Day of Light: 60th anniversary of the laser

The invention of the laser 60 years ago has transformed science and everyday life.

Sixty years after the first laser was operated on 16 May 1960 by Theodore Maiman at Hughes Research Laboratories in California, lasers have revolutionized everyday life as well as science. Lasers are also fundamental for research at the European XFEL. A public event on the European XFEL campus planned to celebrate this anniversary has been postponed to a later date.

When the world’s biggest X-ray laser and one of the planet’s brightest light sources, the European XFEL, started operation in 2017, it was the culmination of several decades of scientific progress in laser and X-ray laser technology. Lasers operating in the visible wavelength range were invented in the 1960s. In these lasers, radiation is generated from electron transitions in atoms or molecules. The light emitted is then continuously amplified between mirrors. This makes it comparatively easy to produce high-quality laser light, and many applications now shape our everyday lives. Examples range from impressive light installations, to high precision surgical instruments, broadband telecommunication, components in the electrical devices we carry in our pockets, and the laser pointer we use during presentations.

Read more on the XFEL website

Image: The optical laser system for pump-probe experiments in the laser lab.

Credit: European XFEL / Jan Hosan

First direct look at how light excites electrons to kick off a chemical reaction

Light-driven reactions are at the heart of human vision, photosynthesis and solar power generation. Seeing the very first step opens the door to observing chemical bonds forming and breaking.

The first step in many light-driven chemical reactions, like the ones that power photosynthesis and human vision, is a shift in the arrangement of a molecule’s electrons as they absorb the light’s energy. This subtle rearrangement paves the way for everything that follows and determines how the reaction proceeds.
Now scientists have seen this first step directly for the first time, observing how the molecule’s electron cloud balloons out before any of the atomic nuclei in the molecule respond.

While this response has been predicted theoretically and detected indirectly, this is the first time it’s been directly imaged with X-rays in a process known as molecular movie-making, whose ultimate goal is to observe how both electrons and nuclei act in real time when chemical bonds form or break.

>Read more on the LCLS at SLAC website

Image: extract, full image here

In search of the lighting material of the future

At the Paul Scherrer Institute PSI, researchers have gained insights into a promising material for organic light-emitting diodes (OLEDs). The substance enables high light yields and would be inexpensive to produce on a large scale – that means it is practically made for use in large-area room lighting. Researchers have been searching for such materials for a long time. The newly generated understanding will facilitate the rapid and cost-efficient development of new lighting appliances in the future. The study appears today in the journal Nature Communications.

The compound is a yellowish solid. If you dissolve it in a liquid or place a thin layer of it on an electrode and then apply an electric current, it gives off an intense green glow. The reason: The molecules absorb the energy supplied to them and gradually emit it again in the form of light. This process is called electroluminescence. Light-emitting diodes are based on this principle.

Read more on the Swiss FEL and Swiss Light Source website

Image: Grigory Smolentsev in front of SwissFEL

Credit: Paul Scherrer Institute/Mahir Dzambegovic

Super laser delivered to European XFEL

High Energy laser will enable study of exoplanet interiors.

A keenly awaited piece of high-tech equipment has been delivered to European XFEL. The high repetition rate, high-energy laser, DiPOLE 100-X, was developed in the UK by scientists and engineers at the Science and Technology Facilities Council’s Central Laser Facility (CFL) as part of the UK contribution to the facility. This unique laser, developed within the framework of the HiBEF user consortium, will be used at the instrument for High-Energy Density (HED) science at European XFEL to generate extreme temperatures and pressures in materials. The atomic structure and dynamics of these extreme states of materials can then be studied using the extremely bright and intense X-ray pulses produced by the European XFEL. This experimental set-up will enable scientists to create conditions similar to the interior of exoplanets with temperatures of up to 10,000°C, and pressures of up to 10,000 tons per square centimeter – similar to the weight of 2000 adult elephants concentrated onto the surface of a postage stamp!

>Read more on the European XFEL website

Image: The HED instrument at European XFEL.
Credit: European XFEL/Jan Hosan

Shaping attosecond waveforms

Scientists show how to control attosecond light pulses at a free-electron laser.

Chemical reactions and complex phenomena in liquids and solids are determined by the movement and rearrangement of electrons. These movements, however, occur on an extremely short timescale, typically only a few hundred attoseconds (1 attosecond =10-18 s or one quintillionth of a second).  Only light pulses of a comparable duration can be used to take snapshots of the dynamics of electrons. An international team of researchers led by Guiseppe Sansone from the University of Freiburg and including scientists from European XFEL have now, for the first time, been able to reliably generate, control and characterize such attosecond light pulses from a free-electron laser.

“These pulses enable us to study the first moment of the electronic response in a molecule or crystal,” explains Sansone. “With the ability to shape the electric field enables us to control electronic movements – with the long-term goal of optimising basic processes such as photosynthesis or charge separation in materials.”

>Read more on the European XFEL website

Image: Scientists have been able to shape the electric field of an attosecond light pulse.
Credit: Jürgen Oschwald and Carlo Callegari

Scientists observe ultrafast birth of free radicals in water

What they learned could lead to a better understanding of how ionizing radiation can damage material systems, including cells.

Understanding how ionizing radiation interacts with water—like in water-cooled nuclear reactors and other water-containing systems—requires glimpsing some of the fastest chemical reactions ever observed.

In a new study conducted at the Department of Energy’s SLAC National Accelerator Laboratory, researchers have witnessed for the first time the ultrafast proton transfer reaction following ionization of liquid water. The findings, published today in Science, are the result of a world-wide collaboration led by scientists at the DOE’s Argonne National Laboratory, Nanyang Technological University, Singapore (NTU Singapore) and the German research center DESY.

The proton transfer reaction is a process of great significance to a wide range of fields, including nuclear engineering, space travel and environmental remediation. This observation was made possible by the availability of ultrafast X-ray free electron laser pulses, and is basically unobservable by other ultrafast methods. While studying the fastest chemical reactions is interesting in its own right, this observation of water also has important practical implications.

>Read more on the LCLS at SLAC website

Image: X-rays capture the ultrafast proton transfer reaction in ionized liquid water, forming the hydroxyl radical (OH) and the hydronium (H3O+) ion. Credit: Argonne National Laboratory

First molecular movies at European XFEL

Scientists show how to use extremely short X-ray pulses to make the first movies of molecular processes at the European XFEL.

In a paper published today in Nature Methods, scientists show how to effectively use the high X-ray pulse repetition rate of the European XFEL to produce detailed molecular movies. This type of information can help us to better understand, for example, how a drug molecule reacts with proteins in a human cell, or how plant proteins store light energy.

Traditional structural biology methods use X-rays to produce snapshots of the 3D structure of molecules such as proteins. Although valuable, this information does not reveal details about the dynamics of biomolecular processes. If several snapshots can be taken in fast enough succession, however, these can be pasted together to make a so-called molecular movie. The high repetition rate of the extremely short X-ray pulses produced by the European XFEL makes it now possible to collect large amounts of data to produce movies with more frames than ever before. An international group of scientists have now worked out how to make optimal use of the European XFEL’s very high X-ray repetition rate to make these molecular movies at the facility in order to reveal unprecedented details of our world.

>Read more on the European XFEL website

Image: Artistic visualisation of a serial crystallography experiment. A stream of crystalline proteins are struck by an optical laser that initiates a reaction. Following a short delay the X-ray laser strikes the crystals. The information recorded about the arrangement of the atoms in the protein is used to reconstruct a model of the structure of the protein.
Credit: European XFEL / Blue Clay Studios

Using European XFEL to shed light on photosynthesis

First membrane protein studied at European XFEL

In a paper now published in Nature Communications an international group of scientists show that the fast X-ray pulse rate produced by the European XFEL can be used to study the structure of membrane proteins such as those involved in the process of photosynthesis. These results open up eagerly awaited experimental opportunities for scientists studying these types of proteins.

Large proteins and protein complexes are difficult to study with traditional structural biology approaches. Large protein complexes, such as those that sit across cell membranes and regulate traffic in and out of cells, are difficult to crystalize and generally only produce small crystals that are hard to analyse. The extremely fast X-ray pulses generated by European XFEL now enable scientists to collect large amounts of data from a stream of small crystals to develop detailed models of the 3D structure of these proteins.

>Read more on the European XFEL website

Image (extract, full illustration in the article): Graphic shows the basic design of a serial femtosecond crystallography experiment at European XFEL. X-ray bursts strike crystallized samples resulting in diffraction patterns that can be reassembled into detailed images.
Credit: Shireen Dooling for the Biodesign Institute at ASU

Breaking up buckyballs is hard to do

A new study shows how soccer ball-shaped molecules burst more slowly than expected when blasted with an X-ray laser beam.

As reported in Nature Physics, an international research team observed how soccer ball-shaped molecules made of carbon atoms burst in the beam of an X-ray laser. The molecules, called buckminsterfullerenes – buckyballs for short ­– consist of 60 carbon atoms arranged in alternating pentagons and hexagons like the leather coat of a soccer ball. These molecules were expected to break into fragments after being bombarded with photons, but the researchers watched in real time as buckyballs resisted the attack and delayed their break-up.

The team was led by Nora Berrah, a professor at the University of Connecticut, and included researchers from the Department of Energy’s SLAC National Accelerator Laboratory and the Deutsches Elektronen-Synchrotron (DESY) in Germany. The researchers focused their attention on examining the role of chemical effects, such as chemical bonds and charge transfer, on the buckyball’s fragmentation.

Using X-ray laser pulses from SLAC’s Linac Coherent Light Source (LCLS), the team showed how the bursting process, which takes only a few hundred femtoseconds, or millionths of a billionth of a second, unfolds over time. The results will be important for the analysis of sensitive proteins and other biomolecules, which are also frequently studied using bright X-ray laser flashes, and they also strengthen confidence in protein analysis with X-ray free-electron lasers (XFELs).

>Read more on the Linear Coherent Light Source at SLAC website

Image: An illustration shows how soccer ball-shaped molecules called buckyballs ionize and break up when blasted with an X-ray laser. A team of experimentalists and theorists identified chemical bonds and charge transfers as crucial factors that significantly delayed the fragmentation process by about 600 millionths of a billionth of a second.
Credit: Greg Stewart/SLAC National Accelerator Laboratory

For additional information: article published on the DESY website

Two years of user operation in numbers

1200 users, 60 experiments and 6 petabytes of data since operation began.

September 1 marks two years since the official opening and start of user operation at European XFEL. With the scheduled expansion from two to six operational instruments, the facility has expanded its experimental capacity and possibilities significantly during the past two years. At the same time, both the performance of the X-ray free-electron laser and instruments was continually improved. The scientific community shows strong interest in experiments at the new facility, with a total of 363 submitted proposals during this period, of which 98 were awarded beamtime. In total, 1200 users from across the world came to Schenefeld for their research. As the facility continues to be developed, even more time will be available for user experiments in the future.

>Read more on the European XFEL website

Image: Laser installation on the European XFEL campus in 2017 highlighting the five underground tunnels.
Credit: The European XFEL (Germany)

Research and tinkering – SwissFEL in 2019

The newest large research facility at the Paul Scherrer Institute, SwissFEL, has been completed. Regular operation began in January 2019.

Henrik Lemke, head of the SwissFEL Bernina research group in the Photon Science Division, gives a first interm report.

Mr. Lemke, you have just published a technical article in which you report on the experience so far with SwissFEL. How would you sum it up?

With SwissFEL, we are entering new territory at PSI. It is one of only five comparable facilities on this scale worldwide. This means we still need to gain experience, because we are doing a lot of things for the first time. On January 1 this year we began regular operation. Research groups from other institutions have already been here, and they have successfully conducted experiments with us, just like PSI researchers themselves. These were already a big success. In parallel to this operation, we are also further optimising the facility and the experimental setup. This will enable us to join ranks with the comparable facilities and, in addition, develop particular methods into specialities of SwissFEL.

>Read more on the SwissFEL website

Image: Lemke at the experiment station Bernina of SwissFEL
Credit: Paul Scherrer Institute/Mahir Dzambegovic

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

A laser for penetrating waves

Research team develops a new principle to generate terahertz radiation

The “Landau-level laser” is an exciting concept for an unusual radiation source. It has the potential to efficiently generate so-called terahertz waves, which can be used to penetrate materials as well as for future data transmission. So far, however, nearly all attempts to make such a laser reality have failed. An international team of researchers has now taken an important step in the right direction: In the journal Nature Photonics (DOI: 10.1038/s41566-019-0496-1), they describe a material that generates terahertz waves by simply applying an electric current. Physicists from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) played a significant role in this project.
Like light, terahertz waves are electromagnetic radiation, in a frequency range between microwaves and infrared radiation. Their properties are of great technological and scientific interest, as they allow fundamental researchers to study the oscillations of crystal lattices or the propagation of spin waves. Simultaneously “terahertz waves are of interest for technical applications because they can penetrate numerous substances that are otherwise opaque, such as clothing, plastics and paper,” Stephan Winnerl from HZDR’s Institute of Ion Beam Physics and Materials Research explains. Terahertz scanners are already used today for airport security checks, detecting whether passengers are concealing dangerous objects under their clothing – without having to resort to harmful X-rays.
>Read more on the FELBE at HZDR website

Image: An international research team has been able to show that it is relatively easy to generate terahertz waves with an alloy of mercury, cadmium and tellurium. To examine the behavior of the electrons in the material, the physicists use the free-electron laser FELBE at HZDR. Circularly polarized terahertz pulses (orange spiral) excite the electrons (red) from the lowest to the next higher energy level (parabolic shell). The energy gap of these so-called Landau levels can be adjusted with the help of a magnetic field. Credit : HZDR / Juniks

How morphing materials store information

Experiments at SLAC’s X-ray laser reveal in atomic detail how two distinct liquid phases in these materials enable fast switching between glassy and crystalline states that represent 0s and 1s in memory devices.

Instead of flash drives, the latest generation of smart phones uses materials that change physical states, or phases, to store and retrieve data faster, in less space and with more energy efficiency. When hit with a pulse of electricity or optical light, these materials switch between glassy and crystalline states that represent the 0s and 1s of the binary code used to store information.
Now scientists have discovered how those phase changes occur on an atomic level.
Researchers from European XFEL and the University of Duisburg-Essen in Germany, working in collaboration with researchers at the Department of Energy’s SLAC National Accelerator Laboratory, led X-ray laser experiments at SLAC that collected more than 10,000 snapshots of phase-change materials transforming from a glassy to a crystalline state in real time.

>Read more on the LCLS at SLAC website

Image: The research team after performing experiments at SLAC’s Linac Coherent Light Source X-ray laser.
Credit: Klaus Sokolowski-Tinten/University of Duisburg-Essen)

Please read also the article published on the EUXFEL website:
Rigid bonds enable new data storage technology

All six European XFEL instruments now operational

User experiments started at instrument for High Energy Density.

The first experiments have now started at the instrument for High Energy Density (HED) experiments. HED is the sixth and thereby last instrument of European XFEL’s current design configuration to start user operation. With six instruments on three SASE beamlines operational, European XFEL now has the capacity to host three times as many user experiments as compared to when operation began in 2017.
HED combines hard X-ray FEL radiation and the capability to generate matter under extreme conditions of pressure, temperature or electric field. HED will be used for studies of matter occurring inside exoplanets, of new extreme-pressure phases and solid-density plasmas, and of structural phase transitions of complex solids in high magnetic fields. The HED instrument is built in close collaboration with the HiBEF consortium led by Helmholtz Zentrum Dresden-Rossendorf (HZDR).
Next operation goals involve further increasing the capabilities and experiment portfolio of the instruments, increasing the amount of beamtime available for users at the six instruments and achieving successful parallel user operation of all three SASE beamlines. Parallel user operation is expected to start later this year.

>Read more on the European XFEL website

Image: The first users at the HED instrument.
Credit: European XFEL

Coherent soft x-ray pulses from an echo-enabled free-electron laser

The free-electron laser (FEL) FERMI is a unique facility, providing users with laser-like pulses in the XUV spectral range. At FERMI, the generation of highly coherent pulses, with tunable spectro-temporal properties, relies on the so-called high-gain harmonic generation (HGHG) technique. In the latter, a (single) infrared seed laser is used to shape the electron-beam properties and trigger the amplification process. Amplification occurs at one selected harmonic, h, of the seed. However, in HGHG, the seed energy required to prepare the electron beam for FEL emission becomes larger and larger for higher harmonics (i.e., shorter FEL wavelengths). For high harmonics, the resulting strong electron-beam energy modulation reduces the FEL gain, limiting the scheme to h<15 (wavelengths of about 10-20 nm) for a single HGHG scheme, or to h of the order of 60-70 (i.e., 4-5 nm), in case of two-stage HGHG. Moreover, at such high h, the sensitivity to the shape of the electron-beam phase space becomes critical and may severely affect the FEL radiation in terms of longitudinal coherence, pulse energy, and shot-to-shot stability. In addition, the HGHG scheme cannot cover the whole harmonic range, as the final harmonic number is a product between the harmonic numbers of the individual stages. Last, but not least, the two-stage setup uses a relatively large portion of the e-beam to accommodate the double seeding process, which makes the implementation double-pulse operation difficult.
The drawbacks of the two-stage HGHG can be overcome by using a recently proposed technique called echo-enabled harmonic generation (EEHG), where the electron-beam is shaped using two seed lasers to enable FEL emission at high harmonics. The method requires a much weaker energy modulation compared to HGHG and is also intrinsically less sensitive to the initial electron-beam imperfections, making it a strong candidate for producing highly stable, nearly fully coherent, and intense FEL pulses, down to soft x-ray wavelengths.

>Read more on the FERMI at Elettra website

Image:  The EEHG scheme together with the e-beam phase space at different stages of the evolution. The 1st seed laser with a wavelength λ1 imprints a sinusoidal energy modulation with an amplitude ΔE<3σE,  σE is the initial uncorrelated energy spread, onto the relativistic e-beam in the 1st modulator. After passing through a strong 1st chicane, the electrons with different energies move relative to each other, resulting in a striated phase space with multiple energy bands. Importantly, the energy spread within a single band is much smaller than σE. The electrons then pass through the 2nd modulator, where their energy is again periodically modulated using a 2nd seed laser with λ2λ1 and ΔE2≈ΔE1. After traversing a weaker 2nd chicane, the e-beam phase space is rotated, transforming the sinusoidal energy modulation into a periodic density modulation, with high-frequency components. As the energy spread within a single band is much smaller than σE, only a moderate ΔE2 is required to reach very high harmonics. The e-beam is then injected into the radiator, tuned to emit light at a high harmonic of the 2nd seed laser.