3,907 research outputs found
When holography meets coherent diffraction imaging
Modern imaging techniques at the molecular scale rely on utilizing novel
coherent light sources like X-ray free electron lasers for the ultimate goal of
visualizing such objects as individual biomolecules rather than crystals. Here,
unlike in the case of crystals where structures can be solved by model building
and phase refinement, the phase distribution of the wave scattered by an
individual molecule must directly be recovered. There are two well-known
solutions to the phase problem: holography and coherent diffraction imaging
(CDI). Both techniques have their pros and cons. In holography, the
reconstruction of the scattered complex-valued object wave is directly provided
by a well-defined reference wave that must cover the entire detector area which
often is an experimental challenge. CDI provides the highest possible, only
wavelength limited, resolution, but the phase recovery is an iterative process
which requires some pre-defined information about the object and whose outcome
is not always uniquely-defined. Moreover, the diffraction patterns must be
recorded under oversampling conditions, a pre-requisite to be able to solve the
phase problem. Here, we report how holography and CDI can be merged into one
superior technique: holographic coherent diffraction imaging (HCDI). An inline
hologram can be recorded by employing a modified CDI experimental scheme. We
demonstrate that the amplitude of the Fourier transform of an inline hologram
is related to the complex-valued visibility, thus providing information on
both, the amplitude and the phase of the scattered wave in the plane of the
diffraction pattern. With the phase information available, the condition of
oversampling the diffraction patterns can be relaxed, and the phase problem can
be solved in a fast and unambiguous manner.Comment: 22 pages, 7 figure
Time-spliced X-ray Diffraction Imaging
Diffraction imaging of non-equilibrium dynamics at atomic resolution is
becoming possible with X-ray free-electron lasers. However, there are
unresolved problems with applying this method to objects that are confined in
only one dimension. Here I show that one-dimensional coherent diffraction
imaging is possible by splicing together images recovered from different delays
in a time-resolved experiment. This is used to image the time and space
evolution of antiferromagnetic order in a complex oxide heterostructure from
measurements of a resonant soft X-ray diffraction peak. Mid-infrared excitation
of the substrate is shown to lead to a magnetic front that propagates at a
velocity exceeding the speed of sound, a critical observation for the
understanding of driven phase transitions in complex condensed matter
Phaseless VLBI mapping of compact extragalactic radio sources
The problem of phaseless aperture synthesis is of current interest in
phase-unstable VLBI with a small number of elements when either the use of
closure phases is not possible (a two-element interferometer) or their quality
and number are not enough for acceptable image reconstruction by standard
adaptive calibration methods. Therefore, we discuss the problem of unique image
reconstruction only from the spectrum magnitude of a source. We suggest an
efficient method for phaseless VLBI mapping of compact extragalactic radio
sources. This method is based on the reconstruction of the spectrum magnitude
for a source on the entire UV plane from the measured visibility magnitude on a
limited set of points and the reconstruction of the sought-for image of the
source by Fienup's method from the spectrum magnitude reconstructed at the
first stage. We present the results of our mapping of the extragalactic radio
source 2200 +420 using astrometric and geodetic observations on a global VLBI
array. Particular attention is given to studying the capabilities of a
two-element interferometer in connection with the putting into operation of a
Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure
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Computational Inverse Problems for Partial Differential Equations
The problem of determining unknown quantities in a PDE from measurements of (part of) the solution to this PDE arises in a wide range of applications in science, technology, medicine, and finance. The unknown quantity may e.g. be a coefficient, an initial or a boundary condition, a source term, or the shape of a boundary. The identification of such quantities is often computationally challenging and requires profound knowledge of the analytical properties of the underlying PDE as well as numerical techniques. The focus of this workshop was on applications in phase retrieval, imaging with waves in random media, and seismology of the Earth and the Sun, a further emphasis was put on stochastic aspects in the context of uncertainty quantification and parameter identification in stochastic differential equations. Many open problems and mathematical challenges in application fields were addressed, and intensive discussions provided an insight into the high potential of joining deep knowledge in numerical analysis, partial differential equations, and regularization, but also in mathematical statistics, homogenization, optimization, differential geometry, numerical linear algebra, and variational analysis to tackle these challenges
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