Publications by authors named "Bastian Pfau"

27 Publications

  • Page 1 of 1

The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication.

Beilstein J Nanotechnol 2021 6;12:304-318. Epub 2021 Apr 6.

Corelab Correlative Microscopy and Spectroscopy, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.

Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3762/bjnano.12.25DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8042487PMC
April 2021

Observation of fluctuation-mediated picosecond nucleation of a topological phase.

Nat Mater 2021 Jan 5;20(1):30-37. Epub 2020 Oct 5.

European XFEL, Schenefeld, Germany.

Topological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41563-020-00807-1DOI Listing
January 2021

Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges.

Phys Rev Lett 2020 Sep;125(12):127201

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2A, 12489 Berlin, Germany.

We systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M_{3,2} absorption resonances using the [email protected] free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16  mJ/cm^{2}/pulse to 10 000  mJ/cm^{2}/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse-implying an increased XUV peak electric field-results in a reduced quenching of the resonant diffraction at the Co M_{3,2} edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevLett.125.127201DOI Listing
September 2020

A tabletop setup for ultrafast helicity-dependent and element-specific absorption spectroscopy and scattering in the extreme ultraviolet spectral range.

Rev Sci Instrum 2020 Sep;91(9):093001

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße 2A, 12489 Berlin, Germany.

Further advances in the field of ultrafast magnetization dynamics require experimental tools to measure the spin and electron dynamics with element-specificity and femtosecond temporal resolution. We present a new laboratory setup for two complementary experiments with light in the extreme ultraviolet (XUV) spectral range. One experiment is designed for polarization-dependent transient spectroscopy, particularly for simultaneous measurements of magnetic circular dichroism (MCD) at the 3p resonances of the 3d transition metals Fe, Co, and Ni. The second instrument is designed for resonant small-angle scattering experiments with monochromatic light allowing us to monitor spin dynamics with spatial information on the nanometer scale. We combine a high harmonic generation (HHG) source with a phase shifter to obtain XUV pulses with variable polarization and a flux of about (3 ± 1) × 10 photons/s/harmonic at 60 eV at the source. A dedicated reference spectrometer effectively reduces the intensity fluctuations of the HHG spectrum to below 0.12% rms. We demonstrate the capabilities of the setup by capturing the energy- and polarization-dependent absorption of a thin Co film as well as the time-resolved small-angle scattering in a magnetic-domain network of a Co/Pt multilayer. The new laboratory setup allows systematic studies of optically induced spin and electron dynamics with element-specificity, particularly with MCD as the contrast mechanism with femtosecond temporal resolution and an unprecedented signal-to-noise ratio.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/5.0013928DOI Listing
September 2020

Achieving diffraction-limited resolution in soft-X-ray Fourier-transform holography.

Ultramicroscopy 2020 Jul 28;214:113005. Epub 2020 Apr 28.

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Str. 2A, Berlin 12489, Germany; Technische Universität Berlin, Institut für Optik und Atomare Physik, Straße des 17. Juni 135, Berlin 10623, Germany.

The spatial resolution of microscopic images acquired via X-ray Fourier-transform holography is limited by the source size of the reference wave and by the numerical aperture of the detector. We analyze the interplay between both influences and show how they are matched in practice. We further identify, how high spatial frequencies translate to imaging artifacts in holographic reconstructions where mainly the reference beam limits the spatial resolution. As a solution, three methods are introduced based on numerical post-processing of the reconstruction. The methods comprise apodization of the hologram, refocusing via wave propagation, and deconvolution using the transfer function of the imaging system. In particular for the latter two, we demonstrate that image details smaller than the source size of the reference beam can be recovered up to the diffraction limit of the hologram. Our findings motivate the intentional application of a large reference-wave source enhancing the image contrast in applications with low photon numbers such as single-shot experiments at free-electron lasers or imaging at laboratory sources.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ultramic.2020.113005DOI Listing
July 2020

Singleshot polychromatic coherent diffractive imaging with a high-order harmonic source.

Opt Express 2020 Jan;28(1):394-404

Singleshot polychromatic coherent diffractive imaging is performed with a high-intensity high-order harmonic generation source. The coherence properties are analyzed and several reconstructions show the shot-to-shot fluctuations of the incident beam wavefront. The method is based on a multi-step approach. First, the spectrum is extracted from double-slit diffraction data. The spectrum is used as input to extract the monochromatic sample diffraction pattern, then phase retrieval is performed on the quasi-monochromatic data to obtain the sample's exit surface wave. Reconstructions based on guided error reduction (ER) and alternating direction method of multipliers (ADMM) are compared. ADMM allows additional penalty terms to be included in the cost functional to promote sparsity within the reconstruction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/OE.28.000394DOI Listing
January 2020

Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet.

Nat Nanotechnol 2018 12 17;13(12):1154-1160. Epub 2018 Sep 17.

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.

Spintronics is a research field that aims to understand and control spins on the nanoscale and should enable next-generation data storage and manipulation. One technological and scientific key challenge is to stabilize small spin textures and to move them efficiently with high velocities. For a long time, research focused on ferromagnetic materials, but ferromagnets show fundamental limits for speed and size. Here, we circumvent these limits using compensated ferrimagnets. Using ferrimagnetic Pt/GdCo/TaO films with a sizeable Dzyaloshinskii-Moriya interaction, we realize a current-driven domain wall motion with a speed of 1.3 km s near the angular momentum compensation temperature (T) and room-temperature-stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature (T). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. Our work shows that high-speed, high-density spintronics devices based on current-driven spin textures can be realized using materials in which T and T are close together.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41565-018-0255-3DOI Listing
December 2018

Imaging Nanometer Phase Coexistence at Defects During the Insulator-Metal Phase Transformation in VO Thin Films by Resonant Soft X-ray Holography.

Nano Lett 2018 06 18;18(6):3449-3453. Epub 2018 May 18.

ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology , Castelldefels , 08860 Barcelona , Spain.

We use resonant soft X-ray holography to image the insulator-metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.8b00458DOI Listing
June 2018

In situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses.

Nat Commun 2018 01 15;9(1):214. Epub 2018 Jan 15.

Max-Born-Institut Berlin, Max-Born-Straße 2a, 12489, Berlin, Germany.

Free-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam is characterized and optimized separately from the actual experiment. Here, we present the simultaneous measurement of XUV diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. Our in situ characterization scheme enables direct monitoring of the sample illumination, providing a basis to optimize and quantitatively understand FEL experiments.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-017-02567-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5768711PMC
January 2018

A general approach to obtain soft x-ray transparency for thin films grown on bulk substrates.

Rev Sci Instrum 2017 Oct;88(10):103701

Institut für Optik und Atomare Physik, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany.

We present a general approach to thin bulk samples to transparency for experiments in the soft x-ray and extreme ultraviolet spectral range. The method relies on mechanical grinding followed by focused-ion-beam milling. It results in a uniformly thin area of high surface quality, suitable for nanoscale imaging in transmission. In a proof-of-principle experiment, nanoscale magnetic bits on a commercial hard drive glass disk are imaged with a spatial resolution below 30 nm by soft x-ray spectro-holography. Furthermore, we demonstrate imaging of a lithographically patterned test object via absorption contrast. Our approach is suitable for both amorphous and crystalline substrates and has been tested for a variety of common epitaxy growth substrates. Lateral thinning areas in excess of 100 μm and a remaining substrate thickness as thin as 150 nm are easily achievable. Our approach allows preserving a previously grown thin film, and from nanofocus electron diffraction, we find no evidence for morphological changes induced by the process, in agreement with numerical simulations of the ion implantation depth distributon. We expect our method to be widely applicable and especially useful for nanoscale imaging of epitaxial thin films.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.5006522DOI Listing
October 2017

Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques.

Nat Nanotechnol 2017 11 2;12(11):1040-1044. Epub 2017 Oct 2.

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii-Moriya interaction (DMI). The last favours homochiral skyrmions, whose motion is driven by spin-orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin-orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI. Also, the non-magnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect. This leads to torques that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field. Although spin-orbit torques led to domain nucleation in continuous films and to stochastic nucleation of skyrmions in magnetic tracks, no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin-orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/nnano.2017.178DOI Listing
November 2017

Generating circularly polarized radiation in the extreme ultraviolet spectral range at the free-electron laser FLASH.

Rev Sci Instrum 2017 May;88(5):053903

Max-Born-Institut Berlin, Max-Born-Str. 2a, 12489 Berlin, Germany.

A new device for polarization control at the free electron laser facility FLASH1 at DESY has been commissioned for user operation. The polarizer is based on phase retardation upon reflection off metallic mirrors. Its performance is characterized in three independent measurements and confirms the theoretical predictions of efficient and broadband generation of circularly polarized radiation in the extreme ultraviolet spectral range from 35 eV to 90 eV. The degree of circular polarization reaches up to 90% while maintaining high total transmission values exceeding 30%. The simple design of the device allows straightforward alignment for user operation and rapid switching between left and right circularly polarized radiation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4983056DOI Listing
May 2017

Multi-color imaging of magnetic Co/Pt heterostructures.

Struct Dyn 2017 Jan 16;4(1):014301. Epub 2017 Feb 16.

Elettra-Sincrotrone Trieste , 34149 Basovizza, Trieste, Italy.

We present an element specific and spatially resolved view of magnetic domains in Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonant small-angle scattering and coherent imaging with Fourier-transform holography reveal nanoscale magnetic domain networks via magnetic dichroism of Co at the M edges as well as via strong dichroic signals at the O and N edges of Pt. We demonstrate for the first time simultaneous, two-color coherent imaging at a free-electron laser facility paving the way for a direct real space access to ultrafast magnetization dynamics in complex multicomponent material systems.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4976004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315665PMC
January 2017

Curved gratings as an integrated photon fluence monitor in x-ray transmission scattering experiments.

Opt Express 2016 Jun;24(12):13091-100

A concept to obtain a measure of the photon flux accepted by a solid sample in single-shot transmission experiments with extreme ultraviolet (XUV) or soft x-ray radiation is demonstrated. Shallow, continuously distorted gratings are used to diffract a constant fraction of the incident photons onto an extended area of a CCD detector. The signal can be tailored to fit the dynamic range of the detector, i.e. matching the scattered intensity of the studied structure of interest. Furthermore, composite grating designs that also allow for the measurement of the spatial photon distribution on the sample are demonstrated. The gratings are directly fabricated by focused ion-beam (FIB) lithography into a Si3N4 membrane that supports the actual sample layer. This allows for rapid fabrication of hundreds of samples, making the concept suitable for systematic studies in destructive single-shot measurements at free-electron laser (FEL) sources. We demonstrate relative photon flux measurements in magnetic scattering experiments with synchrotron and FEL radiation at 59.6 eV photon energy.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/OE.24.013091DOI Listing
June 2016

Holography-guided ptychography with soft X-rays.

Opt Express 2016 Jan;24(2):1840-51

Ptychography is a lensless imaging technique that aims to reconstruct an object from a set of coherent diffraction patterns originating from different and partially overlapping sample illumination areas. For a successful convergence of the iterative algorithms used, the sample scan positions have to be known with very high accuracy. Here, we present a method that allows to directly encode this information in the diffraction patterns without the need of accurate position encoders. Our approach relies on combining ptychography with another coherent imaging method, namely Fourier-transform holography. We have imaged two different objects using coherent soft-X-ray illumination and investigate the influence of experimental and numerical position refinement on the reconstruction result. We demonstrate that holographically encoded positions significantly reduce the experimental and numerical requirements. Our ptychographic reconstructions cover a large field of view with diffraction-limited resolution and high sensitivity in the reconstructed phase shift and absorption of the objects.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/OE.24.001840DOI Listing
January 2016

Indirect excitation of ultrafast demagnetization.

Sci Rep 2016 Jan 6;6:18970. Epub 2016 Jan 6.

Sorbone Universités, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France.

Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/srep18970DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4702181PMC
January 2016

Stimulation of primary osteoblasts with ATP induces transient vinculin clustering at sites of high intracellular traction force.

J Mol Histol 2014 Feb 10;45(1):81-9. Epub 2013 Aug 10.

Institute for Experimental Orthopaedics and Biomechanics, University of Marburg, Marburg, Germany.

Adenosine 5'-triphosphate (ATP), released in response to mechanical and inflammatory stimuli, induces the dynamic and asynchronous protrusion and subsequent retraction of local membrane structures in osteoblasts. The molecular mechanisms involved in the ligand-stimulated herniation of the plasma membrane are largely unknown, which prompted us to investigate whether the focal-adhesion protein vinculin is engaged in the cytoskeletal alterations that underlie the ATP-induced membrane blebbing. Using time-lapse fluorescence microscopy of primary bovine osteoblast-like cells expressing green fluorescent protein-tagged vinculin, we found that stimulation of cells with 100 μM ATP resulted in the transient and rapid clustering of recombinant vinculin in the cell periphery, starting approximately 100 s after addition of the nucleotide. The ephemeral nature of the vinculin clusters was made evident by the brevity of their mean assembly and disassembly times (66.7 ± 13.3 s and 99.0 ± 6.6 s, respectively). Traction force vector maps demonstrated that the vinculin-rich clusters were localized predominantly at sites of high traction force. Intracellular calcium measurements showed that the ligand-induced increase in [Ca(2+)]i clearly preceded the clustering of vinculin, since [Ca(2+)]i levels returned to normal within 30 s of exposure to ATP, indicating that intracellular calcium transients trigger a cascade of signalling events that ultimately result in the incorporation of vinculin into membrane-associated focal aggregates.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s10735-013-9530-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4544565PMC
February 2014

Femtosecond single-shot imaging of nanoscale ferromagnetic order in Co/Pd multilayers using resonant x-ray holography.

Phys Rev Lett 2012 Jun 27;108(26):267403. Epub 2012 Jun 27.

Department of Materials Science and Engineering, Stanford University, Stanford, California 94035, USA.

We present the first single-shot images of ferromagnetic, nanoscale spin order taken with femtosecond x-ray pulses. X-ray-induced electron and spin dynamics can be outrun with pulses shorter than 80 fs in the investigated fluence regime, and no permanent aftereffects in the samples are observed below a fluence of 25 mJ/cm(2). Employing resonant spatially muliplexed x-ray holography results in a low imaging threshold of 5 mJ/cm(2). Our results open new ways to combine ultrafast laser spectroscopy with sequential snapshot imaging on a single sample, generating a movie of excited state dynamics.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevLett.108.267403DOI Listing
June 2012

Method for single-shot coherent diffractive imaging of magnetic domains.

Phys Rev Lett 2012 Jun 29;108(22):223902. Epub 2012 May 29.

Institut für Optik und Atomare Physik, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany.

In preparation for real space studies of magnetic domains in a pump-probe setup at free-electron laser sources, it is necessary to develop an imaging method compatible with the linearly polarized radiation available at these sources. We present results from a prototype experiment performed at the synchrotron source BESSY II, using a modification of existing phase retrieval techniques. Our results show that it is possible to image magnetic domains in real space using linear polarized light, and we introduce the concept of a reliability map of our reconstructions using Gabor transforms.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevLett.108.223902DOI Listing
June 2012

Dynamic redistribution of paxillin in bovine osteoblasts stimulated with adenosine 5'-triphosphate.

J Mol Histol 2012 Oct 5;43(5):571-80. Epub 2012 May 5.

Institut für Experimentelle Orthopädie und Biomechanik, Philipps-Universität Marburg, Marburg, Germany.

Exposure to extracellular 5'-adenosine triphosphate (ATP) is known to induce membrane blebbing. In this study, we investigated the subcellular distribution of the cytoskeletal adaptor protein paxillin in primary bovine osteoblasts upon stimulation with ATP. Cells expressing a fusion protein of green fluorescent protein (GFP) and paxillin were followed by time-lapse video-microscopy after stimulation with 100 μM ATP. Within 100 s, GFP-paxillin became incorporated in numerous de novo formed focal aggregates localized at the cell periphery. The assembly of individual paxillin-containing aggregates occurred with a mean half-life time of <60 s, whereas their disassembly lasted twice as long. Despite the ongoing presence of ATP, the formation of paxillin aggregates was self-limiting within 25 min. Paxillin clustering was preceded by a transient rise in cytoplasmic calcium transients, which peaked already 20 s after adding ATP. The high mobility of paxillin was confirmed by measuring the dissociation rate of GFP-paxillin at mature focal adhesions, demonstrating the presence of a highly mobile fraction with a mean recovery half-life of 8.2 ± 1.2 s, followed by a slower phase (53 ± 20 s). Thus, both the exchange of paxillin at mature focal adhesions and the increase in intracellular calcium concentrations upon ATP stimulation are very rapid processes, which override the time course of ATP-induced paxillin membrane clustering by one to two orders of magnitude. Our data demonstrate that the transient recruitment of paxillin in membrane protuberances is based on the high intracytoplasmic mobility of unbound paxillin molecules and their rapid focal accumulation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s10735-012-9419-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3460167PMC
October 2012

Wavefield back-propagation in high-resolution X-ray holography with a movable field of view.

Opt Express 2010 Aug;18(18):18922-31

Institut für Optik und Atomare Physik, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany.

Mask-based Fourier transform holography is used to record images of biological objects with 2.2 nm X-ray wavelength. The holography mask and the object are decoupled from each other which allows us to move the field of view over a large area over the sample. Due to the separation of the mask and the sample on different X-ray windows, a gap between both windows in the micrometer range typically exists. Using standard Fourier transform holography, focussed images of the sample can directly be reconstructed only for gap distances within the setup's depth of field. Here, we image diatoms as function of the gap distance and demonstrate the possibility to recover focussed images via a wavefield back-propagation technique. The limitations of our approach with respect to large separations are mainly associated with deviations from flat-field illumination of the object.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/OE.18.018922DOI Listing
August 2010

Magnetic imaging at linearly polarized x-ray sources.

Opt Express 2010 Jun;18(13):13608-15

TU Berlin, Institut für Optik und Atomare Physik, Hardenbergstr. 36, Berlin, Germany.

We present a method for high-resolution magnetic imaging at linearly polarized partially coherent x-ray sources. The magnetic imaging was realized via Fourier transform holography. In order to achieve elliptical x-ray polarization, three different filters were designed based on the x-ray magnetic circular dichroism effect. We present proof-of-principle images of magnetic nanostructures and discuss the application of the method for future experiments at free-electron laser sources.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/OE.18.013608DOI Listing
June 2010

Atomic diffusion studied with coherent X-rays.

Nat Mater 2009 Sep 26;8(9):717-20. Epub 2009 Jul 26.

Fakultät für Physik, Universität Wien, Strudlhofgasse 4, 1090 Wien, Austria.

Knowledge of atomic diffusion is a fundamental issue in synthesis and stability of materials. Direct studies of the elementary diffusion event, that is, how the individual atoms 'jump', are scarce, as the available techniques are limited to selected systems. Here we show how by monitoring the spatial and temporal variations of the scattered coherent X-ray intensity the diffusion of single atoms can be studied. This is demonstrated for the intermetallic alloy Cu(90)Au(10). By measuring along several directions in reciprocal space, we can elucidate the dynamical behaviour of single atoms as a function of their neighbourhood. This method, usually referred to as X-ray photon correlation spectroscopy (XPCS), does not rely on specific atomic species or isotopes and can thus be applied to almost any system. Thus, given the advent of the next-generation X-ray sources, XPCS has the potential to become the main method for quantitatively understanding diffusion on the atomic scale.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/nmat2506DOI Listing
September 2009

Digital in-line holography with femtosecond VUV radiation provided by the free-electron laser FLASH.

Opt Express 2009 May;17(10):8220-8

Angewandte Physikalische Chemie, Universität Heidelberg, Heidelberg, Germany.

Femtosecond vacuum ultraviolet (VUV) radiation provided by the free-electron laser FLASH was used for digital in-line holographic microscopy and applied to image particles, diatoms and critical point dried fibroblast cells. To realize the classical in-line Gabor geometry, a 1 microm pinhole was used as spatial filter to generate a divergent light cone with excellent pointing stability. At a fundamental wavelength of 8 nm test objects such as particles and diatoms were imaged at a spatial resolution of 620 nm. In order to demonstrate the applicability to biologically relevant systems, critical point dried rat embryonic fibroblast cells were for the first time imaged with free-electron laser radiation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/oe.17.008220DOI Listing
May 2009

Holographic soft X-ray omni-microscopy of biological specimens.

Opt Express 2009 Apr;17(8):6710-20

Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.

Mask-based digital soft X-ray Fourier transform holography using coherent synchrotron radiation is applied to image diatoms and freeze-dried 3T3 fibroblast cells. At soft X-ray wavelengths between 4 nm and 8 nm we achieve spatial resolutions down to 450 nm over a 33.5 microm field of view. As holography records amplitude and phase of the scattered wavefield, images with different contrast mechanisms can be generated a posteriori by numerical processing of the holographic data ("omni-microscopy"). We present Zernike phase contrast, Schlieren phase contrast, differential interference and gradient contrast images based on amplitude and phase information. We discuss the potential of this approach for destructive single-shot imaging beyond dose limitations of biological samples at coherent femtosecond-pulsed X-ray sources.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1364/oe.17.006710DOI Listing
April 2009

Detrended fluctuation analysis in x-ray photon correlation spectroscopy for determining coarsening dynamics in alloys.

Phys Rev E Stat Nonlin Soft Matter Phys 2006 Oct 6;74(4 Pt 1):041107. Epub 2006 Oct 6.

Fakultät für Physik, Institut für Materialphysik, Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria.

We study the dynamics of precipitate coarsening in phase-separating alloys at late stages of phase separation by x-ray photon correlation spectroscopy (XPCS). For analyzing time series of fluctuating speckle intensities from small-angle scattering of coherent x rays, the method of detrended fluctuation analysis (DFA), which is ideal for determining power-law correlations, is applied. We discuss the application of DFA with respect to XPCS data by means of simulated time series. In particular, the effects of different signal-to-noise ratios are examined. Results from measurements of the two model systems Al-6 at. % Ag at 140 degrees C and Al-9 at. % Zn at 0 degrees C are presented. Since the DFA effectively removes adulterating trends in the data, quantitative agreement with Monte Carlo simulations is obtained. It is verified that two different coarsening mechanisms are predominant in the two systems--coarsening either by diffusion of single atoms or by movement of whole precipitates.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevE.74.041107DOI Listing
October 2006
-->