Research Highlights at FLASH

Published as: “Ultrafast Melting of a Charge-Density Wave in the Mott Insulator 1T-TaS₂”, Physical Review Letters 105, 187401 (2010).

The combination of an optical laser with FEL pulses from FLASH in a pump-probe setup uniquely enables timeresolved core-level photoemission spectroscopy with high temporal and energy resolution. In a proof-ofprinciple experiment, a charge-density wave in the layered strongly correlated electron material 1T-TaS2 is driven out of equilibrium by an intense optical laser pulse and the subsequent nonequilibrium dynamics is probed by FLASH pulses on the sub-picosecond time scale. The results establish FLASH as an ultrafast movie camera for electronic structural dynamics at the surfaces of solids. [More]

Published as: "Sacrificial Tamper Slows Down Sample Explosion in FLASH Diffraction Experiments", Physical Review Letters 104, 064801 (2010).

Coating samples in a sacrificial external layer slows down the rate at which samples are damaged by the ultra-intense X-ray beam provided by the FLASH free electron laser. The extremely intense light produced by FLASH enables coherent imaging using single ultrafast X-ray pulses. The same intense pulse destroys the sample, and diffraction must occur before the X-ray beam significantly alters the sample structure, requiring ever-shorter pulses as resolution is increased. Experiments at FLASH showed that it is possible to delay sample explosion for up to picosecond time duration by coating samples in a sacrificial ‘tamper’ layer. This approach enables the use of longer and more intense pulses for high-resolution single-particle imaging, lowering the barrier to high-resolution structural studies on isolated ... [More]

Published as: “Single-pulse resonant magnetic scattering using a soft x-ray free-electron laser”, Phys. Rev. B 81, 100401 (R) (2010).

Single-pulse resonant magnetic scattering experiments were performed by using soft X-ray pulses generated by the free-electron laser FLASH. A magnetic diffraction pattern was recorded from a Co/Pt multilayer sample at the Co M_2,3 edge with a single 30 femtosecond long FEL pulse, without destroying the sample. [More]

Published as: “Cryptotomography: Reconstructing 3D Fourier Intensities from Randomly Oriented Single-Shot Diffraction Patterns”, Phys. Rev. Lett. 104, 239902 (2010).

A new method called cryptotomography combines two dimensional (2D) diffraction patterns of identical particles, each collected in a random and unknown orientation, to unravel the full three-dimensional (3D) image of the average particle. We carried out the first demonstration of cryptotomography by collecting diffraction patterns of ellipsoidal iron oxide nanoparticles at FLASH. These particles were shot across the FEL beam, in vacuum, using the techniques being developed for single-molecule imaging. This demonstration is an important step towards the 3D imaging of noisy and weakly scattering biological samples at X-ray free electron laser sources. [More]

Published as: “The liquid-liquid phase transition in silicon revealed by snapshots of valence electrons”, Proc. Natl. Acad. Sci. USA 107, 39, 16772–16776 (2010).

Microscopic models for the „anomalies of water“ are still lacking an experimental proof, although anomalous thermodynamic behaviour is common for a class of matter that forms tetrahedral networks - like water, diamond or silicon. Their phase diagrams are very rich, but the exploration of large areas has been limited mostly to theoretical studies. And yet, remainders of those experimentally unaccessible areas contribute to the properties at standard conditions. For example a possible explanation for the anomalies is based on the existence of a liquid-liquid phase transition in the supercooled region. With the combination of ultrashort optical laser pulses and soft X-ray pulses from FLASH, we study the melting dynamics of silicon in detail. We find two distinct melting steps separated ... [More]

Published as: “Near edge X-ray absorption fine structure spectroscopy with X-ray free-electron lasers”, Appl. Phys. Lett. 95, 134102 (2009).

A new method to perform X-ray absorption spectroscopy experiments at a free-electron laser has been developed. Instead of selecting a narrow bandwidth of the incident beam with a grating-monochromator, a dispersive set-up is used. The incident pink radiation from the SASE-FEL is dispersed by the grating and collected by an area detector. A special sample-preparation method has been used in order to measure the intensity and energy distribution of the incident and absorbed beam simultaneously. This method can be improved in the future to perform pump-probe experiments with XAS as the probe with fs-temporal resolution at an FEL. [More]

Published as: "R.R. Fäustlin et al. Observation of Ultrafast Nonequilibrium Collective Dynamics in Warm Dense Hydrogen", Phys. Rev. Lett. 104, 125002 (2010).

The short pulse duration and high intensity of FLASH soft x-ray radiation at DESY allows us to generate and probe highly homogeneous warm dense non-equilibrium hydrogen within a single light pulse. By analyzing the spectrum of the 13.5 nm Thomson scattered light we determine the plasma temperature and density. The results are compared via simulations with different models for impact ionization, which is the main interaction on this early femtosecond time scale during the evolution of the plasma. We find that classical models of this interaction describe our dense plasma conditions better than state of the art theories. This has implications for various fields ranging from planetary astrophysics to inertial confinement fusion. [More]

Published as: “Extreme Ultraviolet Laser Excites Atomic Giant Resonance”, Phys. Rev. Lett. 102, 163002 (2009).

FLASH produces the highest irradiance of extreme ultraviolet (EUV) and soft X-ray pulses in the world. When these pulses are focused even further by a spherical multi-layer mirror developed for EUV lithography, the levels of irradiance can reach 10¹⁶ W cm^-2 [1]. The mechanism of light-matter interaction is not very well understood at these wavelengths and extremely high irradiance levels and needs to be investigated to further our 0understanding about the structure of atomic matter and the photoelectric effect. Our measurements show that the nature of the interaction between the EUV light and matter is heavily dependent on the atomic structure of the target and the excitation of strong resonances. [More]

Published as: Resonant magnetic scattering with femtosecond soft X-ray pulses from a free electron laser operating at 1.59 nm, Phys Rev B 79, 212406 (2009)

We report on a resonant magnetic scattering experiment using soft X-ray pulses generated from the free-electron laser FLASH at DESY. Using the fundamental wavelength of FLASH at 7.97 nm we were able to detect the 5th harmonic at a wavelength of 1.59 nm with an average energy of 4 nJ per pulse. We demonstrated the feasibility of resonant magnetic scattering at an FEL source by using a Co/Pd multilayer as prototype sample that was illuminated with 20 femtosecond-long soft X-ray pulses tuned to the Co L₃ absorption edge at 778.1 eV (1.59nm). [More]

Published as: “Single-shot Terahertz field driven X-ray streak-camera”, Nature Photonics doi: 10.1038/NPHOTON.2009.160.

A few-femtosecond X-ray streak camera for the temporal characterisation of ultrashort X-ray pulses produced by a free-electron laser has been realised at FLASH. In the experiment the electric field of an intense THz pulse is used to accelerate photoelectrons which have been ionised by the FEL pulses. Borrowing its concept from attosecond metrology, the femtosecond X-ray streak camera fills the gap between conventional streak cameras with typical resolutions of hundreds of femtoseconds and streaking techniques operating in the sub fs regime. Its single-shot capability permits to determine the duration and time structure of individual X-ray pulses which at SASE FELs fluctuate from shot to shot. [More]

Published as: “Turning solid aluminium transparent by intense soft X-ray photoionization”, Nature Physics doi:10.1038/nphys1341.

The free-electron laser FLASH at DESY when focussed to ≤1 µm spots reaches record intensities over 10¹⁶ W/cm² in the soft X-ray wavelength regime. In an experimental campaign FLASH, at photon wavelength of 13.5 nm, reached these extreme intensities leading to the saturation of the absorption of an L-shell transition in aluminium: the samples become transparent for soft X-rays (at 92 eV photon energy). This has never been observed before and is an ideal method to create homogenous warm dense matter (WDM), which is highly relevant to planetary science, astrophysics and inertial confinement fusion. [More]

Published as: ''Coherent-pulse 2D Crystallography at Free Electron Lasers'' Phys. Rev. Lett. 102, 035502/1-5 (2009).

Crystallization and radiation damage is presently a bottleneck in protein structure determination. We propose to use two-dimensional (2D) finite crystals and ultrashort Free Electron Laser pulses to reveal the structure of single molecules. This can be especially important for membrane proteins that in general do not form 3D crystals, but easily form 2D crystalline structures. We have demonstrated single pulse train coherent diffractive imaging for a finite 2D crystalline sample, and conclude that this alternative approach to single molecule imaging is a significant step towards revealing the structure of proteins with sub-nanometer resolution at the newly built XFEL sources. [More]

Published as: “Multistep Ionization of Argon Clusters in Intense Femtosecond Extreme Ultraviolet Pulses”, Physical Review Letters 100, 133401 (2008)

The soft X-ray free electron laser FLASH allows for the first time to probe the super intense light pulse – matter interaction at short wavelength, providing the basis for a large variety of studies with femtosecond X-ray pulses. We have performed a photoelectron spectroscopy study complemented with Monte Carlo simulations about the ionization dynamics of atomic clusters at FLASH at power densities exceeding 10¹³ W/cm². We find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. In contrast to earlier studies in the infrared and vacuum ultraviolet regime, there are no indications for efficient plasma heating processes. The current findings have large implications for the ionization and dissociation dynamics of ... [More]

Published as: “Massively parallel X-ray holography", Nature Photonics 2, 560–563 (2008)

The ultrafast pulses from short-wavelength free-electron lasers (FELs), such as FLASH, give us the possibility for imaging processes on the timescale of atomic motions. The upcoming X-ray FELs such as Linac Coherent Light Source and European XFEL will additionally allow us to probe dynamic processes approaching inter-atomic length scales. For many such experiments it is necessary to extract spatially-resolved information from a single pulse since sample damage may prevent stroboscopic methods. We have developed an extremely photonefficient method to form a real-space image of the object. Borrowing concepts from X-ray astronomy, our method of “Massively Parallel Holography” utilizes a special kind of coded pattern called a uniformly redundant array to form a hologram of the object. We showed that the pattern amplified the holographic signal of a bacterial ... [More]

Published as: “Ultrafast single-shot diffraction imaging of nanoscale dynamics”, Nature Photonics 2, 415–419, (2008).

Combining an optical laser with FEL pulses from FLASH in a pump-probe set-up enables imaging with high temporal and spectral resolution. A nanostructure is ablated by an optical laser and this process is followed with lensless coherent diffractive imaging using FLASH pulses. A spatial resolution of better than 50 nm is achieved.  By taking pictures of a succession of exploding targets, a movie can be made following the dynamics of the solid material on a 10-ps timescale. With short-wavelength X-ray FEL sources this method will enable higher spatial resolution imaging and could be used to measure the dynamics of highly correlated systems at nanometer length scales.

[More]

Published as: C. Gahl et al., Nature Photonics 2, 165-169 (2008)

For a fundamental understanding of ultra fast dynamics in chemistry, biology and materials science it has been a longstanding dream to record a molecular movie, where both the atomic trajectories and the chemical state of every atom in matter is followed in real time with time resolved pump/probe spectroscopy. X-ray free-electron lasers (FEL) provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range. To cross-correlate and synchronize the FEL with separate optical lasers we exploit the peak brilliance of the free-electron laser at Hamburg (FLASH) and establish X-ray pulse induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough en route to a molecular movie and – equally important – opens the novel field of femtosecond X-ray induced ... [More]

Published in: Physical Review A 74, 011401(R) (2006)

The Free electron LASer in Hamburg (FLASH) is a unique source for intense, ultra-short extreme-uv (xuv) light pulses. In view of these unprecedented characteristics, an associated time-synchronized optical laser facility opens up new and particularly exciting research opportunities. First characterization of this pump-probe set-up was obtained recently via time-dependent two-photon experiments on rare gases. The process of above threshold ionization (ATI), which is very sensitive to the spatial and temporal overlap of both pulses, was used to demonstrate the capability and the stability of the combined application of two independent femtosecond laser sources, namely the XUV free electron laser and the synchronized optical laser. [More]

Published in: Nature Physics 2, 839 (2006)

Using the FLASH facility we have demonstrated high-resolution coherent diffractive imaging with single soft-X-ray free-electron laser pulses [1]. The intense focused FEL pulse gives a high resolution low-noise coherent diffraction pattern of an object before that object turns into a plasma and explodes. Our experiments are an important milestone in the development of single-particle diffractive imaging with future X-ray free-electron lasers [2,3]. Our apparatus provides a new and unique tool at FLASH to perform imaging of biological specimens beyond conventional radiation damage resolution limits [2, 4] and to acquire images of ultrafast processes initiated by an FEL pulse or other laser pulse. [More]

Published in Nature 420, 482 (2002)

Fourth-generation light sources based on free-electron lasers (FEL) will be a source of intense, short-wavelength radiation for a range of applications, including biological imaging. The first results reported on December 5, 2002 in Nature, from experiments with the FEL at the TESLA test facility at DESY shed light on the interaction of short-wavelength, short-pulse radiation with matter. Here, unexpectedly strong absorption of the laser radiation by Xenon clusters was observed, resulting in the explosion of the clusters and the ejection of high-energy, multiply charged ions. Such a highly nonlinear interaction between matter and soft X-rays below 100 nm has never been seen before. [More]