408 research outputs found
Three-dimensional double helical DNA structure directly revealed from its X-ray fiber diffraction pattern by iterative phase retrieval
Coherent diffraction imaging (CDI) allows the retrieval of the structure of
an isolated object, such as a macromolecule, from its diffraction pattern. CDI
requires the fulfilment of two conditions: the imaging radiation must be
coherent and the object must be isolated. We discuss that it is possible to
directly retrieve the molecular structure from its diffraction pattern which
was acquired neither with coherent radiation nor from an individual molecule,
provided the molecule exhibits periodicity in one direction, as in the case of
fiber diffraction. We demonstrate that by applying iterative phase retrieval
methods to a fiber diffraction pattern, the repeating unit, that is, the
molecule structure, can directly be reconstructed without any prior modeling.
As an example, we recover the structure of the DNA double helix in
three-dimensions from its two-dimensional X-ray fiber diffraction pattern,
Photograph 51, acquired in the famous experiment by Raymond Gosling and
Rosalind Franklin, at a resolution of 3.4 Angstrom
Practical algorithms for simulation and reconstruction of digital in-line holograms
Here we present practical methods for simulation and reconstruction of
in-line digital holograms recorded with plane and spherical waves. The
algorithms described here are applicable to holographic imaging of an object
exhibiting absorption as well as phase shifting properties. Optimal parameters,
related to distances, sampling rate, and other factors for successful
simulation and reconstruction of holograms are evaluated and criteria for the
achievable resolution are worked out. Moreover, we show that the numerical
procedures for the reconstruction of holograms recorded with plane and
spherical waves are identical under certain conditions. Experimental examples
of holograms and their reconstructions are also discussed.Comment: including MATLAB code
Molecular particle-core model and its application to 13C-13C scattering
On the basis of the two-center shell model a theory is developed for the excitation of loosely bound nucleons in heavy ion collisions. These nucleons move in the two-center shell model potential generated by all the nucleons and are described by molecular wave functions. The model is applied to calculate the cross sections for the elastic and inelastic 13C-13C scattering. The cross sections show intermediate structures caused by the excitation of quasibound resonances in the molecular nucleus-nucleus potential. NUCLEAR REACTIONS 13C(13C,13C) molecular wave functions, dynamical two-center shell model, quasimolecular resonances, radial and Coriolis coupling, coupled channel calculations for σ(θ)
Imaging outside the box: Resolution enhancement in X-ray coherent diffraction imaging by extrapolation of diffraction patterns
Coherent diffraction imaging is a high-resolution imaging technique whose
potential can be greatly enhanced by applying the extrapolation method
presented here. We demonstrate enhancement in resolution of a non-periodical
object reconstructed from an experimental X-ray diffraction record which
contains about 10% missing information, including the pixels in the center of
the diffraction pattern. A diffraction pattern is extrapolated beyond the
detector area and as a result, the object is reconstructed at an enhanced
resolution and better agreement with experimental amplitudes is achieved. The
optimal parameters for the iterative routine and the limits of the
extrapolation procedure are discussed.Comment: 12 pages, 4 figure
Resolution enhancement in in-line holography by numerical compensation of vibrations
Mechanical vibrations of components of the optical system is one of the
sources of blurring of interference pattern in coherent imaging systems. The
problem is especially important in holography where the resolution of the
reconstructed objects depends on the effective size of the hologram, that is on
the extent of the interference pattern, and on the contrast of the interference
fringes. We discuss the mathematical relation between the vibrations, the
hologram contrast and the reconstructed object. We show how vibrations can be
post-filtered out from the hologram or from the reconstructed object assuming a
Gaussian distribution of the vibrations. We also provide a numerical example of
compensation for directional motion blur. We demonstrate our approach for light
optical and electron holograms, acquired with both, plane- as well as
spherical-waves. As a result of such hologram deblurring, the resolution of the
reconstructed objects is enhanced by almost a factor of 2. We believe that our
approach opens up a new venue of post-experimental resolution enhancement in
in-line holography by adapting the rich database/catalogue of motion deblurring
algorithms developed for photography and image restoration applications
Inverted Gabor holography principle for tailoring arbitrary shaped three-dimensional beams
It is well known that by modifying the wavefront in a certain manner, the
light intensity can be turned into a certain shape. However, all known light
modulation techniques allow for limited light modifications only: focusing
within a restricted region in space, shaping into a certain class of parametric
curves along the optical axis or bending described by a quadratic-dependent
deflection as in the case of Airy beams. We show a general case of classical
light wavefront shaping that allows for intensity and phase redistribution into
an arbitrary profile including pre-determined switching-off of the intensity.
To create an arbitrary three-dimensional path of intensity, we represent the
path as a sequence of closely packed individual point-like absorbers and
simulate the in-line hologram of the created object set; when such a hologram
is contrast inverted, thus giving rise to a diffractor, it creates the
pre-determined three-dimensional path of intensity behind the diffractor under
illumination. The crucial parameter for a smooth optical path is the sampling
of the predetermined curves, which is given by the lateral and axial resolution
of the optical system. We provide both, simulated and experimental results to
demonstrate the power of this novel method
Reconstruction of purely absorbing, absorbing and phase-shifting, and strong phase-shifting objects from their single-shot in-line holograms
We address the problem of reconstructing phase-shifting objects from their
single shot in-line holograms. We show that a phase-shifting object cannot be
reliably recovered from its in-line hologram by non-iterative reconstruction
routines, and that an iterative reconstruction should be applied. We
demonstrate examples of simulated in-line holograms of objects with the
following properties: purely absorbing, both absorbing and phase shifting, and
strong phase-shifting. We investigate the effects of noise and interference
contrast in holograms on the reconstruction results and discuss details of an
optimal iterative procedure to quantitatively recover the correct absorbing and
phase-shifting properties of the object. We also review previously published
reconstructions of experimental holograms and summarize the optimal parameters
for retrieval of phase-shifting objects from their in-line holograms
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