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

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

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

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

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

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

 

1200 participants at annual users’ meeting

Record number of attendees at the joint DESY and European XFEL event

The joint meeting of users of DESY’s research light sources and the European XFEL X-ray laser once again drew a record number of attendees to Hamburg. Some 1200 participants from nearly 100 institutions from around 30 countries have registered for the three-day event (24-26 January) held at DESY, more than ever before. A particular highlight this year is the beginning of scientific user operation at the European XFEL, from which first results were presented.

“The users’ meeting in Hamburg is the largest in the world for research with X-ray light sources, and we are very proud of that,” emphasised DESY Director Helmut Dosch. “The tremendous interest reflects the importance of these unique research tools for all natural sciences and beyond.” DESY’s research director for photon science, Edgar Weckert, added: “With the X-ray lasers FLASH and European XFEL and the storage-ring-based X-ray source PETRA III, the metropolitan region offers a worldwide unique combination of high-intensity research light sources that serve a wide range of disciplines, from biology and medicine to energy, material and earth science to physics, chemistry and even art history.”

Image: DESY, Axel Heimken

Scientists measure accelerated emission

Grazing light for rapid events

An international team, including scientists from DESY,  has verified a prediction about the quantum-mechanical behaviour of resonant systems made more than 50 years ago. In experiments at SACLA, the Japanese X-ray laser, and at the European Synchrotron Radiation Facility ESRF in France, the group led by Aleksandr Chumakov from ESRF could show a dramatic reduction in the time to emit the first X-ray photon from an ensemble of excited nuclei when the number of X-rays for the excitation was increased. This behaviour is in good agreement with one limit of a superradiant system, predicted by the US physicist Robert Dicke in 1954, as the scientists report in the journal Nature Physics.

One of the broad challenges of science is to understand the behaviour of groups of atoms based on the response of a single atom in isolation, which is usually much simpler. A facet of this is understanding the behaviour of a group of identical oscillators. An analogy is a collection of bells that all have the same tone: one can easily imagine the sound of a single bell struck once – a clear tone ringing out with a volume that decays away over time.

But what happens if one gently taps all the bells in a large collection? Will the tone be the same as a single one? What about the volume? What about the direction – does it matter where you are standing when you listen to the sound? Does it matter if you tap them all at the same time?

>Read more on the FLASH website

“X-ray streaking” allows ultrafast processes to be followed using a single pulse of light

Grazing light for rapid events

An international team of scientists has developed a new experimental method at the FLASH X-ray laser which allows the sequence of events involved in a process to be observed using a single, ultrashort pulse of light from FLASH. Their method is called “X-ray streaking” and enables researchers to observe ultrafast processes continuously, instead of being confined to taking snapshots at discrete intervals using separate X-ray pulses. Apart from the extreme brightness of the FLASH beam, the scientists also made use of an X-ray lens which they introduced into the beamline in a particular configuration, so as to capture a chronological sequence of events using a single X-ray pulse. To demonstrate the functionality of X-ray streaking, they observed the ultrafast demagnetisation of cobalt.

The invention of X-ray lasers has considerably boosted the study of the dynamics of matter. Pump-probe experiments allow artificially induced (“pumped”) processes and reactions to be photographed (“probed”) using an extremely short X-ray pulse at predetermined intervals. Ideally, these photographs, taken with different time delays, can then be assembled to create a film showing the sequence of events during an ultrafast process with a temporal resolution of the order of femtoseconds. One limitation of this otherwise promising experimental technique is, however, that the experiment has to be conducted all over again for each time delay. This means that before each observation, the process of interest must be triggered using the same starting conditions and it must run through the same sequence of events – both of which rule out extreme experimental conditions.

>Read more on the FLASH website

Image Caption: (a,b) Raw images from the reflection and reference detectors respectively. Both the images for the pumped and the un-pumped event are acquired using a single x-ray pulse. (c) Transient reflectivity image (as defined in the text) calculated from the images shown in (a,b). (d) Reshaped transient reflectivity image after calibration of the time window. Article published in Scientific Reports.

 

Scientists demonstrate unparalleled phase control of free-electron laser pulses

Double flashes with attosecond precision

Thanks to a smart mirror scientists can control the phase of X-rays from DESY’s free-electron laser FLASH with attosecond precision. The feat enables new investigations of the interactions of light and matter, as the team headed by DESY scientist Tim Laarmann reports in the journal Nature Communications. An attosecond is a billionth of a billionth of a second. The phase indicates at which point in its rapid oscillation a light wave is at a given point in time or space. Phase-sensitive measurements are important to gain insight of light-matter interactions and require phase-controlled pulses. Although phase control is an established technique in optics, the soft X-rays generated by FLASH oscillate a hundred times faster than visible light, requiring a hundred times better precision.

The scientists have now demonstrated phase control and interferometric autocorrelation at FLASH using pulse pairs created with a smart split-and-delay unit. The successful transfer of a powerful optical method towards short wavelengths paves the way towards utilization of advanced nonlinear methodologies even at partially coherent free-electron lasers that rely on self-amplified spontaneous emission (SASE). Free-electron lasers (FEL) are driven by powerful particle accelerators and produce laser-like light pulses by sending bunches of fast electrons through a magnetic slalom course.

>Read More

FLASHForward accelerates first electron bunches

The plasma accelerator project FLASHForward achieved an important milestone in January 2017.

For the first time, the facility’s high-power laser accelerated electron bunches in a plasma cell. Later in the operational phase, the laser will control the formation of the plasma at FLASH. The group of scientists around DESY’s Jens Osterhoff used the laser to ignite a plasma, from which electrons were accelerated to energies of around 100 mega-electronvolts within a distance of just a few millimetres. This allows important pre-experiments for the planned beam-driven plasma experiment. As of the second half of this year, the FLASHForward scientists want to use the FLASH electron beam to generate a plasma in a plasma cell in order to further accelerate other electron bunches from the FLASH particle accelerator or electron bunches which are formed in the plasma itself.

“The electron bunches that have now been accelerated by the laser in the plasma cell have in many respects very similar properties to those that we are later hoping to accelerate with the FLASH beam in FLASHForward,” explains the project leader Jens Osterhoff.

>Read More

Picture: The FLASHForward scientists accelerated the first electron bunches in such a plasma cell (photo: DESY/ H. Müller-Elsner).

Precise test of quantum physical tunnel effect at DESY’s X-ray laser FLASH

Partnership at a distance: deep-frozen helium molecules

Helium atoms are loners. Only when you cool them to very low temperatures do they form extremely weakly bonded molecules. Yet even in this state, they are able to maintain an extremely large separation from each other thanks to quantum tunnelling. With the help of DESY’s free-electron laser FLASH, Frankfurt nuclear physicists have been able to confirm that the atoms spend more than 75 percent of their time so far apart from each other that their bond can only be explained by means of quantum tunnelling. The scientists have presented their findings in the US journal “Proceedings of the National Academy of Sciences” (PNAS).

The binding energy of a helium molecule is approximately one billionth of the binding energy of everyday molecules like oxygen or nitrogen. On top of this, the molecule is so huge that small viruses or soot particles could actually pass between the atoms. Physicists explain this in terms of quantum tunnelling. They visualise the bond in a classical molecule as a potential well, in which atoms cannot get further apart from each other than by going to opposite “walls”. However, quantum theory also allows atoms to tunnel inside these walls. “It is as if each of them were to dig a shaft without an exit,” explains Reinhard Dörner, a professor at the Institute of Nuclear Physics at the Goethe University in Frankfurt.

 

>Read more on the FLASH website

Cartoon: “When two loners are forced to share a bed, they move well beyond its edges to get away from each other.”
Credit: Peter Evers