Recent Highlights
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Dynamic aspirin - molecular vibrations drive electrons over large distances

30 January 2019

Aspirin is not only an important drug but also an interesting physics model system in which molecular vibrations and electrons are coupled in a particular way. For the first time, x-ray experiments in the ultrashort time domain make electron motions visible in real time. They demonstrate that very small atomic displacements shift electrons over much larger distances within the aspirin molecules.

Aspirin pills (Figure 1a) consist of many small crystallites in which molecules of acetylsalicylic acid form a regular spatial arrangement (Figure 1b). The molecules couple to each other via comparably weak interactions and generate electric fields which exert a force on the electrons of every molecule. Upon excitation of molecular vibrations, the distribution of electrons in space and, thus, the chemical properties should change. While this scenario has been a subject of theoretical work, there has been no experimental demonstration and understanding of the molecular dynamics so far.

Scientists of the Max Born Institute in Berlin, Germany, have now gained the first and direct insight in electrons motions during a coupled vibration of the aspirin molecules. In a recent issue of the journal Structural Dynamics [6,014503 (2019)], they report results of an x-ray experiment in the ultrashort time domain. An ultrashort optical pump pulse induces vibrations of the aspirin molecules with a vibrational period of approximately 1 picosecond (ps, a millionth of a millionth of a second). An ultrashort hard x-ray pulse, which is delayed relative to the pump pulse, is diffracted from the excited powder of crystallites to map the momentary spatial arrangement of electrons via an x-ray diffraction pattern.

The animation in Figure 1c shows the rotational motion of the methyl (CH3) group of an aspirin molecule which arises upon vibrational excitation. In the animation, the atomic displacements are artificially enlarged to make them visible. The methyl rotation is connected with a spatial shift of electrons over the entire aspirin molecule (yellow clouds, so-called isosurface of constant electron density). The periodic electron motions occur in time with the vibrational motions of the atoms and the distances traveled by the electrons are typically 10000 times larger than the atom displacements in the methyl rotation. This behavior demonstrates the hybrid character of the methyl rotation which is comprised of both atomic and electron motions on totally different length scales. The hybrid character originates from the electric interaction between the aspirin molecules and the dynamic minimization of electrostatic energy in the crystallite.

These new results underline the central role of hybrid modes for the stabilization of the crystal structure, in agreement with theoretical calculations. In the case of aspirin, this property favors the so-called form 1 of the crystal structure compared to other molecular arrangements. The strong modulation of the electron distribution by vibrations is relevant for numerous crystal structures in which electric interactions prevail. Vibrational excitations of ferroelectric materials should allow for an ultrafast switching of the macroscopic electric polarization and, thus, lead to new electronic devices for extremely high frequencies.

Aspirin Movie Movie: (a) Aspirin pills. (b) Crystal structure of aspirin representing a regular, spatially periodic arrangement of molecules. (c) The animation illustrates the redistribution of electron density during the rotation of the methyl group with a period of approximately 1 ps. A single aspirin molecule is shown in a ball and stick model, the electron density as a so-called isosurface. The isosurface contains all spatial positions at which the electron density has a particular (fixed) value of 1800 elementary charges per nanometer (1800 e-/nm3). Changes of electron density result in changes of the shape of the isosurface. A shrinking around a particular atom illustrates a loss of electronic charge while an expansion reflects an increase of charge density. In the aspirin molecule, continuous periodic charge motions occur during the methyl rotation, in particular between the atoms of the carbon 6-ring (left) and the COOH carboxy unit (right).

Original publication:
C. Hauf, Hernandez Salvador, A.-A., M. Holtz, M. Woerner, T. Elsaesser
Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction
Struct. Dyn. 6,014503 (2019)/ 1-7 / Chosen as featured article by the Editors

Further information:
Dr. Christoph Hauf, Tel.: 030 6392 1473
Dr. Michael Woerner, Tel.: 030 6392 1470
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400


How Molecules teeter in a laser field

17 January 2019

When molecules interact with the oscillating field of a laser, an instantaneous, time-dependent dipole is induced. This very general effect underlies diverse physical phenomena such as optical tweezers, for which Arthur Ashkin received the Nobel Prize in Physics in 2018, as well as the spatial alignment of molecules by a laser field. Now scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) report on an experiment in the Journal of Physical Chemistry Letters, where the dependence of the driven-dipole response on the bound state of an electron in an methyl iodine molecule is revealed.

The reported work represents the first attosecond transient absorption spectroscopy (ATAS) experiment on a polyatomic molecule. In an ATAS experiment, the absorption of photons in the extreme ultraviolet (XUV) spectral range (provided in the form of an isolated attosecond pulse or an attosecond pulse train) is studied in the presence of an intense infrared laser field, whose relative phase with respect to the XUV radiation is controlled. By performing such an experiment on molecules, the MBI researchers could access a spectral regime, where transitions from the atomic cores to the valence shell can be compared with transitions from the cores to the Rydberg shell. "Initially somewhat surprising, we found that the infrared field affects the weak core-to-Rydberg transitions much more strongly than the core-to-valence transitions, which dominate the XUV absorption," says MBI scientist Lorenz Drescher. The published paper is part of his PhD work at MBI.

Accompanying theory simulations revealed that the Rydberg states dominate the laser-dressed XUV absorption due to their high polarizability. Importantly, the reported experiment offers a glimpse into the future. "By tuning the XUV spectrum to different absorption edges, our technique can map the molecular dynamics from the local perspective of different intra-molecular reporter atoms," explains MBI scientist Dr. Jochen Mikosch. "With the advent of attosecond XUV light sources in the water window, ATAS of light-induced couplings in molecules is anticipated to become a tool to study ultrafast phenomena in organic molecules," he adds. In this wavelength regime, transitions from core-orbitals in nitrogen, carbon and oxygen atoms are located. MBI is at the forefront of developing such light sources, which will allow the researchers to study the building blocks of life.

Abb. 1 (click to enlarge)

Fig. 1: Measured transient change of the XUV absorbance in the 4d-core-to-valence (σ*) and 4d-core-to-Rydberg spectral region in CH3I molecules. Pronounced sub-cycle oscillations at twice the NIR laser frequency are observed in the region of the core-to-Rydberg transitions, while the core-to-valence transitions are only weakly affected by the field. The observed effect is traced back to the higher polarizability of the Ryberg states, which makes them more susceptible to the interaction with the laser field

Original publication:
Lorenz Drescher, Geert Reitsma, Tobias Witting, Serguei Patchkovskii, Jochen Mikosch, Marc J. J. Vrakking
State-Resolved Probing of Attosecond Timescale Molecular Dipoles
J. Phys. Chem. Lett. 10 (2), 265-269 (2019)

Further information:
Dr. Jochen Mikosch, Tel.: 030 6392 1295


New dynamic probes for ions interacting with biomolecules

1st January 2019

Pairs of negatively charged phosphate groups and positive magnesium ions represent a key structural feature of DNA and RNA embedded in water. Vibrations of phosphate groups have now been established as selective probes of such contact pairs and allow for a mapping of interactions and structure on the ultrafast time scales of molecular dynamics.

DNA and RNA are charged polymers that encode genetic information in a double helix structure and act as key player in the biosynthesis of proteins. Their negative charges are located in the molecular backbone, which consists of ionic phosphate (PO2-) and of sugar groups (Figure 1). Stabilization of the macromolecular structures of DNA and RNA requires a compensation of strong repulsive electric forces between the equally charged phosphate groups by ions of opposite, i.e., positive charge. In this context, magnesium Mg2+ ions are particularly relevant as they not only stabilize the structure but also mediate the recognition of external binding partners and act as catalytic centers. Moreover, changes of macromolecular structure via dynamic folding processes are connected with a rearrangement of positive ions embedded in the surrounding water shell.

Positive ions are arranged in different geometries around DNA and RNA: in so-called site-bound or contact-pair geometries, a positive ion is located in direct contact with an oxygen atom of a phosphate group. In contrast, the so-called outer ion atmosphere consists of positive ions separated by at least one layer of water molecules from the phosphate groups. The functional role of the different geometries and the underlying interactions are far from being understood. A deeper insight at the molecular level requires highly sensitive probes which allow for discerning the different ion geometries without disturbing them, and for mapping their dynamics on the ultrafast time scale of molecular motions.

In a recent publication, researchers from the Max-Born-Institute demonstrate that vibrations of phosphate groups represent sensitive and noninvasive probes of ion geometries in a water environment. Dimethylphosphate (DMP, (CH3O)2PO2-), an established model system for the DNA and RNA backbone, was prepared in liquid water with an excess of Mg2+ ions (Figure 2, top) and studied by nonlinear vibrational spectroscopy in the femtosecond time domain (1 fs = 10-15 s). The experiments make use of two-dimensional infrared (2D-IR) spectroscopy, a most sophisticated method for analyzing the ionic interactions and structures on the intrinsic time scale of fluctuating molecular motions.

The experiments map Mg2+ ions in direct contact with a PO2- group via a distinct feature in the 2D-IR spectrum (Figure 2, bottom). The interaction with the Mg2+ ion shifts the asymmetric PO2- stretching vibration to a frequency which is higher than in absence of Mg2+ ions. The lineshape and the time evolution of this new feature reveal fluctuations of the contact ion pair geometry and the embedding water shell on a time scale of hundreds of femtoseconds while the contact pair itself exists for much longer times (∼10-6 s). An in-depth theoretical analysis shows that the subtle balance of attractive electrostatic (Coulomb) forces and repulsive forces due to the quantum-mechanical exchange interaction govern the frequency position of the phosphate vibration.

The ability of 2D-IR spectroscopy to characterize the short-ranged phosphate-ion interaction in solution provides a novel analytical tool that complements currently available structural techniques. An extension of this new approach to DNA and RNA and their ionic environment is most promising and expected to provide new insight in the forces stabilizing equilibrium structures and driving folding processes.

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1:DNA double helix embedded in water (angled small molecules, not to scale). The dark red spheres on the helix surface represent oxygen atoms of the negatively charged PO2- units, the blue spheres positively charged ions in the environment.

Fig. 1 (click to enlarge)

Fig. 2: Top: Molecular structure of a contact ion pair consisting of dimethylphosphate (DMP) and a magnesium ion Mg2+ embedded in water. The arrows mark the elongations of the phosphorus-oxygen bonds in the asymmetric PO2- stretching vibration. Bottom: Two-dimensional infrared (2D-IR) spectra of the asymmetric PO2- stretching vibration measured at a waiting time T=500 fs after vibrational excitation. The vibrational response is shown as a function of the infrared excitation and the detection frequencies and consists of a component P1 from DMP molecules without a magnesium ion in the neighborhood and the contribution P2 from contact ion pairs. The latter is shifted to higher frequencies due to the interaction between PO2- and Mg2+.


Original publication:
Jakob Schauss, Fabian Dahms, Benjamin P. Fingerhut, Thomas Elsaesser:
Phosphate-magnesium ion interactions in water probed by ultrafast two-dimensional infrared spectroscopy
J. Phys. Chem. Lett. 10, 238-243 (2019)

Further information:
Dr. Benjamin Fingerhut, Tel.: 030 6392 1404
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400


5000 times faster than a computer - interatomic light rectifier generates directed electric currents

27 December 2018

The absorption of light in semiconductor crystals without inversion symmetry can generate electric currents. Researchers at the Max-Born-Institute have now generated directed currents at terahertz (THz) frequencies, much higher than the clock rates of current electronics. They show that electronic charge transfer between neighboring atoms in the crystal lattice represents the underlying mechanism.

Solar cells convert the energy of light into an electric direct current (DC) which is fed into an electric supply grid. Key steps are the separation of charges after light absorption and their transport to the contacts of the device. The electric currents are carried by negative (electrons) and positive charge carriers (holes) performing so called intraband motions in various electronic bands of the semiconductor. From a physics point of view, the following questions are essential: what is the smallest unit in a crystal which can provide a photo-induced direct current (DC)? Up to which maximum frequency can one generate such currents? Which mechanisms at the atomic scale are responsible for such charge transport?

The smallest unit of a crystal is the so-called unit cell, a well-defined arrangement of atoms determined by chemical bonds. The unit cell of the prototype semiconductor GaAs is shown in Figure 1(a) and represents an arrangement of Ga and As atoms without a center of inversion. In the ground state of the crystal represented by the electronic valence band, the valence electrons are concentrated on the bonds between the Ga and the As atoms [Figure 1(b)]. Upon absorption of near-infrared or visible light, an electron is promoted from the valence band to the next higher band, the conduction band. In the new state, the electron charge is shifted towards the Ga atoms [Figure 1 (b)]. This charge transfer corresponds to a local electric current, the interband or shift current, which is fundamentally different from the electron motions in intraband currents. Until recently, there has been a controversial debate among theoreticians whether the experimentally observed photo-induced currents are due to intraband or interband motions.

Researchers at the Max-Born-Institute in Berlin, Germany, have investigated optically induced shift currents in the semiconductor gallium arsenide (GaAs) for the first time on ultrafast time scales down to 50 femtoseconds (1 fs = 10-15 seconds). They report their results in the current issue of the journal Physical Review Letters 121, 266602 (2018) . Using ultrashort, intense light pulses from the near infrared (λ = 900 nm) to the visible (λ = 650 nm, orange color), they generated shift currents in GaAs which oscillate and, thus, emit terahertz radiation with a bandwidth up to 20 THz (Figure 2). The properties of these currents and the underlying electron motions are fully reflected in the emitted THz waves which are detected in amplitude and phase. The THz radiation shows that the ultrashort current bursts of rectified light contain frequencies which are 5000 times higher than the highest clock rate of modern computer technology.

The properties of the observed shift currents definitely exclude an intraband motion of electrons or holes. In contrast, model calculations based on the interband transfer of electrons in a pseudo-potential band structure reproduce the experimental results and show that a real-space transfer of electrons over the distance on the order of a bond length represents the key mechanism. This process is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the optical field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.

3d Deutsch
Abb. 1 (click to enlarge)

Fig.1 : (a) Unit cell of the semiconductor gallium arsenide (GaAs). Chemical bonds (blue) connect every Ga atom to four neighboring As atoms and vice versa. Valence electron density in the grey plane of (a) in the (b) ground state (the electrons are in the valence band) and in the (c) excited state (electrons are in the conduction band). Apart from the valence electrons shown, there are tightly bound electrons near the nuclei.

Fig. 1 (click to enlarge)

Fig. 2: The experimental concept is shown in the top. A short pulse in the near-infrared or visible spectral range is sent onto a thin GaAs layer. The electric field of the emitted THz radiation is measured as a function of time (1 ps = 10-12 s). An example of such a THz waveform is shown below. It contains oscillations with a period of 0.08 ps corresponding to a frequency of 12000 GHz=12 THz.

Original publication:
A. Ghalgaoui, K. Reimann, M. Woerner, T. Elsaesser, C. Flytzanis, K.Biermann,
Resonant second-order nonlinear terahertz response of gallium arsenide
Phys. Rev. Lett. 121, 266602 (2018)

Further information:
Dr. Michael Woerner, Tel.: 030 6392 1470
Dr. Ahmed Ghalgaoui, Tel.: 030 6392 1474
Prof. Dr. Klaus Reimann, Tel.: 030 6392 1476
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400


Looking at molecules from two sides with table-top femtosecond soft-X-rays

20 December 2018

X-ray spectroscopy provides direct access into the nature of chemical bonds, from which the outcome of chemical reactions can be understood. For this, intense activities both at x-ray source development and implementation of new measurement methods is pursued by key research labs. Researchers at the MBI have now successfully combined a table-top laser-based extreme high-order harmonic source for short-pulse soft-x-ray absorption spectroscopy in the water window with novel flatjet technology. They are the first to demonstrate the simultaneous probing of carbon and nitrogen atoms in organic molecules in aqueous solution.

X-ray absorption spectroscopy (XAS) monitors unoccupied electronic orbitals with element specificity, from which the electronic structure can be derived. For the majority of organic molecules the soft-X-ray spectral region (100-1000 eV) is relevant, as K-edge transitions of low-Z elements (C, N, and O), and the L-edges of 3d metals are found there. XAS is typically performed at large scale facilities, such as storage rings or free-electron lasers. Table-top laser-based sources have until now only been sparsely used to probe pure materials, e.g., metals and organic films. So far, measurements of the carbon or nitrogen K-edges of organic molecules in dilute aqueous solution have not been reported.

The research team at the MBI has now developed a bright source of femtosecond soft X-ray pulses, making use of the extreme high-order harmonic generation process. Long-wavelength (1.8 µm) driver pulses generated with an amplified Ti:sapphire laser system were used to generate high-order harmonics well above the conventional spectral range, i.e., now extending up to 450 eV. They have combined this source with liquid flatjet technology fully functioning under vacuum conditions. Steady-state absorption spectra of organic molecules and inorganic salts in a thin (~ 1 µm) sheet of aqueous solution can now be measured, throughout the so-called water window region between 200-540 eV (see Fig. 1). In particular, this technique enables the simultaneous local probing at both carbon and nitrogen sites within the molecules. With this the research team demonstrates the feasibility of following multiple sites within molecular systems, with the potential of probing possible correlations between these sites upon molecular rearrangements.

This investigation represents a major step towards the systematic investigation of ultrafast rearrangements of solution phase molecular systems with femtosecond soft X-ray spectroscopy. New insights into ultrafast charge transport processes and photo-induced reactions in chemistry and biology are envisaged to become accessible.

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1: Liquid flatjet (solvated urea) illuminated by a broadband soft X-ray pulse obtained by high-order harmonic generation. The insets show the steady-state absorption of Urea at the C and N K-edges extracted from the measurements.


Original publication:
Carlo Kleine, Maria Ekimova, Gildas Goldsztejn, Sebastian Raabe, Christian Strüber, Jan Ludwig, Suresh Yarlagadda, Stefan Eisebitt, Marc J. J. Vrakking, Thomas Elsaesser,
Erik T. J. Nibbering, and Arnaud Rouzée
"Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source"
Journal of Physical Chemistry Letters, 14 December 2018 (online), https://pubs.acs.org/doi/10.1021/acs.jpclett.8b03420

Further information:
Dr. Arnaud Rouzée, Tel.: 030 6392 1240
Dr. Erik T. J. Nibbering, Tel.: 030 6392 1477


Prof. Dr. Dejan B. Milošević receives a Georg Forster Research Award of the Alexander von Humboldt Foundation

12 December 2018

Dejan Milošević, professor at the Faculty of Science of the University of Sarajevo and member of the Academy of Sciences and Arts of Bosnia and Hercegovina, receives a Georg Forster Research Award of the Alexander von Humboldt Foundation for his seminal contributions to the theory of intense-laser interaction with atoms and molecules and attoscience.

He has maintained a very close collaboration with the Max Born Institute through several institute-linkage programs and many visits ever since his first extended stay as an Alexander von Humboldt fellow almost 20 years ago.

The Georg Forster Research Award is granted to academics from developing or transition countries in recognition of a researcher's entire achievements to date of all disciplines whose fundamental discoveries, new theories, or insights have had a significant impact on their own discipline and beyond and who are expected to continue developing research-based solutions to the specific challenges facing their countries. The award is endowed with an amount of 60,000 Euro.

Dejan Milošević's research has mostly been concerned with the theory of atomic and molecular processes in strong external laser fields. He has been a key figure in the development and application of the Feynman path integral to these problems. Together with his collaborators, especially with his MBI host Dr. Wilhelm Becker, he has shown that the so-called "quantum orbits" afford an ideal tool for the analysis of strong-field processes, combining intuitive appeal with computational ease and precision. The wide applicability of this formalism has become obvious with the advent of attoscience -- a new area of science that has developed over the last decade as an offspring of strong-field physics taking advantage of the advances of laser technology. There is hardly an area of strong-field physics where Dejan Milosevic has not made seminal contributions.

The Max Born Institute looks forward to the enhanced cooperation with Dejan Milošević whose research plans perfectly augment the program of the MBI theory group.

Dr. Wilhelm Becker, 030 6392 1388
Prof. Dr. Misha Ivanov, 030 6392 1210


Atomic jet - the first lens for extreme-ultraviolet light developed

28 November 2018

Scientists from the Max Born Institute (MBI) have developed the first refractive lens that focuses extreme ultraviolet beams. Instead of using a glass lens, which is non-transparent in the extreme-ultraviolet region, the researchers have demonstrated a lens that is formed by a jet of atoms. The results, which provide novel opportunities for the imaging of biological samples on the shortest timescales, were published in Nature.

A tree trunk partly submerged in water appears to be bent. Since hundreds of years people know that this is caused by refraction, i.e. the light changes its direction when traveling from one medium (water) to another (air) at an angle. Refraction is also the underlying physical principle behind lenses which play an indispensable role in everyday life: They are a part of the human eye, they are used as glasses, contact lenses, as camera objectives and for controlling laser beams.

Following the discovery of new regions of the electromagnetic spectrum such as ultraviolet (UV) and X-ray radiation, refractive lenses were developed that are specifically adapted to these spectral regions. Electromagnetic radiation in the extreme-ultraviolet (XUV) region is, however, somewhat special. It occupies the wavelength range between the UV and X-ray domains, but unlike the two latter types of radiation, it can only travel in vacuum or strongly rarefied gases. Nowadays XUV beams are widely used in semiconductor lithography as well as in fundamental research to understand and control the structure and dynamics of matter. They enable the generation of the shortest human made light pulses with attosecond durations (an attosecond is one billionth of a billionth of a second). However, in spite of the large number of XUV sources and applications, no XUV lenses have existed up to now. The reason is that XUV radiation is strongly absorbed by any solid or liquid material and simply cannot pass through conventional lenses.

In order to focus XUV beams, a team of MBI researchers have taken a different approach: They replaced a glass lens with that formed by a jet of atoms of a noble gas, helium (see Fig. 1). This lens benefits from the high transmission of helium in the XUV spectral range and at the same time can be precisely controlled by changing the density of the gas in the jet. This is important in order to tune the focal length and minimize the spot sizes of the focused XUV beams.

In comparison to curved mirrors that are often used to focus XUV radiation, these gaseous refractive lenses have a number of advantages: A "new" lens is constantly generated through the flow of atoms in the jet, meaning that problems with damages are avoided. Furthermore, a gas lens results in virtually no loss of XUV radiation compared to a typical mirror. "This is a major improvement, because the generation of XUV beams is complex and often very expensive," Dr. Bernd Schütte, MBI scientist and corresponding author of the publication, explains.

In the work the researchers have further demonstrated that an atomic jet can act as a prism breaking the XUV radiation into its constituent spectral components (see Fig. 2). This can be compared to the observation of a rainbow, resulting from the breaking of the Sun light into its spectral colors by water droplets, except that the ‘colors’ of the XUV light are not visible to a human eye.

The development of the gas-phase lenses and prisms in the XUV region makes it possible to transfer optical techniques that are based on refraction and that are widely used in the visible and infrared part of the electromagnetic spectrum, to the XUV domain. Gas lenses could e.g. be exploited to develop an XUV microscope or to focus XUV beams to nanometer spot sizes. This may be applied in the future, for instance, to observe structural changes of biomolecules on the shortest timescales.

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1: Focusing of an XUV beam by a jet of atoms that is used as a lens. (Credit: Oleg Kornilov, Lorenz Drescher)

Phase Image
Fig. 2 (click to enlarge)
Fig. 2: Invisible rainbow that is generated by a jet of helium atoms. Light with "colors" close to resonances of helium are either deflected upwards or downwards. (Fig. MBI)

Original publication:
Lorenz Drescher, Oleg Kornilov, Tobias Witting, Geert Reitsma, Nils Monserud, Arnaud Rouzée, Jochen Mikosch, Marc Vrakking, Bernd Schütte
"Extreme-ultraviolet refractive optics"
Nature, 28 November 2018 (online), doi.org/10.1038/s41586-018-0737-2

Further information:
Dr. Bernd Schütte, Tel.: 030 6392 1295
Prof. Dr. M.J. Vrakking , Tel.: 030 6392 1200


Dr. Daniela Rupp received the "Mayor's Young Talent Award" at the "Science Award of the Governing Mayor of Berlin 2018"

8 November 2018

In an award ceremony held at the Berlin town hall ("Rotes Rathaus") on Wednesday November 7th, Dr. Daniela Rupp, the leader of the Junior Research Group "Ultrafast Dynamics Dynamics in Nanoplasma" received the Mayor's Young Talent Award ("Nachwuchspreis") from Mayor Michael Müller, for her pioneering research on imaging the structure and dynamics of nano-scale particles using single-shot coherent diffractive imaging.

In his laudatio, professor-emeritus Ingolf Hertel (the former director of Division A of the MBI) explained how, in her research, Daniela uses both large scale free electron lasers, and lab-based systems such as the high harmonic sources at MBI, to gain unprecedented insight into both the shapes and laser-induced dynamics of rare gas clusters and nano-droplets. A recent example of this work can be found here (MBI Highlight Nature Communications "First imaging of free nanoparticles in laboratory experiment using a high-intensity laser source") . Prior to the current award, Daniela was awarded the 2018 Karl-Scheel Prize and the 2013 Carl Ramsauer Prize, both awarded by the Berlin section of the German Physical Society.


Dr. Daniela Rupp is junior research group leader at the Max Born Institute, Berlin
(Source Senatskanzlei Berlin)


Dr. Daniela Rupp, Tel.: 030 6392 1280




Future Data Storage Technology - Extremely small magnetic nanostructures with invisibility cloak imaged

17 September 2018

In novel concepts of magnetic data storage, it is intended to send small magnetic bits back and forth in a chip structure, store them densely packed and read them out later. The magnetic stray field generates problems when trying to generate particularly tiny bits. Now, researchers at the Max Born Institute (MBI), the Massachusetts Institute of Technology (MIT) und DESY were able to put an "invisibility cloak" over the magnetic structures. In this fashion, the magnetic stray field can be reduced in a fashion allowing for small yet mobile bits. The results were published in "Nature Nanotechnology".

For physicists, magnetism is intimately coupled to rotating motion of electrons in atoms. Orbiting around the atomic nucleus as well as around their own axis, electrons generate the magnetic moment of the atom. The magnetic stray field associated with that magnetic moment is the property we know from e.g. a bar magnet we use to fix notes on pinboard. It is also the magnetic stray field that is used to read the information from a magnetic hard disk drive. In today's hard disks, a single magnetic bit has a size of about 15 x 45 nanometer, about of those would fit on a stamp.

One vision for a novel concept to store data magnetically is to send the magnetic bits back and forth in a memory chip via current pulses, in order to store them at a suitable place in the chip and retrieve them later. Here, the magnetic stray field is a bit of a curse, as it prevents that the bits can be made smaller for even denser packing of the information. On the other hand, the magnetic moment underlying the stray field is required to be able to move the structures around.

The researchers were now able to put an "invisibility cloak" on the magnetic nanostructures and to observe, how small and how fast such structures can get. To this end, different atomic elements with opposite rotation of the electrons were combined in one material. In this way, the magnetic stray field can be reduced or even completely cancelled - the individual atoms, however, still carry a magnetic moment but together appear cloaked.

In spite of this cloaking, the scientists were able to image the tiny structures. Via x-ray holography, they were able to selectively make only the magnetic moments of one of the constituent elements visible - in this way an image can be recorded in spite of the invisibility cloak.

It became apparent, that fine tuning of the strength of the invisibility cloak allows to simultaneously meet two goals which are of importance for potential applications in data storage. "In our images, we see very small, disk-like magnetic structures", says Dr. Bastian Pfau from MBI. "The smallest structures we observed had a diameter of only 10 nanometer". The information density of today's hard disk drives could be significantly increased, if such structures could be used to encode the data. Furthermore, in additional measurements the researchers realized that suitably cloaked bits can be moved particularly fast by short current pulses - an important property for actual use in a memory device. A velocity higher than 1 kilometer per second was reached in the MIT laboratory.

"This is possible as a consequence of quantum physics", explains Prof. Stefan Eisebitt from MBI. "The contribution of the electron's orbit around the nucleus to the magnetic moment is only half as large as the contribution of the electron's spin around its own axis." When combining different atom types with different direction and strength of this rotation in one material, one can cancel the total rotation - physicists talk about the total angular momentum - of the system, while still retaining a small magnetic moment. As the angular momentum leads to a drag when moving the structures via current pulses, this approach allows for high speed motion. Hence, if the strength of the invisibility cloak is adjusted just right, both small size and high speed of the magnetic bit structures can be achieved - an interesting prospect for novel magnetic data storage concepts.

Abb. 1 (click to enlarge)

Fig. 1:In the future, a magnetic skyrmion could encode a "1" in data storage. The skyrmion is made up by the specific arrangement of the magnetic moments of neighboring atoms, represented by arrows in the images. Shown on the right is a skyrmion where neighboring atoms have approximately opposite magnetization, hence cloaking the resulting net magnetic stray field. In this way, smaller diameter skyrmions are stable. Physicists talk about "antiferromagnetic" (AFM) rather than "ferromagnetic" (FM) order between neighboring moments. (Copyright: L. Caretta, M. Huang, MIT)

Original publication:
Lucas Caretta, Maxwell Mann, Felix Büttner, Kohei Ueda, Bastian Pfau, Christian M. Günther, Piet Hessing, Alexandra Churikova, Christopher Klose, Michael Schneider, Dieter Engel, Colin Marcus, David Bono, Kai Bagschik, Stefan Eisebitt and Geoffrey S. D. Beach
"Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet"
Nature Nanotechnology; doi.org/10.1038/s41565-018-0255-3.

Further information:
Prof. Dr. S. Eisebitt, Tel.: 030 6392 1300
Dr. Bastian Pfau, Tel.: 030 6392 1343


Electric polarization in the macroscopic world and electrons moving at atomic scales - a new link from femtosecond x-ray experiments

27 August 2018

Femtosecond x-ray experiments in combination with a new theoretical approach establish a direct connection between electric properties in the macroscopic world and electron motions on the time and length scale of atoms. The results open a new route for understanding and tailoring the properties of ferroelectric materials.

Phenomena in the macroscopic world are described by classical physics while processes at atomic length and time scales are governed by the laws of quantum mechanics. The connection between microscopic and macroscopic physical quantities is far from being trivial and partly unexplained.

The electric polarization is a macroscopic quantity which describes the dipole moment of matter. The polarization originates from the peculiar electron distribution at the atomic scale in polar and ionic materials, among them the most interesting class of ferroelectrics. Their spontaneous electric polarization is widely applied in electronic sensors, memories, and switching devices. The link between polarizations, in particular time dependent ones, and microscopic electron densities is important for understanding and tailoring the properties of ferroelectrics.

Based on a new experimental and theoretical approach, scientists from the Max-Born-Institute have now established a direct quantitative connection between macroscopic electric polarizations and time-dependent microscopic electron densities. As they report in Physical Review B, atomic motions in ferroelectrics are launched by optical excitation and modulate the electron distribution on a femtosecond time scale (1 fs = 10-15 seconds). The resulting dynamics of electron density are mapped by time-resolved x-ray powder diffraction. Such data allow for the generation of temporally and spatially resolved electron density maps from which the momentary macroscopic polarization is derived with the help of a new theoretical concept. The potential of the method is demonstrated with two prototype ferroelectric materials.

The theoretical work extends the existing quantum phase approach for calculating stationary macroscopic polarizations towards ultrafast nonequilibrium dynamics of electron charge and polarization. The theoretical key steps consist in deriving a microscopic current density from time-dependent electron density maps while minimizing the electron kinetic energy, and calculating the macroscopic polarization from the current density. This method is applied to the prototype ferroelectric material ammonium sulfate [(NH4)2SO4, Fig. 1] with the time dependent electron and current densities shown in the movie attached. As a second prototype system, potassium dihydrogen phosphate [KH2PO4] was investigated. The analysis provides macroscopic polarizations and their absolute values as governed by microscopic vibrations.

The results establish ultrafast x-ray diffraction as a unique tool for grasping macroscopic electric properties of complex materials. The broad relevance of this new insight is underlined by the selection of the article as an "Editor's Suggestion".

Abb. 1 (click to enlarge)

Abb. 1:Top: Crystal lattice of ferroelectric ammonium sulfate [(NH4)2SO4] with tilted ammonium (NH4+ tetrahedra (nitrogen: blue, hydrogen: white) and sulfate (SO42-) tetrahedra (sulfur: yellow, oxygen: red). The green arrow shows the direction of the macroscopic polarization P. Blue arrows: local dipoles between sulphur and oxygen atoms. The electron density maps shown in the bottom left panel and the movie are taken in the plane highlighted in grey. Bottom left: Stationary electron density with a high value on the sulfur (red) and smaller values on the oxygen atoms (yellow). Bottom right: Change of local dipoles at a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate sample. An anisotropic shift of charge reduces the dipole pointing to the right and increases the other 3 dipoles.

Movie: Left: Time dependent electron density on the sulfate ion shown in Fig. 1 between delay times of 2.7 ps and 5.1 ps. The change of charge density is shown with an amplitude 100 times larger than the experimental value. Right: Time-dependent current density flowing along the crystal's a axis, as derived from the transient electron density. The current density oscillates with a 90 degrees phase shift relative to the electron density.

Original publication:
Christoph Hauf, Michael Woerner, and Thomas Elsaesser
Macroscopic electric polarization and microscopic electron dynamics: Quantitative insight from femtosecond x-ray diffraction
Phys.Rev. B 98, 054306 (2018, Editor's Suggestion).

Further information:
Dr. Christoph Hauf, Tel.: 030 6392 1473
Dr. Michael Wörner, Tel.: 030 6392 1470
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400


Slow, but efficient: Low-energy electron emission from intense laser cluster interactions

8 August 2018

For the past 30 years intense laser cluster interactions have been seen primarily as a way to generate energetic ions and electrons. In surprising contrast with the hitherto prevailing paradigm, a team of researchers has now found that copious amounts of relatively slow electrons are also produced in intense laser cluster interactions. These low-energy electrons constitute a previously missing link in the understanding of the processes occurring when an intense laser pulse interacts with a nanoscale particle, a situation that is highly relevant for the in-situ imaging of biomolecules on ultrashort timescales.

When a nanoscale particle is exposed to an intense laser pulse, it transforms into a nanoplasma that expands extremely fast, and several phenomena occur that are both fascinating and important for applications. Examples are the generation of energetic electrons, ions and neutral atoms, the efficient production of X-ray radiation as well as nuclear fusion. While these observations are comparably well understood, another observation, namely the generation of highly charged ions, has so far posed a riddle to researchers. The reason is that models predicted very efficient recombination of electrons and ions in the nanoplasma, thereby drastically reducing the charges of the ions.

A team of researchers from the Imperial College London, the University of Rostock, the Max Born Institute, the University of Heidelberg and ELI-ALPS have now helped to solve this riddle. Tiny clusters consisting of a few thousand atoms were exposed to ultrashort, intense laser pulses. The researchers found that the vast majority of the emitted electrons were very slow (see Fig. 1). Moreover, it turned out that these low-energy electrons were emitted with a delay compared to the energetic electrons.

Lead scientist Dr. Bernd Schütte, who performed the experiments at Imperial College in the framework of a research fellowship and who now works at the Max Born Institute, says: "Many factors including the Earth's magnetic field influence the movement of slow electrons, making their detection very difficult and explaining why they have not been observed earlier. Our observations were independent from the specific cluster and laser parameters used, and they help us to understand the complex processes evolving on the nanoscale."

In order to understand the experimental observations, researchers around Professor Thomas Fennel from the University of Rostock and the Max Born Institute simulated the interaction of the intense laser pulse with the cluster. "Our atomistic simulations showed that the slow electrons result from a two-step process, where the second step relies on a final kick that has so far escaped the researchers' attention", explains Fennel. First, the intense laser pulse detaches electrons from individual atoms. These electrons remain trapped in the cluster as they are strongly attracted by the ions. When this attraction diminishes as the particles move farther away from each other during cluster expansion, the scene is set for the important second step. Therein, weakly bound electrons collide with a highly excited ion and thus get a final kick that allows them to escape from the cluster. As such correlated processes are quite difficult to model, the computing resources from the North-German Supercomputing Alliance (HLRN) were essential to solve the puzzle.

The researchers found the emission of slow electrons to be a very efficient process, enabling a large number of slow electrons to escape from the cluster. As a consequence, it becomes much harder for highly charged ions to find partner electrons that they can recombine with, and many of them indeed remain in high charge states. The discovery of the so-called low-energy electron structure can thus help to explain the observation of highly charged ions from intense laser cluster interactions. These findings might be important as low-energy electrons are implicated as playing a major role in radiation damage of biomolecules - of which the clusters are a model.

Senior author Professor Jon Marangos, from the Department of Physics at Imperial, says: "Since the mid-1990's we have worked on the energetic emission of particles (electrons and highly charged ions) from laser-irradiated atomic clusters. What is surprising is that until now the much lower energy delayed electron emission has been overlooked. It turns out that this is a very strong feature, accounting for the majority of emitted electrons. As such, it may play a big role when condensed matter or large molecules of any kind interact with a high intensity laser pulse."

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1: The electron kinetic energy spectrum from Ar clusters interacting with intense laser pulses is dominated by a low-energy structure (orange area). The inset shows the same spectrum on a logarithmic scale, indicating an exponential behavior both for the slow electron emission (red curve) and for the fast electron emission (green curve).

Phase Image
Fig. 2 (click to enlarge)
Fig. 2: Atomistic simulation of the laser-induced cluster explosion.

Original publication:
Bernd Schütte, Christian Peltz, Dane R. Austin, Christian Strüber, Peng Ye, Arnaud Rouzée, Marc J. J. Vrakking, Nikolay Golubev, Alexander I. Kuleff, Thomas Fennel and Jon P. Marangos
"Low-energy electron emission in the strong-field ionization of rare gas clusters"
Physical Review Letters 0031-9007/18/121(6)/063202(6)/ DOI:10.1103/PhysRevLett.121.063202

Further information:
Dr. Bernd Schütte, Tel.: 030 6392 1295
Prof. Dr. Thomas Fennel , Tel.: 030 6392 1245


Concepts for new switchable plasmonic nanodevices: a magneto-plasmonic nanoscale router and a high-contrast magneto-plasmonic disk modulator controlled by external magnetic fields

2nd August 2018

Plasmonic waveguides open the possibility to develop dramatically miniaturized optical devices and provide a promising route towards the next-generation of integrated nanophotonic circuits for information processing, optical computing and others. Key elements of nanophotonic circuits are switchable plasmonic routers and plasmonic modulators. Recently Dr. Joachim Herrmann (MBI) and his external collaborators developed new concepts for the realization of such nanodevices. They investigated the propagation of surface-plasmon-polaritons (SPP) in magneto-plasmonic waveguides. Based on the results of this study they proposed new variants of switchable magneto-plasmonic routers and magneto-plasmonic disk modulators for various nanophotonic functionalities.

In a waveguide based on a metal film with a thickness exceeding the Skin depth and surrounded by a ferromagnetic dielectric an external magnetic field in the transverse direction can induce a significant spatial asymmetry of mode distribution of surface-plasmon-polaritons (SPP). Superposition of the odd and the even asymmetric modes over a certain distance leads to a concentration of the energy on one interface which is switched to the other interface by magnetic field reversal. The requested magnitude of magnetization is exponentially reduced with the increase of the metal film thickness. Based on this phenomenon, the group proposed a new type of waveguide-integrated magnetically controlled switchable plasmonic routers. A configuration of such nanodevice is shown in Fig. 1 consisting of a T-shaped metallic waveguide surrounded by a ferromagnetic dielectric under an external magnetic field inducing a magnetization M. In Fig. 2 numerical results for the plasmon propagation by solving the Maxwell equation show channel switching by the magnetic field reversal with a 99%-high contrast within the optical bandwidth of tens of THz [1]. Here g is the gyration g=χM, χ is the magneto-optical susceptibility and g0 is a characteristic gyration requested to induce a significant mode asymmetry. Magnetic field reversal by integrated electronic circuits can be realized with a repetition rate in the GHz region. Note that up to now there exist only few papers reporting the realization of switchable plasmonic routers based on branched silver nanowires controlled by the polarization of the input light.

In a second paper [2] the group proposed and studied a novel type of ultra-small plasmonic modulator based on a metal-isolator-metal waveguide and a side-coupled magneto-optical disk controlled by an external magnetic field (see Fig.3). The wavenumber change and the transmission of surface-plasmon-polaritons (SPPs) can be tuned by altering the magnetic field and reversible on/off switching of the running SPP modes by a reversal of the direction of the external magnetic field is demonstrated. Resonant enhancement of the magneto-plasmonic modulation by more than 200 times leads to a modulation contrast ratio more than 90% keeping a moderate insertion loss within an optical bandwidth of hundreds of GHz. Numerical simulations by the solution of Maxwell's equations confirm the predictions by the derived analytical formulas of a high-contrast magneto-plasmonic modulation. Fig. 4 shows the distribution of the magnetic field components of the SPPs at a gyration g=0.03 and g=-0.03, respectively. As seen by changing the direction of the external magnetic field, the transmission of the SPPs is switched from an off to an on state via the changed interference pattern in the waveguide.

Abb. 1 (click to enlarge)

Fig 1: Configuration of a switchable plasmonic router consisting of a T-shaped metallic waveguide surrounded by a ferromagnetic dielectric material and under the action of an external magnetic field.

Abb. 2 (click to enlarge)
Fig 2: Numerical results for the distribution of the plasmon intensity demonstrating channel switching.The reversal of the direction of the external magnetic field leads to a switching of the SPP propagation from channel 1 in (a) to channel 2 in (b). The metal waveguide is made from gold and the surrounding ferromagnetic dielectric from Bi-substituted iron garnet (BIG).
Abb. 3 (click to enlarge)
Fig 3: Scheme of the magnetoplasmonic disk resonator side-coupled to a metal-insulator-metal waveguied and controlled by an external magnetic field.
Abb. 4 (click to enlarge)
Fig 4: Magnetic field distribution of the SPPs in a metal waveguide side-coupled to a disk resonator at the wavelength of 748nm. By changing the direction of the magnetic field the transmission of SPPs is switched from an off state (fig 4a) with g=-0.03 to an on state (Fig 4b) with g=0.03.The metal waveguide is made from silver and the ferromagnetic dielectric from Bi-substituted iron garnet (BIG).

Original publication:
Kum-Song Ho, Song-Jin Im, Ji-Song Pae, Chol-Song Ri, Yong-Ha Han and Joachim Herrmann
"Switchable plasmonic routers controlled by external magnetic fields by using magneto-plasmonic waveguides"

[1]Scientific Reports (2018) 8:10584 /DOI:10.1038/s41598.018.28567.8

Ji-Song Pae, Song-Jin IM, Kum-Song Ho, Chol-Song Ri, Sok-Bong Ro and Joachim Herrmann
"Ultracompact high-contrast magneto-optical disk resonator side-coupled to a plasmonic waveguide and switchable by an external magnetic field".

[2] Phys. Rev. B 98, 041406 (R) (2018).

Further information:
Dr. Joachim Herrmann, Tel.: 030 6392 1278


Benjamin Fingerhut receives the ERC Starting Grant

27 July 2018

Dr. Benjamin Fingerhut, junior group leader at the Max Born Institute (MBI), is recipient of the prestigious ERC Starting Grant 2018. The project addresses ultrafast biomolecular dynamics via a non-adiabatic theoretical approach. The award is granted by the European Research Council (ERC) to support excellent researchers at the beginning of their independent research careers.

As of today the European Research Council (ERC) has announced the awardees of its Starting Grants. In the highly competitive selection procedure, the research proposal of Benjamin Fingerhut has been selected for funding. ERC Starting Grants are designed to support excellent early career researchers in establishing their own independent research programme. ERC Starting Grants are awarded to researchers up to 7 years after their PhD to conduct a research programme at a European university or research institute. The grants are awarded under the "excellent science" pillar of Horizon 2020, the EU's research and innovation programme with a funding of up to Euro 1.5 million for a maximum of 5 years.

The successful project is devoted to the fundamental understanding of ultrafast biomolecular vibrational dynamics in the mid-IR/THz spectral region where biologically highly relevant dynamics occur. The innovative non-adiabatic approach addresses fundamental problems, such as proton transfer, vibrational lifetimes and the dissipation of excess energy. The project aims to elucidate ultrafast biomolecular vibrational dynamics in dipolar liquids, within nanoconfined environments and in the vicinity of biological interfaces. As such the non-adiabatic approach to biomolecular vibrational dynamics facilitates insight into transmembrane proton translocation mechanisms which is highly relevant as microscopic foundation of cell respiration.

Dr. Benjamin Fingerhut has joined the MBI in 2014. He is currently funded by an Emmy Noether Early Career Grant of the German Research Foundation (DFG) and has established the Biomolecular Dynamics Junior Research Group at the MBI. The group pursues close collaboration with experimental research conducted at MBI which applies the most advanced methods of femtosecond nonlinear vibrational spectroscopy for mapping the relevant interactions of biomolecular processes. The group's research involves the development of state-of-the-art spectroscopic simulation techniques and their application to the real-time determination of ultrafast structural dynamics of molecular and biomolecular systems. The group combines analytical and computational approaches for novel simulation protocols suited to investigate complex non-adiabatic dynamics.

Further information on Dr. Benjamin Fingerhut can be found at http://staff.mbi-berlin.de/fingerhu/and on ERC Grants https://erc.europa.eu/.

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)
Theory Department T4: Biomolecular Dynamics
Dr. Benjamin P. Fingerhut
Tel. 030 / 6392-1404


What happens when we heat the atomic lattice of a magnet all of a sudden?

13 July 2018

Magnets have fascinated humans for several thousand years and enabled the age of digital data storage. They occur in various flavors. Ferrimagnets form the largest class of magnets and consist of two types of atoms. Similar to a compass needle, each atom exhibits a little magnetic moment, also called spin, which arises from the rotation of the atom's electrons about their own axes. In a ferrimagnet, the magnetic moments point in opposite directions for the two types of atoms (see panel A). Thus, the total magnetization is the sum of all magnetic moments of type 1 (M1), blue arrows) and type 2 (M2), green arrows). Due to the opposite direction, the magnitude of the total magnetization is M1-M2.

When an insulating ferrimagnet is heated, the heat is first deposited in the atomic lattice which causes the atoms to move randomly around their cold positions. Finally, part of the heat also causes random rotation (precession) of the spins around their cold direction. Thus, magnetic order gets lost; the total magnetization (M1-M2) decreases and eventually vanishes if the temperature of the ferrimagnet exceeds a critical temperature, the so-called Curie temperature. Although this process is of fundamental importance, its dynamics are not well understood. Even for the ferrimagnet yttrium iron garnet (YIG), one of the most intensely researched ferrimagnets, it is unknown how long it takes until the heated atomic lattice and the cold magnetic spins reach equilibrium with each other. Previous estimates of this time scale differ from each other by a factor of up to one million.

A team of scientists from Berlin (Collaborative Research Center / Transregio 227 Ultrafast Spin Dynamics, Fritz Haber Institute and Max Born Institute), Dresden (Helmholtz Center), Uppsala (Sweden), St. Petersburg (Russia), and Sendai (Japan) have now revealed the elementary steps of this process. "To instantaneously and exclusively heat up the atomic lattice of a YIG film, we use a very specific and novel kind of stimulus: ultrashort bursts of laser light at terahertz frequencies. With a subsequently arriving visible laser pulse, we can then step-by-step trace the evolution of the initially cold magnetic spins. Essentially, we record a stop-motion movie of how the magnetization evolves." says Sebastian Maehrlein, who conducted the experiments. His colleague Ilie Radu from summarizes: "Our observations are striking. We found that sudden heating of the atomic lattice reduces the magnetic order of the ferrimagnet on two distinct time scales: an incredibly fast scale of only 1 ps and a 100,000 times slower scale of 100 ns."

These two time scales can be understood in analogy to water in a closed pot that is put into a hot oven. The hot air of the oven corresponds to the hot atomic lattice whereas the magnetic spins correspond to the water inside the pot (see panel A). Once the atomic lattice is heated by the terahertz laser burst, the enhanced random oscillations of the atoms lead to a transfer of magnetic order from spin type 1 to spin type 2. Therefore, both the magnetic moments M1 (blue arrows in panel B) and M2 (green arrows) are reduced by exactly the same amount (red arrows). This process evolves on the fast time scale, and the atomic spins are forced to heat up while leaving the total magnetization M1-M2 unchanged, just like water in a closed pot that has to keep its volume.

We know, however, that a heated ferrimagnet not only aims at reducing M1 and M2, but also its total magnetization M1-M2. To do so, part of the spin must be released to the atomic lattice. This situation is again completely analogous to the hot water in a closed pot: the pressure inside the pot increases but is slowly released to the outside through little leaks in the lid (see panel C). This leakage of angular momentum to the atomic lattice is exactly what happens in the ferrimagnet through weak couplings between spins and lattice.

"We now have a clear picture of how the hot atomic lattice and the cold magnetic spins of a ferrimagnetic insulator equilibrate with each other." says Ilie Radu. The international team of researchers discovered that energy transfer proceeds very quickly and leads to a novel state of matter in which the spins are hot but have not yet reduced their total magnetic moment. This "spin overpressure" is released through much slower processes that permit leakage of angular momentum to the lattice. "Our results are also relevant for applications in data storage." Sebastian Maehrlein adds. “The reason is simple. Whenever we want to switch the value of a bit between 0 to 1 in a magnetic storage medium, angular momentum and energy have to finally be transferred between atomic lattice and spins."

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1: Heating a magnet without changing its magnetization. (A) A ferrimagnet consists of two spin sorts of opposite orientation (green and blue arrows). In the experiment, the atomic lattice of the ferrimagnet is heated by an extremely short terahertz laser pulse. This situation is analogous to heating the air (=atomic lattice) inside an oven that contains a pot with water (=spins). (B) Heat is transferred into the spin system and decreases the magnetization of each spin type by exactly the same amount. This process arises because spin is transferred from the blue to the green spin sort. Thus, the magnet is heated without changing its total magnetization! In the pot analogy, heat is transferred from the air outside the pot to the water inside. While the amount of water in the pot has not changed, an overpressure has built up. (C) Finally, the hot spins release their overpressure to the atomic lattice, thereby reducing the total magnetization. In the analogy, water overpressure is released through little leaks in the pot lid. (Source: FHI)

Original publication:
S. F. Maehrlein, I. Radu, P. Maldonado, A. Paarmann, M. Gensch, A. M. Kalashnikova, R. V. Pisarev, M. Wolf, P. M. Oppeneer, J. Barker, T. Kampfrath
Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation.

Sci. Adv. 4, eaar5164 (2018).

Futher Information:
Dr. Ilie Radu, Tel.: 030 6392 1357


Dr. Federico Furch named 2018 OSA Ambassador

21 June 2018

In October of 2017 the Optical Society (OSA) announced the 2018 class of OSA Ambassadors. One member of this class is MBI researcher Dr. Federico Furch, who in the last few years has been responsible for the development of a state-of-the-art 100 kHz OPCPA laser system that is currently being implemented in attosecond experiments. (see Highlight link MBI website).

Furch Osa Ambassador (click to enlarge) Source: MBI

"Some of the OSA Ambassadors at the OSA headquarters in Washington DC, during the Spring leadership meeting. Dr. Federico Furch, second to the left"

As stated in the press release by Chad Stark, president of the OSA Foundation, "OSA Ambassadors are dedicated to supporting OSA's student chapters and local sections, student members and other early career professionals. By sharing their experiences and knowledge, Ambassadors become an important component of OSA's professional development programs and outreach."

All OSA Ambassadors attended the spring leadership meeting at the OSA headquarters in Washington DC, in April of 2018. Moreover, they will attend the student leadership meeting taking place in September, also in Washington. As part of the Ambassadors program Federico has already engaged in activities with students chapters in Berlin, Potsdam, Argentina and Chile. He visited the Ukraine in early June, will visit Argentina and Chile in August, and India in October. During each of these activities he represents not only the OSA, but naturally, the Max Born Institute as well.

Recently, in June, Federico supported the BerlinOptik student chapter and 2016 OSA Ambassador Dr. Aline Dinkelaker in the organization of the career development event "working in Photonics in Berlin," where the Max Born Institute was represented by Prof. Marc Vrakking. He also organized a networking and information event for MBI students, where they were able to interact with the members of the BerlinOptik student chapter and the student chapter at Potsdam University.

Federico's OSA-related activities will continue throughout the year and MBI students that want to know more about OSA and the activities of student chapters are encouraged to contact him.

For more information please consult:
"The Optical Society Announces 2018 Ambassadors"
"2018 Ambassadors"


Dr. Federico Furch, Tel.: 030 6392 1277




"Picture of atomic orbitals featured in NOVA/PBS documentary"

14 June 2018

Pictures of atomic hydrogen orbitals measured by MBI researchers are featured in a new NOVA/PBS documentary (see link). In the video, it is explained how the two-dimensional wavefunction of a hydrogen atom can be visualized by recording a large number of ionized electrons, one at a time, on a 2-dimensional detector. The results were published in Physical Review Letters in 2013 (see Highlight link MBI website). and voted one of the Top 10 breakthroughs in Physics in 2013 (see Highlight link MBI website).

Link to video on Youtube: "What does an atom actually look like"
Source: Greg Kestin PhD, Digital Producer, NOVA/ PBS, Preceptor, Harvard University Physics Department

Link to Original Publication Physical Review Letters 2013: "Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States"

Link to Press Release: "Hydrogen atoms under the magnifying glass: Direct Observation of the Nodal Structures of Electronic States of the Hydrogen Atom"

Link to Top10 breakthrough in Physics 2013: "Internationales Forscherteam des MBI & AMOLF in den "top 10 breakthroughs" in "physicsworld.com" 2013"


Prof. Marc J. Vrakking, Tel.: 030 6392 1200




Dr. Daniela Rupp will receive the Karl Scheel Prize 2018

13 June 2018

The Physical Society of Berlin announced this year's Karl Scheel laureate. We congratulate Dr. Daniela Rupp, who will receive the award on June 22, 2018 at 5 p.m. the Magnus-Haus in Berlin Mitte.


Dr. Daniela Rupp is junior research group leader at the Max Born Institute, Berlin

For more information please consult: "Der Karl-Scheel-Preis der Physikalischen Gesellschaft zu Berlin"


Dr. Daniela Rupp, Tel.: 030 6392 1280




X-Ray Holography reveals Nano-Patchwork during Phase Transition in Vanadium Dioxide

31 May 2018

In the prototypical material VO2, the role of electronic correlation in the phase transition between the insulating and metallic phase have long been debated. Combining spectroscopy and holography with x-rays, an international team of scientists has now observed how tiny patches of different phases evolve during the phase transition.

The interplay of the positions of the atomic nuclei and the spatial distribution and energies of the electrons providing the "glue" holding them together defines the properties of the materials that make up our world. Of particular interest to researchers is to understand the mechanisms at play in phase transitions. For example, vanadium dioxide (VO2) is insulating at low temperatures and becomes metallic above about 65°C. The change in the electronic structure (insulating vs. metallic) is accompanied by a change of the crystal structure from a monoclinic (M1) structure to a rutile (R) structure, with a minute change of atomic positions (See Fig.1). The driving forces for this phase transition have been a matter of long standing debate, specifically the role of electronic correlation in thin VO2 films, where it had been reported that the material turns metallic at slightly lower temperatures even before the atoms rearrange to the R-structure.

A team of scientists from the Max Born Institute and the Technical University in Berlin, the ICFO and ALBA in Spain as well as the Vanderbildt University in the USA have now investigated this phase transition in thin VO2 films using x-ray spectro-holography. This technique allows to probe the electronic structure with 40 nm spatial resolution and can thus shed light on the role of inhomogeneity in the mechanism of the phase transition on the nanoscale. In the journal Nano Letters the researchers report that defects in the VO2 film can locally change the pathway of the phase transition. The picture that emerges from temperature series of spectro-holographic images through the phase transition as shown in Fig. 2 is the following:

When raising the temperature, growth of metallic regions in R-structure start from nanoscale defects. At these defects, "misplaced atoms" generate a strain in their neighborhood that reduce the energy required for the M1 to R transition to occur. In turn, the volume mismatch between these two phases locally generates a new strain field, triggering the growth of domains in yet another, different monoclinic phase called M2 in adjacent regions. This effect hence leads to a coexistence of different phases of the material on the nanometer length scale, as seen e.g. as a stripe pattern at 335°K (62°C) in Fig. 2. At higher temperature, these still insulating M2 phases will ultimately also transform into the metallic R phase - just like some of the M1 phase patches will do directly. The pathway for the insulator to metal phase transition is thus not homogeneous throughout the thin VO2 film, but varies spatially. Researchers have been blind to the inhomogeneity on this small lengthscale in the past and may thus have come to wrong conclusions by averaging over these regions in their experiments. In particular, in this new work no evidence for reduced electronic correlations or a new monoclinic yet metallic phase below the phase transition temperature is seen, as has been discussed in the past. The results highlight the importance of combining spatial and spectroscopic resolution and will serve as the basis to study the dynamics of laser-driven phase transitions in materials with electronic correlation.

3d Deutsch
Fig. 1 (click to enlarge)

Fig. 1: Crystal structures for the insulating monoclinic phases M1 and M2 as well as for the metallic R rutile structure. Minute changes in atomic positions have a large effect on the material properties. Vanadium atoms are shown in orange, oxygen atoms in blue. Connecting lines are meant as guide to the eye.

Phase Image
Fig. 2 (click to enlarge)
Fig. 2: Images of the phase separation occurring when heating a 75 nm thin VO2 film. The images were acquired via x-ray spectro-holography and are displayed in false color to indicate the different regions: red = defect region, black = M1 phase, blue = M2 phase, green = R phase. Note that some sample regions transition directly from M1 to R (e.g. cross marker) while others transition via the intermediate M2 phase (e.g. diamond marker).

Original publication:
Luciana Vidas, Christian M. Günther, Timothy A. Miller, Bastian Pfau, Daniel Perez-Salinas, Elías Martínez, Michael Schneider, Erik Gührs, Pierluigi Gargiani, Manuel Valvidares, Robert E. Marvel, Kent A. Hallman, Richard F. Haglund, Jr., Stefan Eisebitt, and Simon Wall
Imaging Nanometer Phase Coexistence at Defects During the Insulator-Metal Phase Transformation in VO2 Thin Films by Resonant Soft X-ray Holography

Nano Letters, Article ASAP, DOI: 10.1021/acs.nanolett.8b00458.

Further information:
Prof. Dr. Stefan Eisebitt, Tel.: 030 6392 1300
Dr. Bastian Pfau , Tel.: 030 6392 1343


Laser-driven electron recollision remembers molecular orbital structure

4 May 2018

Scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin combined state-of-the-art experiments and numerical simulations to test a fundamental assumption underlying strong-field physics. Their results refine our understanding of strong-field processes such as high harmonic generation (HHG) and laser-induced electron diffraction (LIED). The results have been published in "Science Advances".

Strong infrared laser pulses can extract an electron from a molecule (ionization), accelerate it away into free space, then turn it around (propagation), and finally collide it with the molecule (recollision). This is the widely used three-step model of strong-field physics. In the recollision step, the electron may, for example, recombine with the parent ion, giving rise to high harmonic generation, or scatter elastically, giving rise to laser-induced electron diffraction.

One of the commonly used assumptions underlying attosecond physics is that, in the propagation step, the initial structure of the ionized electron is "washed out", thus losing the information on the originating orbital. So far, this assumption was not experimentally verified in molecular systems.

A combined experimental and theoretical study at the Max Born Institute Berlin investigated the strong-field driven electron recollision dynamics in the 1,3-trans-butadiene molecule. In this molecule, the interaction with the strong laser field leads mainly to the ionization of two outermost electrons exhibiting quite different densities, see Figure 1. The state-of-the-art experiments and simulations then allowed the scientists to measure and calculate the high-angle rescattering probability for each electron separately. These probabilities turned out to be quite different both in the measurements and in the simulations. These observations clearly demonstrate that the returning electrons do retain structural information on their initial molecular orbital.

Schell Electron
Fig. 1 (click to enlarge)

Fig. 1: Continuum electronic wavepackets for strong-field ionization channel 1 and 2 in 1,3-trans-butadiene shortly after ionization. | Fig. MBI | Abb. MBI

Original publication:
"Molecular orbital imprint in laser-driven electron recollision"
Felix Schell, Timm Bredtmann, Claus Peter Schulz, Serguei Patchkovskii, Marc J. J. Vrakking and Jochen Mikosch
Science Advances, Vol. 4, no. 5, eaap8148 (2018), DOI: 10.1126/sciadv.aap8148

Dr. Jochen Mikosch, Tel. 030 / 6392 1295
Dr. Timm Bredtmann, Tel. 030 / 6392 1251


Freeing electrons to better trap them

16 April 2018

For the first time, researchers from UNIGE and MBI in Berlin have placed an electron in a dual state - neither freed nor bound - thus confirming a hypothesis from the 1970s.
Atoms are composed of electrons moving around a central nucleus they are bound to. The electrons can also be torn away, overcoming the confining force of their nucleus, using the powerful electric field of a laser. Half a century ago, the theorist Walter Henneberger wondered if it was possible to free an electron from its atom with the laser field, but still make it stay around the nucleus. Many scientists considered this hypothesis to be impossible. However, it was recently successfully confirmed by physicists from the University of Geneva (UNIGE), Switzerland, and the Max Born Institute (MBI) in Berlin, Germany. For the first time, they managed to control the shape of the laser pulse to keep an electron both free and bound to its nucleus, and were at the same time able to regulate the electronic structure of this atom dressed by the laser. What's more, they also made these unusual states amplify laser light. They also identified a no-go area. In this area nicknamed "Death Valley", physicists lose all their power over the electron. These results shatter the usual concepts related to the ionisation of matter. The results have been published in the journal Nature Physics.

Since the 1980s, many experiments have tried to confirm the hypothesis advanced by the theorist Walter Henneberger: an electron can be placed in a dual state that is neither free nor bound. Trapped in the laser, the electron would be forced to pass back and forth in front of its nucleus, and would thus be exposed to the electric field of both the laser and the nucleus. This dual state would make it possible to control the motion of the electrons exposed to the electric field of both the nucleus and the laser, and would let the physicists to create atoms with "new", tunable by light, electronic structure. But is this really possible?

Leveraging the natural oscillations of the electron
The more intense a laser is, the easier should it be to ionise the atom - in other words, to tear the electrons away from the attracting electric field of their nucleus and free them into space. "But once the atom is ionised, the electrons don't just leave their atom like a train leaves a station, they still feel the electric field of the laser", explains Jean-Pierre Wolf, a professor at the applied physics department of the UNIGE Faculty of Sciences. "We thus wanted to know if, after the electrons are freed from their atoms, it is still possible to trap them in the laser and force them to stay near the nucleus, as the hypothesis of Walter Henneberger suggests", he adds.

The only way to do this is to find the right shape for the laser pulse to be applied, to impose oscillations on the electron that are exactly identical, so that its energy and state remain stable. "The electron does naturally oscillate in the field of the laser, but if the laser intensity changes these oscillations also change, and this forces the electron to constantly change its energy level and thus its state, even leaving the atom. This is what makes seeing such unusual states so difficult", adds Misha Ivanov, a professor at the theoretical department of MBI in Berlin.

Modulating laser intensity to avoid Death Valley
The physicists tested different laser intensities so that the electron freed from the atom would have steady oscillations. They made a surprising discovery. "Contrary to natural expectations that suggest that the more intense a laser is, the easier it frees the electron, we discovered that there is a limit to the intensity, at which we can no longer ionise the atom", observes Misha Ivanov. "Beyond this threshold, we can control the electron again". The researchers dubbed this limit "Death Valley", following the suggestion of Professor Joe Eberly from the University of Rochester.

Confirming an old hypothesis to revolutionise physics theory
By placing the electron in a dual state which is neither free nor bound, the researchers found a way to manipulate these oscillations as they like. This enables them to directly work on the electronic structure of the atom. After several adjustments, for the first time, physicists from UNIGE and MBI were able to free the electron from its nucleus, and then trap it in the electric field of the laser, as Walter Henneberger suggested. "By applying an intensity of 100 trillion watts per cm2, we were able to go beyond the Death Valley threshold and trap the electron near its parent atom in a cycle of regular oscillations within the electric field of the laser", Jean-Pierre Wolf says enthusiastically. As a comparison, the intensity of the sun on the earth is approximately 100 watts per m2.

"This gives us the option of creating new atoms dressed by the field of the laser, with new electron energy levels", explains Jean-Pierre Wolf. "We previously thought that this dual state was impossible to create, and we've just proved the contrary. Moreover, we discovered that electrons placed in such states can amplify light. This will play a fundamental role in the theories and predictions on the propagation of intense lasers in gases, such as air", he concludes.

Abb. (click to enlarge)

Schematic illustration of the Kramers Henneberger potential formed by a mixture of the atomic potential and a strong laser field. Source: UNIGE - Xavier Ravinet

Original publication:
"Amplification of intense light fields by nearly free electrons"
Mary Matthews, Felipe Morales, Alexander Patas, Albrecht Lindinger, Julien Gateau, Nicolas Berti, Sylvain Hermelin, Jérôme Kasparian, Maria Richter, Timm Bredtmann, Olga Smirnova, Jean-Pierre Wolf and Misha Ivanov
Nature Physics (2018), DOI:10.1038/s41567-018-0105-0

Prof. Dr. Misha Ivanov, Tel. 030 / 6392 1210


From insulator to conductor in a flash

16 April 2018

A clever combination of novel technologies enables us to study promising materials for the electronics of tomorrow. Over the past decades, computers have become faster and faster and hard disks and storage chips have reached enormous capacities. But this trend cannot continue forever: we are already running up against physical limits that will prevent silicon-based computer technology from attaining any impressive speed gains from this point on. Researchers are particularly optimistic that the next era of technological advancements will start with the development of novel information-processing materials and technologies that combine electrical circuits with optical ones. Using short laser pulses, a research team led by Misha Ivanov of the Max Born Institute in Berlin together with scientists from the Russian Quantum Center in Moscow have now shed light on the extremely rapid processes taking place within these novel materials. Their results have appeared in the prestigious journal "Nature Photonics".

Of particular interest for modern material research in solid state physics are "strongly correlated systems", so called for the strong interactions between the electrons in these materials. Magnets are a good example of this: the electrons in magnets align themselves in a preferred direction of spin inside the material, and it is this that produces the magnetic field. But there are other, entirely different structural orders that deserve attention. In so-called Mott insulators for example, a class of materials now being intensively researched, the electrons ought to flow freely and the materials should therefore be able to conduct electricity as well as metals. But the mutual interaction between electrons in these strongly correlated materials impedes their flow and so the materials behave as insulators instead.

By disrupting this order with a strong laser pulse, the physical properties can be made to change dramatically. This can be likened to a phase transition from solid to liquid: as ice melts, for example, rigid ice crystals transform into free-flowing water molecules. Very similarly, the electrons in a strongly correlated material become free to flow when an external laser pulse forces a phase transition in their structural order. Such phase transitions should allow us to develop entirely new switching elements for next-generation electronics that are faster and potentially more energy efficient than present-day transistors. In theory, computers could be made around a thousand times faster by "turbo-charging" their electrical components with light pulses.

The problem with studying these phase transitions is that they are extremely fast and it is therefore very difficult to "catch them in the act". So far, scientists have had to content themselves with characterising the state of a material before and after a phase transition of this kind. Researchers Rui E. F. Silva, Olga Smirnova, and Misha Ivanov of the Berlin Max Born Institute, however, have now devised a method that will, in the truest sense, shed light on the process. Their theory involves firing extremely short, tailored laser pulses at a material - pulses that can only recently be produced in the appropriate quality given the latest developments in lasers. One then observes the material's reaction to these pulses to see how the electrons in the material are excited into motion and, like a bell, emit resonant vibrations at specific frequencies, as harmonics of the incident light.

"By analysing this high harmonic spectrum, we can observe the change in the structural order in these strongly correlated materials 'live' for the first time," says first author of the paper Rui Silva of the Max Born Institute. Laser sources capable of targetedly triggering these transitions have only been available since very recently. The laser pulses namely have to be amply strong and extremely short - on the order of femtoseconds in duration (millionths of a billionth of a second).

In some cases, it takes only a single oscillation of light to disrupt the electronic order of a material and turn an insulator into a metal-like conductor. The scientists at the Berlin Max Born Institute are among the world's leading experts in the field of ultrashort laser pulses.

"If we want to use light to control the properties of electrons in a material, then we need to know exactly how the electrons will react to light pulses," Ivanov explains. With the latest-generation laser sources, which allow full control over the electromagnetic field even down to a single oscillation, the newly published method will allow deep insights into the materials of the future.

Fig. (click to enlarge)

High harmonic spectroscopy of light-induced phase transition. The vertical red line shows when the laser electric field (yellow oscillating curve) crosses the threshold field, destroying the insulating phase of the material. The top panel shows the average number of doublon-hole pairs per site (blue) and the decay of the insulating field-free ground state (red). (Source: MBI)

Original publication:
"High harmonic spectroscopy of ultrafast many-body dynamics in strongly correlated systems"

R. E. F. Silva, Igor V. Blinov, Alexey N. Rubtsov, O. Smirnova & M. Ivanov
Nature Photonics, (2018) (online), DOI: 10.1038/s41566-018-0129-0


Prof. Dr. Misha Ivanov, Tel. 030 / 6392 1210
Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340
Dr. R.E.F. Silva, Tel.: 030 6392 1239


Wiggling atoms switch the electric polarization of crystals

12 April 2018

Ferroelectric crystals display a macroscopic electric polarization, a superposition of many dipoles at the atomic scale which originate from spatially separated electrons and atomic nuclei. The macroscopic polarization is expected to change when the atoms are set in motion but the connection between polarization and atomic motions has remained unknown. A time-resolved x-ray experiment now elucidates that tiny atomic vibrations shift negative charges over a 1000 times larger distance between atoms and switch the macroscopic polarization on a time scale of a millionth of a millionth of a second.

Ferroelectric materials have received strong interest for applications in electronic sensors, memories, and switching devices. In this context, fast and controlled changes of their electric properties are essential for implementing specific functions efficiently. This calls for understanding the connection between atomic structure and macroscopic electric properties, including the physical mechanisms governing the fastest possible dynamics of macrosopic electric polarizations.

Researchers from the Max-Born-Institute in Berlin have now demonstrated how atomic vibrations modulate the macroscopic electric polarization of the prototype ferroelectric ammonium sulphate [Fig. 1] on a time scale of a few picoseconds (1 picosecond (ps) = 1 millionth of a millionth of a second). In the current issue of the journal Structural Dynamics [5, 024501 (2018)], they report an ultrafast x-ray experiment which allows for mapping the motion of charges over distances on the order of the diameter of an atom (10-10m = 100 picometers) in a quantitative way. In the measurements, an ultrashort excitation pulse sets the atoms of the material, a powder of small crystallites, into vibration. A time-delayed hard x-ray pulse is diffracted from the excited sample and measures the momentary atomic arrangement in form of an x-ray powder diffraction pattern. The sequence of such snapshots represents a movie of the so-called electron-density map from which the spatial distribution of electrons and atomic vibrations are derived for each instant in time ([Fig. 2], [Movie]).

The electron density maps show that electrons move over distances of 10-10m between atoms which are more than a thousand times larger than their displacements during the vibrations [Fig. 3]. This behavior is due to the complex interplay of local electric fields with the polarizable electron clouds around the atoms and determines the momentary electric dipole at the atomic scale. Applying a novel theoretical concept, the time-dependent charge distribution in the atomic world is linked to the macroscopic electric polarization [Fig. 3]. The latter is strongly modulated by the tiny atomic vibrations and fully reverses its sign in time with the atomic motions. The modulation frequency of 300 GHz is set by the frequency of the atomic vibrations and corresponds to a full reversal of the microscopic polarization within 1.5 ps, much faster than any existing ferroelectric switching device. At the surface of a crystallite, the maximum electric polarization generates an electric field of approximately 700 million volts per meter.

The results establish time-resolved ultrafast x-ray diffraction as a method for linking atomic-scale charge dynamics to macroscopic electric properties. This novel strategy allows for testing quantum-mechanical calculations of electric properties and for characterizing a large class of polar and/or ionic materials in view of their potential for high-speed electronics.

Fig. 1 (click to enlarge)

Fig. 1: Crystal lattice of ferroelectric ammonium sulfate [(NH4)2SO4] with tilted ammonium (NH4+) tetrahedra (nitrogen: blue, hydrogen: white) and sulfate (SO42-) tetrahedra (sulfur: yellow, oxygen: red). The green arrow shows the direction of macroscopic polarization P. Blue arrows: local dipoles between sulphur and oxygen atoms. The electron density maps shown in the bottom left panel, in Fig. 2, and the movie are taken in the plane shown in grey. Bottom left: Stationary electron density of sulfur and oxygen atoms, displaying high values on the sulfur (red) and smaller values on the oxygens (yellow). Bottom right: Change of local dipoles at a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. An anisotropic shift of charge reduces the dipole pointing to the right and increases the other 3 dipoles.

Fig. 2 (click to enlarge)
Fig 2: (a) Stationary electron density in the grey plane shown in Fig. 1. (b) Change of electron density at a delay time of 2.8 picoseconds (ps) after excitation of the ammonium sulfate crystallites. The circles mark the atomic positions, the black arrows indicate the transfer of electronic charge between one of the oxygen atom and the SO3 group of a single sulfate ion. The vibrational displacements of the atoms are smaller than the line thickness of the circles and, thus, invisible on this length scale. (c) The reverse charge transfer occurs at a delay time of 3.9 ps.
Movie (click to enlarge)
Movie: The movie shows the entire temporal evolution of the electron density map.
Fig. 3 (click to enlarge)
Fig 3: Upper panel: Change of the S-O bond length as a function of the delay time. The maximum change of 0.1 pm is 1000 times smaller than the bond length itself, i.e., the atomic motions cannot be observed in Fig. 2. Middle panel: Charge transfer from one oxygen atom to the SO3 group of the sulfate ion (left black arrows in Fig. 2) as a function of delay time. Lower panel: Change of the macroscopic polarization P along the c axis which is the sum of all microscopic dipole changes of the local S-O dipoles within the sulfhate ions (red and blue arrows in Fig. 1 bottom right).

Original article:
Christoph Hauf, Antonio-Andres Hernandez Salvador, Marcel Holtz, Michael Woerner, and Thomas Elsaesser, Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction, Struct. Dyn. 5, 024501 (2018).

Further information:
Dr. Michael Wörner, Tel.: 030 6392 1470
Dr. Christoph Hauf, Tel.: 030 6392 1473
Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400




X-ray snapshots of reacting acids and bases - Erik T. J. Nibbering receives an ERC Advanced Grant for groundbreaking basic research

9 April 2018

Dr. Erik T. J. Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin receives an Advanced Grant from the European Research Council (ERC). Goal with this prestigious award is to investigate and elucidate the elementary steps of aqueous proton transfer dynamics between acids and bases. The ERC Advanced Grant is endowed with 2.5 million euro and awarded to well-established top researchers in Europe pursuing scientifically excelling projects.

Dr. Erik T. J. Nibbering, head of the department "Femtosecond Spectroscopy of Molecular Systems" at MBI, has a major track record in time-resolved spectroscopy of ultrafast chemical reactions, in particular proton transfer between acids and bases, electron transfer in donor-acceptor complexes, and trans/cis isomerization. In recent years his activities have focused on the dynamics of the hydrogen bond structure of photoacid-base complexes and of hydrated protons.

How acids and bases react in water is a question raised since the pioneering days of modern chemistry. Recent decades have witnessed an increased effort in elucidating the microscopic mechanisms of proton exchange between acids and bases and the important mediating role of water in this. With ultrafast spectroscopy it has been shown that the elementary steps in aqueous proton transfer occur on femtosecond to picosecond time scales (1 femtosecond = 10-15 s = 1 millionth of a billionth of a second). Aqueous acid-base neutralization predominantly proceeds in a sequential way via water bridging acid and base molecules. These ultrafast experiments probing molecular transitions in the ultraviolet, visible and mid-infrared spectral ranges, though, only provide limited insight into the electronic structure of acids, bases and the water molecules accommodating the transfer of protons in the condensed phase. Soft-x-ray absorption spectroscopy (XAS), probing transitions from inner-shell levels to unoccupied molecular orbitals, is a tool to monitor electronic structure with chemical element specificity.

The aim is now to develop steady-state and time-resolved soft-x-ray spectroscopy of acids and bases. Here novel liquid flatjet technology is utilized with soft-x-ray sources at synchrotrons as well as table-top laser-based high-order harmonic systems. Resolving the electronic structural dynamics of elementary steps of aqueous proton transport will furthermore elucidate the role of mediating water in bulk solution, and in specific conditions such as hydrogen fuel cells or trans-membrane proteins.

Further information on Dr. Erik T. J. Nibbering can be found at http://staff.mbi-berlin.de/nibberin/ and on ERC Grants at http://erc.europa.eu/.

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)
Dr. Erik T. J. Nibbering
Phone +49 / 30 / 6392-1477




A spinning top of light

27 February 2018

Short, rotating pulses of light reveal a great deal about the inner structure of materials. An international team of physicists led by Prof. Misha Ivanov of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now developed a new method for precisely characterising such extremely short light pulses. The research results have been published in Nature Communications.

Not all light is equal: depending on how it is prepared, it can exist in very different forms. Not only can we choose different wavelengths or colours but, as an electromagnetic wave, light can also exhibit different forms of oscillation. It can occur in different polarisations, for example - either linearly polarised or circularly polarised, where the oscillations of the electromagnetic fields follow a line or go round in circles, respectively. Above all, extremely short pulses of polarised light waves are excellent for studying many different types of materials. We have methods for producing such pulses, but these methods are already pushing the limits of technical feasibility and the light pulses are not always produced with the desired properties.

A new method now allows us to characterise these short light pulses with unprecedented precision. The trouble starts with the fact that the processes of interest taking place inside matter, which we would like to study with our light pulses, are extremely short-lived. Accordingly, the light pulses have to be similarly short, in the range of around 100 attoseconds (billionths of a billionth of a second). In this unimaginably short timespan, a light wave can only undergo a few rotations. Even using the latest laser methods to produce such ultrashort pulses, it can easily happen that the light wave will not come out rotating the right way.

The concept for the new method can be described as follows: one fires an extremely short, high-energy and circularly polarised light pulse at an atom or a solid body where, upon being absorbed, the light pulse knocks an electron out of the body. This electron then carries information about the light wave itself and can furthermore reveal clues as to the properties of the sample being examined. Because the light pulses are circularly polarised, the ejected electrons also fly off with a rotating motion.

"You can compare the electrons being ejected with a one-armed sprinkler, which either continues turning in the direction you want it to, or which keeps stuttering and even changing its direction," says Misha Ivanov, Head of the Theory Department of the Max Born Institute. If the sprinkler is allowed to run for a while, then it will wet the grass in a full circle - irrespective of whether it rotates consistently or not. So, merely looking at the grass will not reveal whether the sprinkler has been turning exactly the way it was desired or not. "But if a gusty wind comes along, then we can distinguish whether the sprinkler has been turning regularly or irregularly," Ivanov says. If the wind blows alternately from the left or right each time the arm of the sprinkler faces left or right, then the patch of wet grass will not be circular, but rather elliptical in shape. A sprinkler rotating completely irregularly would magically conjure up an ellipse on the grass stretched in the wind direction, while a regularly rotating sprinkler will display a tilted ellipse.

This "wind" is added into the experiment in the form of an infrared laser pulse whose oscillations are perfectly synchronised with the ultrashort pulses. The infrared radiation accelerates the electron either to the left or right - just like the wind blows the water droplets.

"By measuring the electrons, we can then determine whether the light pulse possessed the desired consistent rotation or not," says the first author of the publication in "Nature Communications", researcher Álvaro Jiménez-Galán of the Max Born Institute. "Our method allows one to characterise the properties of the ultrashort light pulses with unprecedented precision," Jiménez-Galán adds. And the more precisely these light pulses are characterised, the more detailed information can be derived about the electron's place of origin within an exotic material.

This is of special significance when it comes to studying a whole series of novel materials. These could include superconductors, which can conduct electricity without electrical resistance, or topological materials that exhibit exotic behaviour, the research of which earned a Nobel Prize in Physics in 2016. Materials like these could be used to make a quantum computer, for example, or could allow superfast, energy-efficient processors and memory chips to be built into normal computers and smartphones.

The new sprinkler method still only exists in theory for the moment, but ought to be implementable in the near future. "Our requirements are fully within the latest state of the art, so there is nothing to preclude this from being realised soon," Ivanov asserts.


Fig. 1 (click to enlarge)


Left alone, the circular sprinkler distributes the water evenly, and the grass grows in a circular pattern regardless of whether the sprinkler rotates clock-wise, counter-clockwise, or randomly. If wind blows, the grass is watered unevenly, as seen in its growth. If the wind blows in a regular pattern, changing its strength with clock-work precision, the asymmetry in the grass growth allows us to reconstruct the properties of the sprinkler, distinguishing the precision-made, regularly rotating sprinkler from a randomly oscillating cheap version.

In our micro-world setup, the sprinkler is the short pulse (in blue), lasting only about 10-16 sec, with its electric field rotating even faster in an unknown pattern. The wind is a linearly polarized and precisely controlled infrared laser field (in red). The grass is the measured photoelectron angular distribution (in green). The asymmetry in the latter allows us to reconstruct, for the first time, the polarization properties of the ultra-short pulse that lasts about 10-16 sec. (Figure credit: Felipe Morales und Álvaro Jiménez-Galán)

Original Publication: Nature Communications
Álvaro Jiménez-Galán, Gopal Dixit, Serguei Patchkovskii, Olga Smirnova, Felipe Morales & Misha Ivanov: Attosecond recorder of the polarization state of light
Nature Communications, volume 9, Article number: 850 (2018), doi:10.1038/s41467-018-03167-2


Prof. Dr. Misha Ivanov, Tel.: 030 6392 1340

Prof. Dr. Álvaro Jiménez-Galán, Tel.: 030 6392 1340

Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340




C'mon electrons, let's do the twist! Twisting electrons can tell right-handed and left-handed molecules apart.

19 February 2018

Identifying right-handed and left-handed molecules is a crucial step for many applications in chemistry and pharmaceutics. An international research team (CELIA-CNRS/INRS/Berlin Max Born Institute/SOLEIL) has now presented a new original and very sensitive method. The researchers use laser pulses of extremely short duration to excite electrons in molecules into twisting motion, the direction of which reveals the molecules' handedness. The research results appear in Nature Physics.

Are you right handed or left handed? No, we aren't asking you, dear reader; we are asking your molecules. It goes without saying that, depending on which hand you use, your fingers will wrap either one way or the other around an object when you grip it. It so happens that this handedness, or "chirality", is also very important in the world of molecules. In fact, we can argue that a molecule's handedness is far more important than yours: some substances will be either toxic or beneficial depending on which "mirror-twin" is present. Certain medicines must therefore contain exclusively the right-handed or the left-handed twin.

The problem lies in identifying and separating right-handed from left-handed molecules, which behave exactly the same unless they interact with another chiral object. An international research team has now presented a new method that is extremely sensitive at determining the chirality of molecules.

We have known that molecules can be chiral since the 19th century. Perhaps the most famous example is DNA, whose structure resembles a right-handed corkscrew. Conventionally, chirality is determined using so-called circularly polarised light, whose electromagnetic fields rotate either clockwise or anticlockwise, forming a right or left "corkscrew", with the axis along the direction of the light ray. This chiral light is absorbed differently by molecules of opposite handedness. This effect, however, is small because the wavelength of light is much longer than the size of a molecule: the light's corkscrew is too big to sense the molecule's chiral structure efficiently.

The new method, however, greatly amplifies the chiral signal. "The trick is to fire a very short, circularly polarized laser pulse at the molecules," says Olga Smirnova from the Max Born Institute. This pulse is only some tenths of a trillionth of a second long and transfers energy to the electrons in the molecule, exciting them into helical motion. The electrons' motion naturally follows a right or left helix in time depending on the handedness of the molecular structure they reside in.

Their motion can now be probed by a second laser pulse. This pulse also has to be short to catch the direction of electron motion and have enough photon energy to knock the excited electrons out of the molecule. Depending on whether they were moving clockwise or anticlockwise, the electrons will fly out of the molecule along or opposite to the direction of the laser ray.

This lets the experimentalists of CELIA to determine the chirality of the molecules very efficiently, with a signal 1000 times stronger than with the most commonly used method. What's more, it could allow one to initiate chiral chemical reactions and follow them in time. It comes down to applying very short laser pulses with just the right carrier frequency. The technology is a culmination of basic research in physics and has only been available since recently. It could prove extremely useful in other fields where chirality plays an important role, such as chemical and pharmaceutical research.

Having succeeded in identifying the chirality of molecules with their new method, the researchers are now thinking already of developing a method for laser separation of right- and left-handed molecules. Text: Dirk Eidemueller/Forschungsverbund Berlin e.V. - Translation: Peter Gregg


Fig. (click to enlarge)

Fig: Following excitation by an ultra-short circularly polarised laser pulse, electrons follow a right or left helix depending on the handedness of the molecular structure they reside in. Source: Samuel Beaulieu

Original Publication: Nature Physics
S. Beaulieu, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, A. G. Harvey, F. Légaré, Z. Mašin, L. Nahon, A. F. Ordonez, S. Petit, B. Pons, Y. Mairesse, O. Smirnova and V. Blanchet: Photoexcitation Circular Dichroism in Chiral Molecules
Nature Physics (2018) online, doi:10.1038/s41567-017-0038-z.


Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340




Flexibility and arrangement - the interaction of ribonucleic acid and water

16 January 2018

Ribonucleic acid (RNA) plays a key role in biochemical processes which occur at the cellular level in a water environment. Mechanisms and dynamics of the interaction between RNA and water were now revealed by vibrational spectroscopy on ultrashort time scales and analyzed by in-depth theory

Ribonucleic acid (RNA) represents an elementary constituent of biological cells. While deoxyribonucleic acid (DNA) serves as the carrier of genetic information, RNA displays a much more complex biochemical functionality. This includes the transmission of information in the form of mRNA, RNA-mediated catalytic function in ribosomes, and the encoding of genetic information in viruses. RNA consists of a sequence of organic nucleobase molecules which are held together by a so-called backbone consisting of phosphate and sugar groups. Such a sequence can exist as a single strand or in a paired double-helix geometry. Both forms are embedded in a water shell and their phosphate and sugar groups are distinct docking points for water molecules. The structure of the water shell fluctuates on a time scale of a few tenth of a picosecond (1 ps = 10-12 s = 1 millionth of a millionth of a second). The interactions of RNA and water and their role for the formation of three-dimensional RNA structures are only understood insufficiently and difficult to access by experiment.

Scientists from the Max Born Institute have now observed the interaction of RNA with its water shell in real time. In their new experimental method, vibrations of the RNA backbone serve as sensitive noninvasive probes of the influence of neighboring water molecules on the structure and dynamics of RNA. The so-called two-dimensional infrared spectroscopy allows for mapping the time evolution of vibrational excitations and for determining molecular interactions within RNA and between RNA and water. The results show that water molecules at the RNA surface perform tipping motions, so-called librations, within a fraction of a picosecond whereas their local spatial arrangement is preserved for a time range longer than 10 ps. This behavior deviates strongly from that of neat water and is governed by the steric boundary conditions set by the RNA surface. Individual water molecules connect neighboring phosphate groups and form a partly ordered structure which is mediated by their coupling to the sugar units.

The librating water molecules generate an electrical force by which the water fluctuations are transferred to the vibrations of RNA. The different backbone vibrations display a diverse dynamical behavior which is determined by their local water environment and reflects its heterogeneity. RNA vibrations also couple mutually and exchange energy among themselves and with the water shell. The resulting ultrafast redistribution of excess energy is essential for avoiding a local overheating of the sensitive macromolecular structure. This complex scenario was analyzed by detailed theoretical calculations and simulations which, among other results, allowed for the first complete and quantitative identification of the different vibrations of the RNA backbone. Comparative experiments with DNA reveal similarities and characteristic differences between these two elementary biomolecules, showing a more structured water arrangement around RNA. The study highlights the strong potential of non-invasive time-resolved vibrational spectroscopy for unraveling the interplay of structure and dynamics in complex biomolecular systems on molecular length and time scales.


Fig. 1 (click to enlarge)

Fig. 1: Left: Structure of a RNA double helix. The blue spheres represent sodium counterions. Right: Enlarged segment of the sugar-phosphate backbone of RNA, including bridging water molecules. Vibrations of the RNA backbone serve as sensitive real time probes for mapping the influence of the neighboring water molecules on RNA's structure and dynamics.

Fig. 2 (click to enlarge)
Fig. 2: Two-dimensional vibrational spectra of RNA (upper panel) and DNA (lower panel) in the frequency range of the sugar-phosphate vibrations of the backbone. The RNA spectrum displays additional bands (contours) along the frequency diagonal ν13 and a more complex distribution of off-diagonal peaks. In addition to the frequency positions the line shapes of the individual bands (contours) give insight in details of the interactions with neighboring water molecules.

Original publication:
E. M. Bruening, J. Schauss, T. Siebert, B. P. Fingerhut, T. Elsaesser: Vibrational Dynamics and Couplings of the Hydrated RNA Backbone: A Two-Dimensional Infrared Study.
J. Phys. Chem. Lett. 9, 583-587 (2018). DOI: 10.1021/acs.jpclett.7b03314.


Dr. Benjamin Fingerhut, Tel.: 030 6392 1404

Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400




Instant x-ray footprints

15 January 2018

MBI scientists together with colleagues from Italy have established a way to detect the exact x-ray fluence footprint generated on a sample by a free electron laser pulse.

Free electron lasers (FELs) deliver intense and coherent x-ray pulses - a prerequisite to investigate and exploit non-linear processes in the interaction of x-rays with matter. Analogous to the development of nonlinear optics after the invention of lasers in the optical regime, the application of these processes is expected to have a widespread impact on numerous research fields. A pivotal parameter in this context is the fluence of the radiation deposited on the sample during a single ultrashort pulse - in essence, the number of photons hitting the sample per unit area for a given photon energy. Unfortunately, this fluence can vary at FELs from shot to shot both in its total amount as well as in the way it is distributed in a focal spot. Simply put, there is a footprint of x-rays on the sample which can vary in both shape and intensity. This makes quantitative experiments on nonlinear processes at x-ray wavelengths very challenging, as they are inherently sensitive to the precise fluence distribution.

Scientists from MBI and the Italian research institutes ELETTRA and IOM have now demonstrated a method, which allows to take a snapshot picture of the fluence distribution impinging on the sample while at the same time recording the scattering signal of interest generated by that very same FEL shot. The approach relies upon the fabrication of very shallow grooves of only a few nanometer depth into the membrane holding the sample. Via a tailored two-dimensional distortion, this groove pattern forms a diffractive optical element that is designed to image the footprint of the incident x-ray beam on a two-dimensional detector. In the figure below, the beam footprint on the sample is visible (in two conjugate copies) as a spot with a checkerboard of many side maxima and minima, while the magnetic scattering from this sample with ferromagnetic domains is visible as a ring on the very same detector. Using this approach, scientists can now relate a scattering signal from a specimen to the exact incident fluence footprint on this sample, as both originate from the identical x-ray pulse.

Furthermore, the use of the grating structure alone - without a sample - turned out to be extremely helpful when aligning the x-ray optics of the FEL or a sample relative to the focal position. Together with the detector, the distorted grating provides instant feedback on the beam shape when placed into the x-ray beam. The new method is already now routinely used at the FERMI free electron laser for alignment purposes.


Fig. 1 (click to enlarge)

Fig. 1: 2D Detector image downstream of the sample showing both the ring-shaped magnetic scattering as well as the fluence map ("footprint") of x-rays on the sample generating this magnetic scattering.

Original publikation:
M. Schneider, C. M. Günther, B. Pfau, F. Capotondi, M. Manfredda, M. Zangrando, N. Mahne, L. Raimondi, E. Pedersoli, D. Naumenko, and S. Eisebitt, In situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses, Nature Communications 9, 214 (2018).


Prof. Dr. Stefan Eisebitt, Tel.: 030 6392 1300



Marc Vrakking named Editor-in-Chief of the Journal of Physics B

1 January 2018

Prof. Marc Vrakking, director of Division A of the Max Born Institute, has been named Editor-in-Chief of the Journal of Physics B from January 1st 2018. In this capacity he succeeds Prof. Paul Corkum of the University of Ottawa, who served as Editor-in-Chief since 2011.

The Journal of Physics B is one of the publications of the Institute of Physics, and is a prominent journal serving the Atomic, Molecular and Optical (AMO) Physics community. In particular in the emerging fields of strong field physics, attosecond science and free electron laser science the journal has a prominent role, as is manifest by numerous important papers (including Topical Reviews, Tutorials, Special Issues and Roadmaps) that the journal continuously publishes. As such, the journal is strongly aligned with the research program in Division A of the Max Born Institute.


Prof. Marc Vrakking Tel. (030) 6392 1200


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