finely
controlled. Modeling and simulations of the conformational evolution of the
polymer chains during electrostatic elongation of semidilute solutions
demonstrate that the molecules stretch almost fully within less than 1 mm from
jet start, increasing polymer axial orientation at the jet center. The
nanoscale mapping of the local dichroism of individual fibers by polarized
near-field optical microscopy unveils for the first time the presence of an
internal spatial variation of the molecular order, namely the presence of a
core with axially aligned molecules and a sheath with almost radially oriented
molecules. These results allow important and specific fiber properties to be
manipulated and tailored, as here demonstrated for the polarization of emitted
light.Comment: 45 pages, 10 figures, Macromolecules (2014
Dense Fiber Modeling for 3D-Polarized Light Imaging Simulations
3D-Polarized Light Imaging (3D-PLI) is a neuroimaging technique used to study
the structural connectivity of the human brain at the meso- and microscale. In
3D-PLI, the complex nerve fiber architecture of the brain is characterized by
3D orientation vector fields that are derived from birefringence measurements
of unstained histological brain sections by means of an effective physical
model.
To optimize the physical model and to better understand the underlying
microstructure, numerical simulations are essential tools to optimize the used
physical model and to understand the underlying microstructure in detail. The
simulations rely on predefined configurations of nerve fiber models (e.g.
crossing, kissing, or complex intermingling), their physical properties, as
well as the physical properties of the employed optical system to model the
entire 3D-PLI measurement. By comparing the simulation and experimental
results, possible misinterpretations in the fiber reconstruction process of
3D-PLI can be identified. Here, we focus on fiber modeling with a specific
emphasize on the generation of dense fiber distributions as found in the human
brain's white matter. A new algorithm will be introduced that allows to control
possible intersections of computationally grown fiber structures
Design and fabrication of blazed gratings for a waveguide-type head mounted display
In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real-world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies. We report the design of blazed gratings for green light (lambda = 543 nm) and a diffraction angle of 43 degrees. The blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are fabricated using grayscale electron beam lithography. We outline the subsequent replication in a polymer waveguide material with ultraviolet nanoimprint lithography and confirm a throughput efficiency of 17.4%. We finally show the in- and outcoupling of an image through two blazed gratings appearing sharp and non-distorted in the environment. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen
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