Papers by Lahsen Assoufid
Physical Review Materials, Dec 1, 2020
Anomalous diffraction of Bijvoet pairs (BPs) is a fundamental method of structure analyses in cry... more Anomalous diffraction of Bijvoet pairs (BPs) is a fundamental method of structure analyses in crystallography. We demonstrate that the diffraction intensity ratio of a BP always intrinsically equals the two structure factors' squared modulus ratio instead of the modulus ratio that has been believed to govern dynamical diffraction. High-resolution experiments of perfect quartz precisely proved this principle in pure dynamical diffraction. The underlying mechanism is that the elementary diffraction functions of BPs always obey the squared modulus ratio rule, which is universally valid for both kinematical and dynamical diffraction in any diffraction geometry. This fundamental mechanism clarifies one of the long-lasting ambiguities in x-ray crystallography and paves the way for various applications of high-resolution diffraction of BPs, particularly for large high-quality crystals.
A neural-network machine learning model is developed to control a bimorph adaptive mirror to achi... more A neural-network machine learning model is developed to control a bimorph adaptive mirror to achieve and preserve aberration-free coherent X-ray wavefronts at synchrotron radiation and free electron laser beamlines. The controller is trained on a mirror actuator response directly measured at a beamline with a real-time single-shot wavefront sensor, which uses a coded mask and wavelet-transform analysis. The system has been successfully tested on a bimorph deformable mirror at the 28-ID IDEA beamline of the Advanced Photon Source at Argonne National Laboratory. It achieved a response time of a few seconds and maintained desired wavefront shapes (e.g., a spherical wavefront) with sub-wavelength accuracy at 20 keV of X-ray energy. This result is significantly better than what can be obtained using a linear model of the mirror’s response. The developed system has not been tailored to a specific mirror and can be applied, in principle, to different kinds of bending mechanisms and actuators.
Optica, 2022
X-ray phase-contrast imaging has become indispensable for visualizing samples with low absorption... more X-ray phase-contrast imaging has become indispensable for visualizing samples with low absorption contrast. In this regard, speckle-based techniques have shown significant advantages in spatial resolution, phase sensitivity, and implementation flexibility compared with traditional methods. However, the computational cost associated with data inversion has hindered their wider adoption. By exploiting the power of deep learning, we developed a speckle-based phase-contrast imaging neural network (SPINNet) that significantly improves the imaging quality and boosts the phase retrieval speed by at least 2 orders of magnitude compared to existing methods. To achieve this performance, we combined SPINNet with a coded-mask-based technique, an enhanced version of the speckle-based method. Using this scheme, we demonstrate the simultaneous reconstruction of absorption and phase images on the order of 100 ms, where a traditional correlation-based analysis would take several minutes even with a ...
Journal of Synchrotron Radiation, 2022
Rigorous dynamical theory calculations show that four-beam diffraction (4BD) can be activated onl... more Rigorous dynamical theory calculations show that four-beam diffraction (4BD) can be activated only by a unique photon energy and a unique incidence direction. Thus, 4BD may be used to precisely calibrate X-ray photon energies and beam positions. Based on the principles that the forbidden-reflection 4BD pattern, which is typically an X-shaped cross, can be generated by instant imaging using the divergent beam from a point source without rocking the crystal, a detailed real-time high-resolution beam (and source) position monitoring scheme is illustrated for monitoring two-dimensional beam positions and directions of modern synchrotron light sources, X-ray free-electron lasers and nano-focused X-ray sources.
In this paper, we report progress to develop adaptive X-ray mirrors using magnetically smart mate... more In this paper, we report progress to develop adaptive X-ray mirrors using magnetically smart materials or MSM for short. These adaptive X-ray mirrors work using the stress difference a magnetically smart material (MSM) provides under the influence of an external magnetic field. We used silicon substrates coated with Terfenol-D R © (MSM), to act as the actuating layer, and NiCo as our magnetically hard material to retain the surface profile of the mirror after magnetization. Profile measurement of 5mm x 20mm films were done using a Fizcam 2000 interferometer under different conditions. The goal was to show that the samples could be shaped with the help of an external magnetic field, and that the remnant field in the NiCo holds the deflection after the external field has been removed. For this project, we measured three different samples showing a deflection up to 1.34m under 0.12 Tesla; Two of the samples successfully hold some of their deflection under the remnant magnetic field cre...
An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demon... more An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demonstrated key technology prototypes and software modeling tools required for new high-coherent flux beamline optical systems. New free electron laser (FEL) and diffraction-limited storage ring (DLSR) light sources demand wavefront preservation from source to sample to achieve and maintain optimal performance. Fine wavefront control was achieved using a novel, roomtemperature cooled mirror system called REAL (resistive element adjustable length) that combines cooling with applied, spatially variable auxiliary heating. Single-grating shearing interferometry (also called Talbot interferometry) and Hartmann wavefront sensors were developed and used for optical characterization and alignment on several beamlines, across a range of photon energies. Demonstrations of non-invasive hard x-ray wavefront sensing were performed using a thin diamond single-crystal as a beamsplitter.
Journal of Imaging, 2021
Near-field X-ray speckle tracking has been used in phase-contrast imaging and tomography as an em... more Near-field X-ray speckle tracking has been used in phase-contrast imaging and tomography as an emerging technique, providing higher contrast images than traditional absorption radiography. Most reported methods use sandpaper or membrane filters as speckle generators and digital image cross-correlation for phase reconstruction, which has either limited resolution or requires a large number of position scanning steps. Recently, we have proposed a novel coded-mask-based multi-contrast imaging (CMMI) technique for single-shot measurement with superior performance in efficiency and resolution compared with other single-shot methods. We present here a scanning CMMI method for the ultimate imaging resolution and phase sensitivity by using a coded mask as a high-contrast speckle generator, the flexible scanning mode, the adaption of advanced maximum-likelihood optimization to scanning data, and the multi-resolution analysis. Scanning CMMI can outperform other speckle-based imaging methods, ...
Optics Express, 2020
This article describes the development and testing of a novel, water-cooled, active optic mirror ... more This article describes the development and testing of a novel, water-cooled, active optic mirror system (called “REAL: Resistive Element Adjustable Length”) that combines cooling with applied auxiliary heating, tailored to the spatial distribution of the thermal load generated by the incident beam. This technique is theoretically capable of sub-nanometer surface figure error control even at high power density. Tests conducted in an optical metrology laboratory and at synchrotron X-ray beamlines showed the ability to maintain the mirror profile to the level needed for the next generation storage rings and FEL mirrors.
Journal of Synchrotron Radiation, 2019
The full radiation from the first harmonic of a synchrotron undulator (between 5 and 12 keV) at t... more The full radiation from the first harmonic of a synchrotron undulator (between 5 and 12 keV) at the Advanced Photon Source is microfocused using a stack of beryllium compound refractive lenses onto a fast-moving liquid jet and overlapped with a high-repetition-rate optical laser. This micro-focused geometry is used to perform efficient nonresonant X-ray emission spectroscopy on transient species using a dispersive spectrometer geometry. The overall usable flux achieved on target is above 1015 photons s−1 at 8 keV, enabling photoexcited systems in the liquid phase to be tracked with time resolutions from tens of picoseconds to microseconds, and using the full emission spectrum, including the weak valence-to-core signal that is sensitive to chemically relevant electronic properties.
Journal of Applied Crystallography, 2018
Errors about left-handed (laevorotatory) quartz and right-handed (dextrorotatory) quartz in the p... more Errors about left-handed (laevorotatory) quartz and right-handed (dextrorotatory) quartz in the paper by Huang, Gog, Kim, Kasman, Said, Casa, Wieczorek, Hönnicke & Assoufid [J. Appl. Cryst. (2016), 51, 140–147] are corrected.
Journal of visualized experiments : JoVE, Jan 11, 2016
A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray so... more A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is reported. The measurements were demonstrated at the 1-BM bending magnet beamline of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). By using a 2-D checkerboard π/2 phase-shift grating, transverse coherence lengths were obtained along the vertical and horizontal directions as well as along the 45° and 135° directions to the horizontal direction. Following the technical details specified in this paper, interferograms were measured at different positions downstream of the phase grating along the beam propagation direction. Visibility values of each interferogram were extracted from analyzing harmonic peaks in its Fourier Transformed image. Consequently, the coherence length along each direction can be extracted from the evolution of visibility as a function of the grating-to-detector distance. The simultaneous measu...
AIP Conference Proceedings, 1997
Department of Energy. The US. Government retains for itself, and others acting on its behalf, a p... more Department of Energy. The US. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
Optics letters, 2016
The feasibility of an off-axis x-ray reflection zone plate to perform wavelength-dispersive spect... more The feasibility of an off-axis x-ray reflection zone plate to perform wavelength-dispersive spectroscopy, on-axis point focusing, and two-dimensional imaging is demonstrated by means of one and the same diffractive optical element (DOE) at a synchrotron radiation facility. The resolving power varies between 3×10<sup>1</sup> and 4×10<sup>2</sup> in the range of 7.6 keV to 9.0 keV, with its maximum at the design energy of 8.3 keV. This result is verified using an adjustable entrance slit, by which horizontal (H) and vertical (V) focusing to 0.85 μm(H) and 1.29 μm(V) is obtained near the sagittal focal plane of the astigmatic configuration. An angular and axial scan proves an accessible field of view of at least 0.6 arcmin × 0.8 arcmin and a focal depth of ±0.86 mm. Supported by the grating efficiency of around 17.5% and a very short pulse elongation, future precision x-ray fluorescence and absorption studies of transition metals at their K-edge on an ultrashor...
Optics and Photonics News, 2015
Bringing the brightness and power of vast synchrotron and free-electron laser sources to the scal... more Bringing the brightness and power of vast synchrotron and free-electron laser sources to the scale of the local lab and clinic marks an important next frontier-and could transform the landscape of X-ray science and technology.
Journal of Synchrotron Radiation, 2014
A new method for beamline simulation combining ray-tracing and wavefront propagation is described... more A new method for beamline simulation combining ray-tracing and wavefront propagation is described. The `Hybrid Method' computes diffraction effects when the beam is clipped by an aperture or mirror length and can also simulate the effect of figure errors in the optical elements when diffraction is present. The effect of different spatial frequencies of figure errors on the image is compared withSHADOWresults pointing to the limitations of the latter. The code has been benchmarked against the multi-electron version ofSRWin one dimension to show its validity in the case of fully, partially and non-coherent beams. The results demonstrate that the code is considerably faster than the multi-electron version ofSRWand is therefore a useful tool for beamline design and optimization.
Review of Scientific Instruments, 1996
We describe a horizontally focusing curved-crystal monochromator that invokes a 4-point bending s... more We describe a horizontally focusing curved-crystal monochromator that invokes a 4-point bending scheme and a liquid-metal cooling bath. The device has been designed for dispersive diffraction and spectroscopy in the 5–20 keV range, with a predicted focal spot size of ≤100 μm. To minimize thermal distortions and thermal equilibration time, the 355×32×0.8 mm crystal will be nearly half submerged in a bath of Ga-In-Sn-Zn alloy. The liquid metal thermally couples the crystal to the water-cooled Cu frame, while permitting the required crystal bending. Calculated thermal profiles and anticipated focusing properties are discussed.
Physical Review B, 2007
A first-principles calculation is performed to investigate the interaction between adamantane and... more A first-principles calculation is performed to investigate the interaction between adamantane and an atomicforce-microscope tip. By holding the tip at different distances from adamantane, three scans across two surfaces, one with a carbon atom at the center and the four other equivalent atoms at the corners, and the other with three equivalent carbon atoms in the front and three other atoms in the back forming a hexagon shape, reveal the detailed morphology of adamantane. For the first scan surface, a huge potential energy change is observed when the tip is close to adamantane, which results from the strong interaction from two hydrogen atoms attached to the center carbon atom. On the second scan surface, a radial scan shows the maximum force constant of 2 hartrees/ Å 2 . This is proof of the hardness of adamantane. The rotational scan along the second surface reveals a systematic change in the potential energy as the tip is moved away from adamantane. Due to the existence of two types of carbon atoms in adamantane, the original potential maxima are shifted 60°to new maxima. Between these maxima, there is a flat region. Finally, an x-y force scan over the above two surfaces is performed, where two distinctive images of hydrogen atoms are found. These results are detectable experimentally.
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Papers by Lahsen Assoufid