496 research outputs found
Spontaneous-emission rates in finite photonic crystals of plane scatterers
The concept of a plane scatterer that was developed earlier for scalar waves
is generalized so that polarization of light is included. Starting from a
Lippmann-Schwinger formalism for vector waves, we show that the Green function
has to be regularized before T-matrices can be defined in a consistent way.
After the regularization, optical modes and Green functions are determined
exactly for finite structures built up of an arbitrary number of parallel
planes, at arbitrary positions, and where each plane can have different optical
properties. The model is applied to the special case of finite crystals
consisting of regularly spaced identical planes, where analytical methods can
be taken further and only light numerical tasks remain. The formalism is used
to calculate position- and orientation-dependent spontaneous-emission rates
inside and near the finite photonic crystals. The results show that emission
rates and reflection properties can differ strongly for scalar and for vector
waves. The finite size of the crystal influences the emission rates. For
parallel dipoles close to a plane, emission into guided modes gives rise to a
peak in the frequency-dependent emission rate.Comment: 18 pages, 6 figures, to be published in Phys. Rev.
Analytical modeling of light transport in scattering materials with strong absorption
We have investigated the transport of light through slabs that both scatter
and strongly absorb, a situation that occurs in diverse application fields
ranging from biomedical optics, powder technology, to solid-state lighting. In
particular, we study the transport of light in the visible wavelength range
between and nm through silicone plates filled with YAG:Ce
phosphor particles, that even re-emit absorbed light at different wavelengths.
We measure the total transmission, the total reflection, and the ballistic
transmission of light through these plates. We obtain average single particle
properties namely the scattering cross-section , the absorption
cross-section , and the anisotropy factor using an analytical
approach, namely the P3 approximation to the radiative transfer equation. We
verify the extracted transport parameters using Monte-Carlo simulations of the
light transport. Our approach fully describes the light propagation in phosphor
diffuser plates that are used in white LEDs and that reveal a strong absorption
() up to , where is the
slab thickness, is the absorption mean free path. In
contrast, the widely used diffusion theory fails to describe this parameter
range. Our approach is a suitable analytical tool for industry, since it
provides a fast yet accurate determination of key transport parameters, and
since it introduces predictive power into the design process of white light
emitting diodes
Exploiting speckle correlations to improve the resolution of wide-field fluorescence microscopy
Fluorescence microscopy is indispensable in nanoscience and biological
sciences. The versatility of labeling target structures with fluorescent dyes
permits to visualize structure and function at a subcellular resolution with a
wide field of view. Due to the diffraction limit, conventional optical
microscopes are limited to resolving structures larger than 200 nm. The
resolution can be enhanced by near-field and far-field super-resolution
microscopy methods. Near-field methods typically have a limited field of view
and far-field methods are limited by the involved conventional optics. Here, we
introduce a combined high-resolution and wide-field fluorescence microscopy
method that improves the resolution of a conventional optical microscope by
exploiting correlations in speckle illumination through a randomly scattering
high-index medium: Speckle correlation resolution enhancement (SCORE). As a
test, we collect two-dimensional fluorescence images of 100-nm diameter
dye-doped nanospheres. We demonstrate a deconvolved resolution of 130 nm with a
field of view of 10 x 10 \text{\mu m}^2
Scattering Lens Resolves sub-100 nm Structures with Visible Light
The smallest structures that conventional lenses are able to optically
resolve are of the order of 200 nm. We introduce a new type of lens that
exploits multiple scattering of light to generate a scanning nano-sized optical
focus. With an experimental realization of this lens in gallium phosphide we
have succeeded to image gold nanoparticles at 97 nm optical resolution. Our
work is the first lens that provides a resolution in the nanometer regime at
visible wavelengths.Comment: 4 pages, 3 figure
Non-Imaging Speckle Interferometry forHigh Speed Nanometer-Scale Position Detection
We experimentally demonstrate a non-imaging approach to displacement
measurement for complex scattering materials. By spatially controlling the wave
front of the light that incidents on the material we concentrate the scattered
light in a focus on a designated position. This wave front acts as an unique
optical fingerprint that enables precise position detection of the illuminated
material by simply measuring the intensity in the focus. By combining two
optical fingerprints we demonstrate position detection along one dimension with
a displacement resolution of 2.1 nm. As our approach does not require an image
of the scattered field, it is possible to employ fast non-imaging detectors to
enable high-speed position detection of scattering materials.Comment: 4 pages, 3 figure
Grazing as a conservation management tool:Responses of voles to grazer species and densities
Grazing is a widely applied conservation management tool, but the optimal regime for biodiversity conservation is still unknown. The effects of grazers on small mammals are not yet fully understood and mostly restricted to studies which compare grazed with ungrazed areas. We determined the effect of different livestock grazers and densities and a rotation regime, on voles in a conservation area in The Netherlands. We used a 7-year grazing experiment with horse and cattle grazing at two densities namely 0.5 and 1 animal ha(-1) (equivalent to 0.4 and 0.8 LSU), including a rotation regime i.e. 1 year summer grazing with 1 cattle ha(-1) followed by 1 ungrazed year. We recorded vole activity signs as a measure for presence (i.e. presence of burrow entrances, droppings, runways and plant clippings) in circular 2 m(2) plots along transects. Low grazer densities, regardless of species, corresponded to higher vole presence. Vole presence tended to be greater with cattle grazing than with horse grazing, but the difference was not significant. The increase in vole presence was greater in the rotation regime than with low or high density cattle grazing. The different vole activity signs provided similar results to each other with the exception of burrow entrances, suggesting that this measure is less accurate in predicting vole presence. Hence, voles clearly responded to the different grazing regimes. Our results have high relevance for conservation, in particular in systems where small mammals contribute to important ecological processes (e.g. bioturbation, seed dispersal) and play a crucial role in the survival of (iconic) higher trophic level taxa such as raptors or mammalian predators. In such systems, conservation management may best implement low-density cattle or rotation grazing. (C) 2018 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved
Процес «Спілки Визволення України» та зростання селянського опору в умовах суцільної колективізації
Мета даної роботи полягає у з’ясуванні механізму використання матеріалів
процесу «СВУ» на території сучасної Чернігівщини, пропагандистських цілях та
реакції на нього з боку як населення, лояльного до влади, так і селян, які вперто
чинили опір політиці колективізації
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