Category: FLASH (DESY)

Watching superheated silver nanoparticles bubbling

Watching superheated silver nanoparticles bubbling

Matter under extreme conditions, especially at extreme temperatures and pressures, plays an important role in many fields. These range from astrophysics and geology over inertial fusion reactor studies to applied research on material processing by laser ablation. Due to the complex behaviour of matter under such conditions the underlying interactions are not yet fully understood. In recent years isolated nanoparticles, which form well-defined crystalline structures at lower temperatures, have proven to be suitable test objects for the study of such questions.

A team of researchers including DESY scientists around Daniela Rupp (ETH Zurich), Thomas Möller (TU Berlin) and Bernd v. Issendorff (Univ. Freiburg), as well as several groups from Univ. Rostock (Ingo Barke, Karl-Heinz Meiwes-Broer, Thomas Fennel), has applied time-resolved soft X-ray diffraction at the CAMP endstation at FLASH to study the dynamics of superheated silver nanoparticles in a laser pump – FEL probe experiment. They observed that silver nanoparticles, when strongly laser-heated via their plasmon resonance, exhibit a wide range of phenomena, from melting over cavitation-induced bubble-like expansion to explosion, as a function of the excitation strength. “Depending on the heating conditions and the temporal evolution, we could identify different classes of diffraction images, stemming from faceted, round, hollow, fragmenting or sometimes finally exploding nanoparticles”, Alessandro Colombo (ETH Zurich), one of the three first authors, explains.

Read more on DESY website

Image: Diffraction image of a hollow silver nanoparticle (left) measured at FLASH, along with its reconstruction (top right) and a corresponding picture from MD simulations (bottom right).

Credit: T. Reichenbach, Univ. Freiburg

Molecular movie of gold nanoparticle oscillations driven by displaced electrons

Molecular movie of gold nanoparticle oscillations driven by displaced electrons

Photocatalysis, sensors, solar cells: Plasmons promise a variety of applications if the processes triggered by optical excitation in the nanoparticles can be controlled. A research team from Hamburg and Berlin reports experimental observations of a so-called molecular movie that cannot be explained by established models in Nano Letters. The team including researchers from DESY provides a new theoretical model that explains the dynamics of excited gold nanoparticles observed in their experiments.

Plasmons are collective electron oscillations associated with highly localised fields. The decay of these oscillations after optical excitation is currently the subject of intense debate. Researchers assume that very energetic “hot” electrons are generated in the process which lose their energy by electron-electron scattering into a “warm” electron gas. The gas heats up the particle which eventually releases the excess energy into the environment. The efficiency of the energy transfer between the “hot electron”, “warm electron”, and “warm particle” stages is important for applications wanting to make use of these processes. In particular, the energy transfer from the warm electron gas to the nanoparticle appears to be so efficient that the particle is heated extremely quickly. In the process, it expands explosively, causing it to oscillate collectively, like a breathing sphere. However, so far direct experimental studies resolving the breathing oscillation have been missing.

For their study, researchers from the Departments of Physics and Chemistry at Universität Hamburg, the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), the CFEL at DESY, and TU Berlin joined forces. Led by Holger Lange, Jochen Küpper, and Kartik Ayyer, who all conduct research in the Cluster of Excellence “CUI: Advanced Imaging of Matter”, and Andreas Knorr from Berlin, the team combined theory and experiment for an accurate description of the dynamics of excited gold nanoparticles.

Using single-particle X-ray diffractive imaging (SPI), performed at DESY’s FLASH facility, and transient absorption spectroscopy (TA), the researchers determined both the structural size and the electron temperature of the nanoparticles after optical excitation as a function of time. They observed that the particles already expanded with the optical excitation pulse, much faster than previously assumed. This observation directly proved the need for an immediate excitation source other than the temperature rise and associated expansion of the particle.

Read more on the DESY website

Image: Optical excitation of gold nanoparticles directly sets the particle into an oscillatory motion in which the particle periodically expands and contracts.

Credit: Univ. Hamburg/H. Lange

Electrons and photons in a twin pack

Electrons and photons in a twin pack

Resonant two-photon ionisation of helium measured with angular resolution

Using a new experimental method, physicists from the Max Planck Institute for Nuclear Physics in Heidelberg investigated the resonant two-photon ionisation of helium with improved spectral resolution and angular resolution. For this purpose, they utilised a reaction microscope in combination with a high-resolution extreme-ultraviolet (EUV) photon spectrometer developed at the Institute. The measurements have been performed at the Free Electron Laser in Hamburg (FLASH), a brilliant radiation source, delivering intense EUV laser flashes. This allows the events from each individual laser flash to be analysed in terms of photon energy, yielding spectrally high-resolution data sets.

Helium, as the simplest and most accessible multi-electron system, is ideally suited for fundamental theoretical and experimental studies. Here, the mutual electrical repulsion of the two electrons plays an essential role – it accounts for a good third of the total binding energy. Of particular and fundamental interest is the interaction with photons (the quanta of light). Researchers from the groups around Christian Ott and Robert Moshammer in the division of Thomas Pfeifer at the Max Planck Institute for Nuclear Physics in Heidelberg have investigated the resonant two-photon ionisation of helium in detail at the free-electron laser FLASH of DESY in Hamburg.

Read more on the DESY website

Image: Fig. 2: Spectrum of photons unsorted (top) and sorted by peak position (bottom).

Electronic quantum dance in molecules

Electronic quantum dance in molecules

Scientists watch moving charge density in real-time

An international research team led by DESY scientist Tim Laarmann has for the first time been able to monitor the quantum mechanically evolving electron charge distribution in glycine molecules via direct real-time measurement. The results – obtained at DESY´s brilliant free-electron laser FLASH – are published in the scientific journal Science Advances. Better knowledge of the quantum effects in the motion of electrons at the molecular level can pave the way to controlling, optimising, and engineering ionising radiation to be used for example in radiotherapy for cancer treatment.

“The amino acid glycine is an abundant basic building block of proteins and plays part in the recognition sites on cell membranes and enzymes,“ says Laarmann. “Due to its compact nature and tendencies to form hydrogen bonds it facilitates protein folding in biomolecular reactions. Stand-alone, it is utilized as an inhibiting neurotransmitter in the central nervous system.” Glycine has also been found in space and is therefore a first signature of extra-terrestrial life. The molecular reactivity in the harsh astronomical environments is an important aspect, and in particular how isolated molecules interact with ionizing radiation is a key question in astrochemistry.

Read more on DESY website

Image: In the prump-probe experiment the glycine molecule is first ionised by the high intensity X-ray pulse from DESY’s free-electron laser FLASH (left). This induces a correlated motion of the valence electrons and holes, depicted by red an d blue lobes. After a variable time delay from 0 to 175 femtoseconds the probe pulse samples the state of the glycine ion and electron motion through further ionisation and measurement of the ionisation products (right). In this example, a time delay of 10 femtoseconds is depicted, which shows two extrema of the oscillatory electron/hole motion, i.e. a half period of the electron coherence.

Credit: DESY, David Schwickert

Photon Science: A career of creativity & intriguing questions awaits

Photon Science: A career of creativity & intriguing questions awaits

Markus Ilchen is a physicist at FLASH, the world’s first short wavelength free-electron laser. FLASH is located at DESY in Hamburg. The DESY campus is a ‘small city’ of science offering a versatile and vibrant culture for a wide variety of professions and scientific disciplines. In his #LightSourceSelfie, Markus gives you a peek into some of the highlights on campus, describing some of its history and how FLASH’s unique capabilities will help him to study the chirality (handedness) of molecules. Contributing to solving the mystery behind what chirality does in our universe, drives him and his colleagues.

For those starting out in photon science, Markus has this advice, “Enjoy the great choice! But still of course find your sweet spot. Find your place where you have fun; where you can be yourself; where you can work with nice people; where you are working on intriguing questions; where you can be creative and enjoy the freedom of science in a way that, for one, it keeps you up at night but in a good way.”

Microscopic origins of electrical conductivity in superheated solids revealed

Microscopic origins of electrical conductivity in superheated solids revealed

Scientists used terahertz radiation for measurements of strongly excited material

In-depth understanding of the electrical conductivity of matter is the key to many cutting-edge research and applications, ranging from phase-change memory in microelectronics to magnetospheres rooted in planetary interiors due to the motion of the conductive fluid. Unique states of material created by ultrafast table-top lasers or free-electron lasers (FEL) allow us to gain insight into atomic levels. However, it also requires sub-picosecond resolution to capture the details on the timescale of atomic motion. Therefore, in conductivity measurements it prevents the use of contact diagnostics such as multimeter and four-point-probe. Although ultrafast optical or X-ray measurements can provide information on high frequency electrical conductivity, they require complex models to extrapolate the intrinsic direct current (DC) conductivity of material.

The terahertz radiation (1 THz= 1012 Hz (cycles per second)) offers a unique solution to tackle this dilemma. The THz electromagnetic wave behaves like DC electric-field to the sample because the oscillation of its electric field is slow compared to the electron momentum relaxation frequencies in solid and liquid materials (typically 1013Hz or larger), and the width of each THz cycle is short enough to resolve sub-picosecond dynamics. Nevertheless, to measure the conductivity of strongly excited materials in the irreversible regime still requires high brightness THz radiation in order to penetrate the dense electron cloud as well as high sensitivity to detect the THz temporal profile in a single shot.

An international research team, led by scientists from the SLAC National Accelerator Laboratory and DESY, have recently measured the electrical conductivity of strongly heated material using the THz FEL radiation at FLASH. In this study, gold nano-foil samples were heated by the FLASH extreme ultraviolet (XUV) FEL pulses to electron temperatures up to 16,000 °C. As the thermal energy transfers from the electrons to the ions, the sample transits from cold to superheated solid and eventually melts into warm dense liquid. The researchers have determined the DC electrical conductivity by measuring the transmitted THz electric field through the heated samples. The multi-cycle THz pulses from FLASH provide continuous measurements with temporal resolutions better than 500 femtoseconds.

Read more on the DESY website

Image: Artist’s impression: origins of the electrical conductivity in superheated solids measured with THZ radiation at FLASH at DESY

Credit: Z. Chen, SLAC

Study at FLASH: XUV lasing from exploding noble-gas nanoclusters

Study at FLASH: XUV lasing from exploding noble-gas nanoclusters

New mechanism of XUV light amplification

An international team of scientists, headed by Nina Rohringer from DESY and Unversität Hamburg, has succeeded in getting bursts of laser-like extreme ultraviolet (XUV) emission from noble-gas clusters in the transient warm dense matter state. Xenon clusters were irradiated by DESY’s free-electron laser FLASH, and the resulting strongly amplified fluorescence signal was analysed by a high-resolution spectrometer. Theoretical modeling of the process indicates that the clusters, transformed to a nanometer-sized plasma (‘nanoplasma’), enable the creation of population inversion by means of electron-ion collisions. The transient but sizeable population inversion of the ensemble of clusters enables amplification of spontaneous emission in a single pass of the emitted XUV radiation. This study, performed at the CAMP station of the FLASH beamline BL1 at DESY, is published in Physical Review A and is highlighted as an Editors’ Suggestion.

>Read more on the FLASH website

Image: Excited noble-gas clusters stimulate lasting emission in the forward direction. (Credit: Original publication in Phys. Reb. A (2020))

Record participation at user meetings of the Hamburg research light sources

Record participation at user meetings of the Hamburg research light sources

More than 1300 participants from 28 countries have registered

For this year’s users’ meetings of the Hamburg X-ray light sources, more participants have registered than ever before: More than 1300 scientists from 28 countries will come to discuss research with DESY’s X-ray source PETRA III, the free-electron laser in Hamburg FLASH and the X-ray laser European XFEL for three days starting this Wednesday. The jointly organised users’ meetings of DESY and European XFEL are the largest gathering of this kind worldwide.

“The steadily increasing number of participants from Germany and abroad shows the great importance of the Hamburg research light sources for the national and international scientific community,” says DESY’s Director for Photon Science, Edgar Weckert. “Hamburg is one of the X-ray capitals of the world.” The brilliant X-ray light from the powerful particle accelerators provides detailed insights into the structure and dynamics of matter at the atomic level. It can be used, for example, to decipher the structure of biomolecules, illuminate innovative materials, film chemical reactions and simulate and study the conditions inside planets and stars.

At the European X-ray laser European XFEL, all six scientific experiment stations are in operation since June. “Our users’ experiences and expertise are crucial for shaping the future of our science and facility”, says European XFEL managing director Robert Feidenhans’l. “The annual users’ meeting, therefore, is an extremely valuable opportunity for users and scientists who work at our facilities to share their experiences of doing experiments at the instruments, and talk about ideas for further development.” In 2019, 890 scientists from 255 institutes in 28 countries participated in experiments at the facility.

> Read more on the PETRA III and FLASH website

> Please find here another article on the European XFEL website

Picture: The jointly organised users’ meetings are the largest gathering of this kind worldwide.
Credit: DESY, Marta Mayer

60 years of DESY – From Hamburg particle accelerator to global research centre

60 years of DESY – From Hamburg particle accelerator to global research centre

Germany’s largest accelerator centre turns 60 on 18 December 2019

The story of DESY began on 18 December 1959 with the signing of a contract in Hamburg’s town hall. It is a story of success, for global research and for Germany as a science hub! For the past 60 years, fundamental research has been carried out at DESY in Hamburg-Bahrenfeld – which was joined in 1991 by a second DESY site in Zeuthen. In those 60 years, DESY has become a world leader in accelerator technology, structure research, particle physics and astroparticle physics. During these 60 years, DESY has developed pioneering technologies, which have been used by scientists from all over the world to make outstanding advances. Among other things, the gluon was discovered and the structure of ribosomes was determined at DESY.
“It is now a question of the big challenges of our times,” says DESY’s director Professor Helmut Dosch. “We have developed a new generation of research tools in the form of so-called X-ray lasers. These afford fundamental insights in medicine and in materials engineering, for example, which will help shape the world of tomorrow.” DESY offers unique conditions for this: the combination of the radiation sources PETRA III, FLASH and European XFEL means that international scientists can carry out experiments using high-intensity X-rays. In addition to this, DESY offers structure researchers and businesses from all over the world a unique “toolbox” in the form of supplementary methods for manufacturing, processing and examining nano-samples and nanomaterials. DESY’s second site in Zeuthen is also an international magnet as a growing centre of excellence in astroparticle physics. Zeuthen operates the only accelerator in Brandenburg and is one of the largest scientific institutions in the region.

>Read more on the DESY website

Image: Part of the DESY staff in Hamburg holds the DESY-60 logo
Credit: DESY/H. Müller-Elsner

Superfluorescent emission in the UV range

Superfluorescent emission in the UV range

Free-electron laser FLASH coaxes superfluorescent emission from the noble gas xenon

Scientists have for the first time induced superfluorescence in the extreme ultraviolet range. Superfluorescence, or superradiance, could be used to build a laser that does not require an optical resonator. The team headed by DESY’s lead scientist Nina Rohringer used DESY’s free-electron laser FLASH to stimulate xenon, a noble gas, inside a narrow tube, causing it to emit coherent radiation, like a laser. The research team is now presenting its work in the journal Physical Review Letters.

“The phenomenon of superfluorescence was first discovered in the microwave range in the 1970s, and then demonstrated for infrared and optical wavelengths too,” explains Rohringer. “In the meantime, superfluorescence has also been observed in the X-ray domain, and at one time this mechanism was believed to be a promising candidate for building X-ray lasers. Until now, however, superfluorescence had not been demonstrated in the extreme ultraviolet, or XUV, range.”

In superfluorescence, the incident light is amplified and emitted along the axis of the medium as a narrow beam of coherent radiation, like in a laser. To produce superfluorescence in the XUV spectrum, the incoming light needs to have enough energy to knock the electrons out of the inner shell of the atoms that make up the lasing medium. Redistribution within the electron shell (Auger decay) leads to a situation in which more particles find themselves in an excited state than in an unexcited state. Physicists refer to this as population inversion.

>Read more on the FLASH at DESY website

Image: The xenon superfluorescence shows up as a bright line (yellow) superimposed on the averaged free-electron laser spectrum (purple, averaged over many shots).
Credit: European XFEL, Laurent Mercadier

When is a laser a real laser?

When is a laser a real laser?

Pulsed lasers are intense and coherent light sources, and the latest category is that of Free Electron Lasers, such as FERMI. First order coherence is a familiar phenomenon, and is manifested for example in diffraction phenomena. This represents the correlation between the amplitudesof a wave at different points in space (transverse coherence) or time (longitudinal coherence.) However, a high degree of first order coherence is not enough to define a laser, according to the Nobel laureate Roy Glauber, who stated that a laser can be defined as a source that is coherent in all orders. The higher order correlations are between intensityat different points in time and space. How are these correlations measured? For this one has to look at the statistics of the photons.
Glauber’s work was inspired by the famous Hanbury Brown and Twiss experiment, in which coincidences of photons (i.e. correlations) were measured of photons coming from distant stars. By varying the distance between two detectors, they were able to determine the degree of coherence of the star, and extract other information. This is the key to measuring the second order coherence of a light source: the intensity of light at different points is measured in coincidence, and statistical analysis is made. This experiment is considered by many as initiating the whole field of quantum optics. Now a team led by Ivan Vartaniants (DESY, Hamburg, and the National Research Nuclear University, Moscow) has performed a Hanbury Brown and Twiss experiment at FERMI. Instead of the two discrete photodetectors used originally, a CCD detector was used. Since all of the photons arrive in less than 100 fs, there is no need to use coincidence methods: the signal is naturally synchronised.

>Read more on the FERMI at Elettra Sincrotrone Trieste website

Figure 1.  Difference between chaotic and coherent light sources. (a) photon correlation map for FERMI operated in seeded mode. (b) corresponding spectrum. (c) correlation map for FERMI operated in Self Amplified Stimulated Emission mode (the mode of operation of most Free Electron Lasers). (d) corresponding spectrum.
Credit: Reprinted from O. Yu. Gorobtsov et al, Nature Communications 9 (2018) 4498. (Copyright Nature Publishing Group)

First experiments reveal unknown structure of antibiotics killer

First experiments reveal unknown structure of antibiotics killer

DESY-led international collaboration obtains first scientific results from European XFEL

An international collaboration led by DESY and consisting of over 120 researchers has announced the results of the first scientific experiments at Europe’s new X-ray laser European XFEL. The pioneering work not only demonstrates that the new research facility can speed up experiments by more than an order of magnitude, it also reveals a previously unknown structure of an enzyme responsible for antibiotics resistance. “The groundbreaking work of the first team to use the European XFEL has paved the way for all users of the facility who greatly benefit from these pioneering experiments,” emphasises European XFEL managing director Robert Feidenhans’l. “We are very pleased – these results show that the facility works even better than we had expected and is ready to deliver new scientific breakthroughs.” The scientists present their results, including the first new protein structure solved at the European XFEL, in the journal Nature Communications.

“Being at a totally new class of facility we had to master many challenges that nobody had tackled before,” says DESY scientist Anton Barty from the Center for Free-Electron Laser Science (CFEL), who led the team of about 125 researchers involved in the first experiments that were open to the whole scientific community. “I compare it to the maiden flight of a novel aircraft: All calculations and assembly completed, everything says it will work, but not until you try it do you know whether it actually flies.”

The 3.4 kilometres long European XFEL is designed to deliver X-ray flashes every 0.000 000 220 seconds (220 nanoseconds). To unravel the three-dimensional structure of a biomolecule, such as an enzyme, the pulses are used to obtain flash X-ray exposures of tiny crystals grown from that biomolecule. Each exposure gives rise to a characteristic diffraction pattern on the detector. If enough such patterns are recorded from all sides of a crystal, the spatial structure of the biomolecule can be calculated. The structure of a biomolecule can reveal much about how it works.

>Read more on the DESY website and on the European XFEL website

Image: Artist’s impression of the experiment: When the ultra-bright X-ray flashes (violet) hit the enzyme crystals in the water jet (blue), the recorded diffraction data allow to reconstruct the spatial structure of the enzyme (right).
Credit: DESY/Lucid Berlin

Helmholtz Association supports ATHENA

Helmholtz Association supports ATHENA

ATHENA (“Accelerator Technology HElmholtz iNfrAstructure”) is a new research and development platform focusing on accelerator technologies and drawing on the resources of all six Helmholtz accelerator institutions (DESY, Jülich Research Centre, Helmholtz Centre Berlin, Helmholtz Centre Dresden-Rossendorf HZDR, KIT and GSI with the Helmholtz Institute of Jena). The Helmholtz Association has now decided to pay almost 30 million euros towards ATHENA as a strategic development project. “This decision demonstrates the Helmholtz Association’s strong commitment to developing and supplying ground-breaking new accelerator technologies for solving the future challenges faced by society,” says Helmut Dosch, who is the Chairman of DESY’s Board of Directors and also the spokesperson for the Helmholtz Association’s research division Matter.

Together, these centres want to set up two German flagship projects in accelerator research based on innovative plasma-based particle accelerators and ultramodern laser technology: an electron accelerator at DESY in Hamburg and a hadron accelerator at HZDR. At both facilities, a range of different fields of application are to be developed, ranging from a compact free-electron laser, through novel medical uses to new applications in nuclear and particle physics. As soon as they have reached the necessary level of maturity to be put to practical use in a particular area, new compact devices could be built for use in other Helmholtz centres, as well as in universities and hospitals.

>Read more on the Bessy II at HZB website or the DESY website
Third light source generates first X-ray light

Third light source generates first X-ray light

European XFEL starts operation of its third light source, exactly a year after the first X-ray light was generated in the European XFEL tunnels. The third light source will provide light for the MID (Materials Imaging and Dynamics) and HED (High Energy Density Science) instruments scheduled to start user operation in 2019. All three light sources, successfully run in parallel for the first time on the anniversary of European XFEL’s first light, will eventually provide X-rays for at least six instruments. At any one time, three of these six instruments can simultaneously receive X-ray beam for experiments. “The operation of the third light source, and the generation of light from all sources in parallel, are important steps towards our goal of achieving user operation on all six instruments” said European XFEL Managing Director Robert Feidenhans’l. “I congratulate and thank all those involved in this significant accomplishment. It was a tremendous achievement to get all three light sources to generate light within the space of one year.”

To generate flashes of X-ray light, electrons are first accelerated to near the speed of light before they are moved through long rows of magnets called undulators. The alternating magnetic fields of these magnets force the electrons on a slalom course, causing the electrons to emit light at each turn. Over the length of the undulator, the produced light interacts back on the electron bunch, thereby producing a particularly intense light. This light accumulates into intensive X-ray flashes. This process is known as ‘self-amplified spontaneous emission’, or SASE. European XFEL has three SASE light sources. The first one, SASE 1, taken into operation at the beginning of May 2017, provides intense X-ray light to the instruments SPB/SFX (Single Particles, Clusters and Biomolecules and Serial Femtosecond Crystallography) and FXE (Femtosecond X-ray Experiments), the first instruments available for experiments and operational since September 2017. The second light source, SASE 3, was successfully taken into operation in February 2018 and will provide light for the instruments SQS (Small Quantum Systems) and SCS (Spectroscopy and Coherent Scattering), scheduled to start user operation in November 2018. SASE 1 and SASE 3 can be run simultaneously – high speed electrons first generate X-ray light in SASE 1, before being used a second time to produce X-ray light of a longer wavelength in SASE 3. Now, exactly a year after the first laser light was generated in the European XFEL tunnels, the third light source, SASE 2, is operational. SASE 2 will generate X-ray light for the MID (Materials Imaging and Dynamics) and HED (High Energy Density Science) instruments scheduled to start user operation in 2019. The MID instrument will be used to, for example, understand how glass forms on an atomic level, and for the study of cells and viruses with a range of imaging techniques. The HED instrument will enable the investigation of matter under extreme conditions such as that inside exoplanets, and to investigate how solids react in high magnetic fields.

>Read more on the European XFEL website and the article on the DESY website.

Image: All three light sources, SASE 1,2 and 3, are now operational and have been successfully run in parallel for the first time.
Credit: DESY/European XFEL

Scientists observe nanowires as they grow

Scientists observe nanowires as they grow

X-ray experiments reveal exact details of self-catalysed growth for the first time

At DESY’s X-ray source PETRA III, scientists have followed the growth of tiny wires of gallium arsenide live. Their observations reveal exact details of the growth process responsible for the evolving shape and crystal structure of the crystalline nanowires. The findings also provide new approaches to tailoring nanowires with desired properties for specific applications. The scientists, headed by Philipp Schroth of the University of Siegen and the Karlsruhe Institute of Technology (KIT), present their findings in the journal Nano Letters. The semiconductor gallium arsenide (GaAs) is widely used, for instance in infrared remote controls, the high-frequency components of mobile phones and for converting electrical signals into light for fibre optical transmission, as well as in solar panels for deployment in spacecraft.

To fabricate the wires, the scientists employed a procedure known as the self-catalysed Vapour-Liquid-Solid (VLS) method, in which tiny droplets of liquid gallium are first deposited on a silicon crystal at a temperature of around 600 degrees Celsius. Beams of gallium atoms and arsenic molecules are then directed at the wafer, where they are adsorpted and dissolve in the gallium droplets. After some time, the crystalline nanowires begin to form below the droplets, whereby the droplets are gradually pushed upwards. In this process, the gallium droplets act as catalysts for the longitudinal growth of the wires. “Although this process is already quite well established, it has not been possible until now to specifically control the crystal structure of the nanowires produced by it. To achieve this, we first need to understand the details of how the wires grow,” emphasises co-author Ludwig Feigl from KIT.

>Read more on the FLASH and PETRA III at DESY website

Image: A single nanowire, crowned by a gallium droplet, as seen with the scanning electron microscope (SEM) of the DESY NanoLab.
Credit: DESY, Thomas Keller

Record number of participants at User Meeting

Record number of participants at User Meeting

Celebrating a year of glorious firsts and outlining future developments

“Welcome to the first European XFEL user meeting with actual users!” said Martin Meedom Nielsen, head of the European XFEL council as he opened the three day event on 24 January in front of a packed lecture hall on the DESY campus in Hamburg. With 1200 registered participants from ca. 100 institutions from 30 countries, this year’s joint European XFEL and DESY photon science users’ meeting, the first since operation began, was the biggest yet.

Meedom Nielsen and European XFEL Managing Director Robert Feidenhans’l started the meeting by summarizing the achievements and developments of the last year and thanking everyone who had contributed to the facility’s success. “It has been a fantastic year,” said Feidenhans’l looking back on his first year as director of the facility, “a tough year and we have worked really hard, but a fantastic year.” “2017 was a year of glorious firsts” said Meedom Nielsen, highlighting especially the facility’s inauguration in September and the beams of laser light that shone across the city to mark the occasion. “Hamburg was shining for European XFEL, and European XFEL was shining back” he said.

>Read more on the European XFEL website

 Photo Credit: European XFEL