288 research outputs found
Coherent Diffraction Imaging of Single 95nm Nanowires
Photonic or electronic confinement effects in nanostructures become
significant when one of their dimension is in the 5-300 nm range. Improving
their development requires the ability to study their structure - shape, strain
field, interdiffusion maps - using novel techniques. We have used coherent
diffraction imaging to record the 3-dimensionnal scattered intensity of single
silicon nanowires with a lateral size smaller than 100 nm. We show that this
intensity can be used to recover the hexagonal shape of the nanowire with a
28nm resolution. The article also discusses limits of the method in terms of
radiation damage.Comment: 5 pages, 5 figure
Calculation of an optimized telescope apodizer for Terrestrial Planet Finder coronagraphic telescope
One of two approaches to implementing NASA's Terrestrial Planet Finder is to
build a space telescope that utilizes the techniques of coronagraphy and
apodization to suppress diffraction and image exo-planets. We present a method
for calculation of a telescope's apodizer which suppresses the side lobes of
the image of a star so as to optimally detect an Earth-like planet. Given the
shape of a telescope's aperture and given a search region for a detector, we
solve an integral equation to determine an amplitude modulation (an apodizer)
which suppresses the star's energy in the focal plane search region. The method
is quite general and yields as special cases the product apodizer reported by
Nisenson and Papaliolios (2001) and the Prolate spheroidal apodizer of Kasdin
et al (2002), and Aime et al (2002). We show computer simulations of the
apodizers and the corresponding point spread functions for various
aperture-detector configurations.Comment: 16 Pages, 9 figures, Accepted for publication in June issue of PAS
Simulation of superresolution holography for optical tweezers
Optical tweezers manipulate microscopic particles using foci of light beams. Their performance is therefore limited by diffraction. Using computer simulations of a model system, we investigate the application of superresolution holography for two-dimensional (2D) light shaping in optical tweezers, which can beat the diffraction limit. We use the direct-search and Gerchberg algorithms to shape the center of a light beam into one or two bright spots; we do not constrain the remainder of the beam. We demonstrate that superresolution algorithms can significantly improve the normalized stiffness of an optical trap and the minimum separation at which neighboring traps can be resolved. We also test if such algorithms can be used interactively, as is desirable in optical tweezers
Inversion of the Diffraction Pattern from an Inhomogeneously Strained Crystal using an Iterative Algorithm
The displacement field in highly non uniformly strained crystals is obtained
by addition of constraints to an iterative phase retrieval algorithm. These
constraints include direct space density uniformity and also constraints to the
sign and derivatives of the different components of the displacement field.
This algorithm is applied to an experimental reciprocal space map measured
using high resolution X-ray diffraction from an array of silicon lines and the
obtained component of the displacement field is in very good agreement with the
one calculated using a finite element model.Comment: 5 pages, 4 figure
Thermodynamic Properties of Generalized Exclusion Statistics
We analytically calculate some thermodynamic quantities of an ideal -on
gas obeying generalized exclusion statistics. We show that the specific heat of
a -on gas () vanishes linearly in any dimension as when
the particle number is conserved and exhibits an interesting dual symmetry that
relates the particle-statistics at to the hole-statistics at at low
temperatures. We derive the complete solution for the cluster coefficients
as a function of Haldane's statistical interaction in
dimensions. We also find that the cluster coefficients and the virial
coefficients are exactly mirror symmetric (=odd) or antisymmetric
(=even) about . In two dimensions, we completely determine the closed
forms about the cluster and the virial coefficients of the generalized
exclusion statistics, which exactly agree with the virial coefficients of an
anyon gas of linear energies. We show that the -on gas with zero chemical
potential shows thermodynamic properties similar to the photon statistics. We
discuss some physical implications of our results.Comment: 24 pages, Revtex, Corrected typo
Solution to the twin image problem in holography
While the invention of holography by Dennis Gabor truly constitutes an
ingenious concept, it has ever since been troubled by the so called twin image
problem limiting the information that can be obtained from a holographic
record. Due to symmetry reasons there are always two images appearing in the
reconstruction process. Thus, the reconstructed object is obscured by its
unwanted out of focus twin image. Especially for emission electron as well as
for x- and gamma-ray holography, where the source-object distances are small,
the reconstructed images of atoms are very close to their twin images from
which they can hardly be distinguished. In some particular instances only,
experimental efforts could remove the twin images. More recently, numerical
methods to diminish the effect of the twin image have been proposed but are
limited to purely absorbing objects failing to account for phase shifts caused
by the object. Here we show a universal method to reconstruct a hologram
completely free of twin images disturbance while no assumptions about the
object need to be imposed. Both, amplitude and true phase distributions are
retrieved without distortion
Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16
The parabolic arc phenomenon visible in the Fourier analysis of the
scintillation spectra of pulsars provides a new method of investigating the
small scale structure in the ionized interstellar medium (ISM). We report
archival observations of the pulsar B1133+16 showing both forward and reverse
parabolic arcs sampled over 14 months. These features can be understood as the
mutual interference between an assembly of discrete features in the scattered
brightness distribution. By model-fitting to the observed arcs at one epoch we
obtain a ``snap-shot'' estimate of the scattered brightness, which we show to
be highly anisotropic (axial ratio >10:1), to be centered significantly off
axis and to have a small number of discrete maxima, which are coarser the
speckle expected from a Kolmogorov spectrum of interstellar plasma density. The
results suggest the effects of highly localized discrete scattering regions
which subtend 0.1-1 mas, but can scatter (or refract) the radiation by angles
that are five or more times larger.Comment: 14 pages, 4 figures, submitted to Astrophysical Journa
A unified evaluation of iterative projection algorithms for phase retrieval
Iterative projection algorithms are successfully being used as a substitute
of lenses to recombine, numerically rather than optically, light scattered by
illuminated objects. Images obtained computationally allow aberration-free
diffraction-limited imaging and the possibility of using radiation for which no
lenses exist. The challenge of this imaging technique is transfered from the
lenses to the algorithms. We evaluate these new computational ``instruments''
developed for the phase retrieval problem, and discuss acceleration strategies.Comment: 12 pages, 9 figures, revte
Evolutionary determination of experimental parameters for ptychographical imaging
The Ptychographical Iterative Engine (PIE) algorithm is a recently developed novel method of Coherent Diffractive Imaging (CDI) that uses multiple overlapping diffraction patterns to reconstruct an image. This method has successfully produced high quality reconstructions at both optical and X-ray wavelengths but the need for accurate knowledge of the probe positions is currently a limiting factor in the production of high resolution reconstructions at electron wavelengths. This paper examines the shape of the search landscape for producing optimal image reconstructions in the specific case of electron microscopy and then shows how evolutionary search methods can be used to reliably determine experimental parameters in the electron microscopy case (such as the spherical aberration in the probe and the probe positions)
Accurate phase retrieval of complex point spread functions with deep residual neural networks
Phase retrieval, i.e. the reconstruction of phase information from intensity
information, is a central problem in many optical systems. Here, we demonstrate
that a deep residual neural net is able to quickly and accurately perform this
task for arbitrary point spread functions (PSFs) formed by Zernike-type phase
modulations. Five slices of the 3D PSF at different focal positions within a
two micron range around the focus are sufficient to retrieve the first six
orders of Zernike coefficients.Comment: 8 pages, 4 figure
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