Archive: Highlights at MBI
Highlights 2017

Physicist Alexandre Mermillod-Blondin receives the Thomas Alva Edison Patent Award

2 November 2017

The Research & Development Council of New Jersey has granted the 2017 Thomas Alva Edison Patent Award in the category of Technology Transfer to Craig B. Arnold, Euan McLeod, Alexandre Mermillod-Blondin, and Chris Theriault in recognition of their contributions to Tunable Acoustic Gradient Index of Refraction Lens and System.

Quelle MBI
Quelle: MBI (click to enlarge)

Tunable acoustic gradient index of refraction lenses (aka TAG lenses) are high-speed fluid lenses with the ability to scan an extended range of focii on the sub-microsecond timescale for various applications like direct imaging or material processing. Their principle relies on the generation of soundwaves in a liquid with the help of a piezoelectric element to create a programmable index of refraction profile. This refractive index profile behaves like a simple aspheric lens element with reduced wavefront aberrations.


Additional Informationen:

Dr. Alexandre Mermillod-Blondin Tel. (030) 6392 1214


Lightwave controlled nanoscale electron acceleration sets the pace

30 October 2017

Extremely short electron bunches are key to many new applications including ultrafast electron microscopy and table-top free-electron lasers. A German team of physicists from Rostock University, the Max Born Institute in Berlin, the Ludwig-Maxmilians-Universität Munich, and the Max Planck Institute of Quantum Optics in Garching has now shown how electrons can be accelerated in an extreme and well-controlled way with laser light, while crossing a silver particle of just a few nanometers. Of particular importance for potential applications is the ability to manipulate the acceleration process, known as a swing-by maneuver from space travel, with the light waveform. This could facilitate an all-optical generation of attosecond electron pulses.

Quelle MBI
Abb. 1 (click to enlarge)

Fig. 1: The waveform-controlled laser pulse creates a plasmon-enhanced near-field that drives the forward acceleration of an electron during its passage through the nanometer-sized metal cluster.


When metal clusters, small nanoparticles consisting of just a few thousand atoms, are exposed to intense laser light, electrons inside the particle are excited to a swinging collective motion. The electron cloud's motion, a plasmon, can be excited resonantly with light of a suitable color leading to very high amplitudes and an enhanced electric field inside the cluster. In the experiment, which was conducted at the Institute of Physics in Rostock, a team of researches around Prof. Thomas Fennel has now deliberately exploited this enhanced near-field. With so-called two-color laser pulses the scientists tailored the plasmonic field via the waveform of the light field. This led to a controlled slingshot-type acceleration of electrons traversing the nanoparticle within only one optical cycle. These experimental results, together with their interpretation by a theoretical model, were now published in the journal Nature Communications.

In their study, the researchers demonstrated that electronic processes in clusters can be controlled with the waveform of laser light. The few nanometer-sized clusters serve as ideal experimental and theoretical model systems for investigating new physical effects in the light matter interaction of nanostructures. "In our experiment we could show that electrons can gain energies of up to one kiloelectron volt within just one optical cycle in the nanoaccelerator. This corresponds to an enhancement of more than one order of magnitude with respect to the strong-field ionization of atoms", describes Dr. Josef Tiggesbäumker from the Institute of Physics in Rostock, who has developed the setup for the experiments together with first author Dr. Johannes Passig from the team around cluster physicist Prof. Karl-Heinz Meiwes-Broer. "The acceleration of electrons via near-field-assisted forward scattering can be switched with attosecond precision (1 attosecond = 1 billionth of a billionth of a second) by tailoring the light waveform.", adds Prof. Matthias Kling from the Ludwig-Maximilians-Universität Munich and the Max Planck Institute of Quantum Optics in Garching, who provided the technology for the generation of the phase-controlled laser pulses. "The control with just and only the laser light paves new ways for the intensely researched area of light-based particle acceleration", sums up Fennel from the University Rostock and the Max Born Institute in Berlin, who developed the concept for the study. The researchers plan to realize the acceleration principle in multiple stages in the future to investigate its potential applications in laser-driven grating accelerators.

Furch NOPA Fig. 2: Prof. Thomas Fennel from the University of Rostock and the Max Born Institute in Berlin.

Original puclikation: Nature Communication 8, 1181 (2017) doi:10.1038/s41467-017-01286-w
"Nanoplasmonic electron acceleration by attosecond-controlled forward rescattering in silver clusters"
Johannes Passig, Sergey Zherebtsov, Robert Irsig, Mathias Arbeiter, Christian Peltz, Sebastian Göde,Slawomir Skruszewicz, Karl-Heinz Meiwes-Broer, Josef Tiggesbäumker, Matthias F. Kling, Thomas Fennel


Prof. Dr. Thomas Fennel Tel. (030) 6392 1245


A new type of third-order nonlinearity in magneto-plasmonic structures

27 October 2017

Studies of nonlinear phenomena in magneto-plasmonic waveguides is of great interest, not only due to their fundamental importance but also because of potential applications in integrated nanoplasmonic devices for diverse functionality in chip-scale plasmonic communication systems. Recently, Dr. Joachim Herrmann (MBI) and his external collaborators predicted a new type of ultrafast nonlinearity of surface-plasmon-polaritons (SPP) in planar magneto-plasmonic structures, based on the inverse Faraday effect. Specifically they show that SPPs with a significant longitudinal component of the electric field can create, via the inverse Faraday effect (IFE), an effective transverse magnetic field in magnetic thin layers. Its response to the plasmon propagation leads to a strong ultrafast third-order nonlinearity. They estimate that the new nonlinearity exceeds the optical Kerr effect of typical dielectric materials by five orders of magnitude and that of gold by two orders of magnitude.

A magnetic material irradiated by circularly polarized light induces a magnetization along the wave vector. This effect is called the inverse Faraday effect (IFE). The sign of the light-induced magnetization is determined by the helicity of the incident light wave, and the magnetization vanishes for linearly polarized waves. Left- and right-handed circular polarization induces magnetization of opposite signs.

While light cannot penetrate into a thin metallic layer, under appropriate conditions surface plasmon-polaritons (SPP) can be excited. These hybrid electron-photon excitations move along the metal surface. Since SPPs exhibit a longitudinal component of the electric field, even for a linearly polarized input beam a magnetization can be induced by the plasmons generated by the incident light, regardless that the plasmons are not circularly polarized in the conventional sense.

To quantify this new type of IFE-based nonlinear susceptibility in a planar plasmonic structure including a ferromagnetic layer, the group used the Lorentz reciprocity theorem, deriving analytical expressions. The new nonlinearity plays a similar role in the plasmonic propagation as the optical Kerr effect, but it originates from a different physical mechanism and differs from the traditional Kerr-related nonlinear susceptibility in magnitude, frequency dependence and the dependence on material parameters.

The scheme of a ferromagnetic dielectric/metallic interface is shown in Fig. 1. The wavelength dependence of the nonlinear propagation coefficient can be seen in Fig.2 and the power dependence of the nonlinear phase shift in Fig.3.

Magneto-plasmonic structures as building blocks open doors to a new class of plasmonic devices on the nanoscale, such as optical phase modulators, isolators and optical clocks that will satisfy key applications in nanoscale information networks. The new results are very promising for important applications in this field.

Fig. 1 (click to enlarge)

Fig. 1: Scheme of the ferromagnetic dielectric/metal interface

Fig. 2 (click to enlarge)
Fig. 2: Wavelength dependence for the IFE-related nonlinear susceptibility for the interface between gold and a ferromagnetic dielectric
Abb. 3 (click to enlarge) Abb. 3: Power dependence of the nonlinear phase shift at a wavelength of 1550 nm and a propagation distance of 1000nm.

Original publication: Phys. Rev. B96, 165437 (2017), https://doi.org/10.1103/PhysRevB.96.165437.
Song-Jin Im, Chol-Song Ri, Kum-Song Ho, Joachim Herrmann
"Third-order nonlinearity by the inverse Faraday effect in planar magnetoplasmonic structures"


Dr. Joachim Herrmann, Tel.: 030 6392 1278




Physicist Lisa Torlina receives Marthe Vogt Award

27. October 2017

The Forschungsverbund Berlin e.V. (FVB) is granting this year's Marthe Vogt Award to Dr Lisa Torlina for her doctoral dissertation in quantum mechanics. In the course of her work at the Max Born Institute, Lisa Torlina successfully addressed unanswered questions on basic research in physics. To do so, she developed a theoretical framework for interpreting interactions between electrons and light pulses. Since 2001, the Marthe Vogt Award has been granted to women junior researchers specialising in areas of natural science investigated at FVB institutes. The doctoral dissertation must have been completed at a research facility in Berlin or Brandenburg. The award is valued at Euro 3,000.

Quelle MBI
Dr. Lisa Torlina - Foto: Herbort

"I always wanted to know how the world works," says Lisa Torlina, commenting on her research. So, she started studying how atoms and molecules interact with light pulses, and investigating the electron dynamics that are generated - big questions that basic research in physics has so far failed to answer. "Her doctoral thesis produced groundbreaking insights into problems that have been under discussion for decades," says her supervisor, Professor Dr Olga Smirnova of MBI. She mentored her progress as a doctoral student at the Leibniz Graduate School, "Dynamics in New Light". Under MBI management, the school supported doctoral candidates working on ultrashort and ultraintense light pulses during the period 2011-2015.


Groundbreaking insights in basic research

When the young researcher starts talking about electrons and their path through potential barriers, it soon becomes clear that she has turned a passion into a profession. Back in high school in Australia where she grew up, she was always attracted to natural sciences. "If you follow the rules in mathematics, you always know that you will arrive at the correct result, even if that result may seem counterintuitive at first", says Lisa Torlina. In the field she is now engaged in, the objects of her investigations often behave in ways that you might not expect. If you throw a ball against a wall, for example, it shoots back at you. But when electrons hit an obstacle, there is a chance that they will tunnel through it. These movements are visualised by shining light pulses on atoms and electrons and looking at how the electrons respond, almost like in a photo. The electrons move so fast that their motion is measured in billionths of billionths of seconds, known as attoseconds. At the Theory Department of MBI the scientists modelled the process theoretically.

If you want to interpret observations of this kind, you need a very good theoretical description, and this is what Lisa Torlina's framework has delivered: her "Analytical R-Matrix Approach (ARM)" shows how to disentangle different effects coming from the interplay of electron interaction with light and with the atomic nucleus. It proves, for instance, that there is no passing of time when an electron initially bound inside an atom breaks through a potential barrier that keeps it from becoming free. In order to acquire this knowledge, Lisa Torlina spent a lot of time at her desk: only after months of calculations, with results that threw up new problems of their own, did she manage to test her theory on specific cases. "That's how it goes in research," says the scientist, remembering working on her doctorate. Today, she achieves much better results with her ARM tool than with previously established methods.

After completing her doctorate, Lisa Torlina became a postdoc at MBI - during her time at MBI she has seen eight publications in prestigious journals. Lisa Torlina speaks at international conferences and has now won the Marthe Vogt Award, a huge honour for the physicist. "Lisa Torlina is undoubtedly one of the most talented doctoral candidates I have ever worked with," says her supervisor, relishing the junior researcher's success (Text: Alessa Wendland)

Marthe Vogt Award Ceremony The award ceremony will take place at 7 pm on Wednesday, 8 November at the Leibniz Association Building, Chausseestraße 111, in Berlin as part of the Berlin Science Week 2017. It is open to anyone who is interested and is free of charge. If you would like to attend, please send an e-mail to preisverleihung@fv-berlin.de or phone 030 / 6392-3339.

Additional Informationen:


Dr. Lisa Torlina Tel. (030) 6392 1364


MBI researchers tackle long-standing problem of few-femtosecond internal conversion

18 October 2017

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. In the study by Galbraith et al., published in Nature Communications this week, MBI researchers characterize one of the fastest internal conversion processes in a molecule studied to date.

When Horst Köppel published his first article on the benzene ion molecule in 1987, Martin Galbraith was just born. Köppel, a professor from Heidelberg, had found the perfect testbed for the ensuing development of a new theoretical methodology, the so-called multi-configurational time-dependent Hartree method, for which he and his colleagues became famous over the next few decades. The omnipresent benzene molecule, whose backbone is a ring consisting of six carbon atoms, turns out to be the perfect compromise between complexity and chemical relevance. That's why the theory specialists from Heidelberg studied it in more and more detail, publishing more than 30 highly cited scientific papers over the years, as they matured the theory into a cutting-edge tool for computational chemistry, which is now being used by researchers all over the word.

One thing was out of reach though until now: The experimental verfication of the theoretical results in a time-resolved experiment. The predicted dynamics in the benzene ion was simply too fast - on a timescale of only about 10 femtoseconds, the tiny time interval one obtains when dividing one second by 1014, a number consisting of a 1 and 14 zeros.

While working on his PhD thesis at the Max Born Institute, Martin Galbraith and his coworkers have now pushed the experimental limits to the point where measurements of the extremely fast dynamics in the benzene molecule became technically feasible. "The development of few-cycle laser pulses and the creation of attosecond pulse trains consisting of only a few bursts allowed us to devise a photochemical experiment with unprecedented time resolution", says Dr. Jochen Mikosch, who headed the scientific effort. Researchers in the division of Prof. Marc Vrakking applied a dedicated spectral filter in their experiment, which makes it possible to create a defined superposition of electronic states in the molecule. Extremely short time constants could be measured, which were interpreted in terms of population transfer via two sequential conical intersections. Conical intersections are often described as molecular funnels, where different potential energy surfaces intersect. These are of particular interest since the usually distinct timescales for electronic and nuclear motion become comparable. Conical intersections play a crucial role in biochemical processes such as the stability of DNA with respect to UV light and the first steps of vision in animals and humans.

The scientific study published now in Nature Communications results from a collaboration of the MBI researchers with the theory groups of Prof. Horst Köppel and Alexander Kuleff from the University of Heidelberg. "I am very excited that after so many years of our calculations on the benzene ion being merely a theoretical benchmark, a detailed comparison of theory and experiment is now possible and validates our approach.", says Horst Köppel. The published work includes even more advanced theoretical modeling by Dr. Simona Scheit from the group in Heidelberg and shows excellent agreement between theory and computation.

Molecular dynamics near conical intersections play a key role in diverse very active fields of research in modern chemistry. Importantly, the previously inaccessible dynamics within the first few femtoseconds of the photochemical process are often crucial. Hence, Dr. Jochen Mikosch from the MBI is optimistic about the future prospects and concludes: "By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules."

Fig. 1 (click to enlarge

Fig. 1: Schematic overview of the lowest eight electronic component states of the benzene cation, depicted as potential energy V in eV as a function of a dimensionless effective nuclear coordinate Qeff. The violet arrows represent the ionization by the pump pulse, the orange arrows the excitation by the probe pulse. The dashed black line corresponds to the appearance energy for dissociation producing C4H3+. The dashed-dotted green curves are a cartoon drawing of the time-evolution of a cation originally transferred to the E state and then undergoing a series of internal conversion processes to the D and subsequently to the B states, via the conical intersections indicated in the figure.

Fig. 2 (click to enlarge)
Fig. 2: Experimentally measured C4H3+ fragment yield as a function of the pump-probe delay (red dots). The bold black line is a biexponential fit to the data, the dashed lines represent the contributions from two timescales that correspond to crossings of two sequential conical intersections. The inset displays a long range pump-probe scan of C4H3+.

Original publication: Nature Communications 8, 1018 (2017), doi:10.1038/s41467-017-01133-y.
M. C. E. Galbraith, S. Scheit, N. V. Golubev, G. Reitsma, N. Zhavoronkov, V. Despre, F. Lepine, A. I. Kuleff, M. J. J. Vrakking, O. Kornilov, H. Köppel, and J. Mikosch
"Few-Femtosecond Passage of Conical Intersections in the Benzene Cation"


Dr. Jochen Mikosch, Tel.: 030 6392 1295




New Method for Generating Magnetic Swirls

2 October 2017

Magnetic swirls called skyrmions are considered to be a promising potential means of achieving more efficient data storage technology and are currently the subject of intense research. Scientists have now discovered a method to generate such skyrmions in a way which can be directly integrated into memory chips and which functions reliably up to the gigahertz range. Using current pulses, the researchers generated nanoswirls at predetermined positions and then moved them in a controlled way. They used x-ray holography to image and directly observe the skyrmions. The researchers from Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), Massachusetts Institute of Technology (MIT), as well as other German research institutions reported their findings in "Nature Nanotechnology".

The scientists generated the skyrmions in a sandwich structure consisting of platinum, magnesium oxide, and a magnetic alloy consisting of cobalt, iron, and boron. Dr. Felix Büttner from MIT explains: "Due to the spin Hall effect, which has its roots in quantum mechanics, and a particular interaction of the atoms at the interfaces between the materials, skyrmions can be generated via pulses of electrical current. With our method, this can be realized directly in so-called racetrack structures at positions that we can determine in advance, which is of course essential for writing data in a controlled manner." These racetrack structures are nanometer-thin wires consisting of stacks of magnetic materials. The scientists controlled the exact position where the magnetic swirls were generated by adding a small constriction in the racetrack wire.

At the Deutsches Elektronen-Synchrotron (DESY) in Hamburg the scientists have used x-ray radiation to establish that such special skyrmion magnetic swirls were indeed generated and moved along the racetrack wire via current pulses. "X-ray holography is an extremely sensitive method for detecting these very small magnetic structures. The magnetic swirls can be imaged with a resolution of about 20 nanometers", explains Dr. Bastian Pfau from the MBI team.

In their investigations, the researchers were able to observe how skyrmions were generated with single current pulses and were then subsequently moved along the racetrack wire with additional pulses. The study was concerned with understanding the fundamental mechanisms underlying these processes: What happens in the few-nanometer-thin layers of material and at their interfaces in particular when single current pulses with a duration in the range of nanoseconds are sent through the wire? During the current pulses, how do electrons from the platinum layer influence the magnetization in the adjacent cobalt alloy so that skyrmions with a particular rotation direction are generated? Towards this end, the team compared the experimental observations to micromagnetic computer simulations, which emulate these processes. "The insight into the microscopic mechanism will significantly help us to improve the concepts and materials for future data storage technologies", says Büttner.

Storing data in three dimensions

"Our data is in the cloud" - when we say that, we often forget that ultimately the data reside on hard disk drives in the large data centers of companies like Google or Facebook. The single bits of data are encoded in the magnetization of thin magnetic films. A mechanically moving read/write head uses magnetic field pulses to write the bits onto a fast-rotating magnetic disk, the actual hard disk. To be able to store more data in the same volume in the future, scientists are working on transitioning from this inherently two-dimensional concept to a three-dimensional storage approach. In such racetrack memory devices, the information is also encoded in magnetization patterns, but now in a wire-like structure. There, they can be moved back and forth very quickly along this wire - aptly dubbed the "racetrack" - to be stored or retrieved. In contrast to today's hard disk technology, this can happen without using any moving mechanical parts, solely by applying very short current pulses. As the racetrack wires can be packed tightly together in three dimensions like a bunch of many parallel straws, this concept enables high data storage densities.

One candidate for representing the single bits in these racetracks are the nanometer-small swirls of magnetization, the skyrmions. Researchers are fascinated by them, as they can be moved with electrical current, while they are very stable at the same time. The presence or absence of a skyrmion would represent the bits "0" and "1" in this technology. So far, quite complex setups have been required to generate single skyrmions in a controlled manner - in contrast to the newly reported approach, which thus opens new perspectives for data storage technology.

Streubild Heliumnanotroepfchen

Fig. 1 (click to enlarge)

Fig. 1: Schematic representation of a racetrack wire, consisting of a stack of 45 layers, each only about one nanometer in thickness. In the schematic diagram, only 3 of the 45 layers are shown. Skyrmions (shown in blue) are generated in this particular material system behind the constriction formed by the small notches, as soon as strong current pulses are sent through the wire. Additional weaker current pulses can then be used to move the skyrmions along the wire to store them. The presence or absence of a skyrmion encodes the bits "1" or "0". The background shows part of an x-ray hologram, as used by the researchers to image the skyrmions. (Graphic by Moritz Eisebitt)

Original Publication
Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques
Felix Büttner, Ivan Lemesh, Michael Schneider, Bastian Pfau, Christian M. Günther,
Piet Hessing, Jan Geilhufe, Lucas Caretta, Dieter Engel, Benjamin Krüger, Jens Viefhaus, Stefan Eisebitt and Geoffrey S. D. Beach. Nature Nanotechnology.
DOI 10.1038/nnano.2017.178

Full text access via SharedIt: http://rdcu.be/wnQn

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)

Prof. Dr. Stefan Eisebitt
Email eisebitt@mbi-berlin.de
Phone +49 30 6392 1300

Dr. Bastian Pfau
Email bastian.pfau@mbi-berlin.de
Phone +49 30 6392 1343


High power within 4 cycles - demonstration of record parameters in the generation of ultrashort infrared pulses

21 September 2017

A novel light source provides infrared pulses of 75 femtoseconds duration at a wavelength of 5 micrometer and a repetition rate of 1 kilohertz. A multi-stage optical parametric amplifier in combination with a compact short-pulse laser system serves for the generation of very high peak powers in the range of 8 gigawatts. This infrared source holds potential for a broad range of applications in ultrafast science and, in particular, for generating extremely short hard x-ray pulses.

Ultrashort optical pulses are an important tool of basic reserach and a key ingredient of numerous optical technologies. The infrared spectral range at wavelengths beyond 1 µm (1 µm = 10-6 m = one millionth of a meter) is not only relevant in fiber-based optical communications; light with wavelengths between 1 and 300 µm is also used in optical analytics, sensors, and imaging technologies. A particular challenge consists in the generation of extremely short pulses in which the optical waves oscillate a few times only, in the limiting case only once. The generation of such "few-cycle" pulses requires a precise control of the optical phases and the propagation conditions. Sources providing intense few-cycle infrared pulses of high intensity and stability are a central topic of current laser research.

In the journal Optics Letters, a team of scientists from the Max-Born-Institute in Berlin and the company BAE Systems, Nashua, NH, USA, reports a new light source providing infrared pulses with record parameters. The highly compact system is based on the method of optical parametric chirped pulse amplification (OPCPA) in which a weak ultrashort infrared pulse is amplified in a nonlinear crystal by interaction with an intense pump pulse of a shorter wavelength. In the present light source, pump pulses of a 2 µm wavelength and a 10 ps duration drive a three-stage parametric amplifier with a pump energy of up to 20 mJ. A novel light modulator is implemented for optimal compression of the amplified pulses centered at a wavelength of 5 µm. The amplified pulses display a pulse energy of ~1 mJ and a duration of 75 fs, corresponding to a peak power of some 8 GW within the 4 optical cycles. The highly stable infrared pulses are generated with a 1 kHz repetition rate and show excellent optical beam parameters. The output power and repetition rate are scalable and can be optimized for different applications.

The results were recognized as an "Editor's pick" by Optics Letters and hold strong potential for opening new areas of application in ultrafast science, e.g., for studying (bio)molecular vibrational dynamics, low-frequency excitations in solids, and/or generating ultrashort pulses at short wavelengths. The new infrared source is presently being implemented in a laboratory source for hard x-ray pulses with a 100 fs duration and kilohertz repetition rates.

Fig. 1 (click to enlarge)

Fig. 1: Experimental setup of the 3-stage parametric amplifier. Three nonlinear ZnGeP2 crystals (ZGP I-III) serve as the amplifying media. The optical paths are shown in false colors.

Fig. 2 (click to enlarge)
Fig. 2: Temporal intensity envelope of the infrared pulses (blue) of a 75 fs duration (~4 optical cycles) at a central wavelength of 5 µm. Inset: Spectrally and temporally resolved pulse structure from a FROG measurement (FROG: Frequency Resolved Optical Gating).

Original article:
L. von Grafenstein, M. Bock, D. Ueberschaer, K. Zawilski, P. Schunemann, U. Griebner und T. Elsaesser, 5 µm few-cycle pulses with multi-gigawatt peak power at a 1 kHz repetition rate,
Optics Letters 42, 3796-3799 (2017).

Additional information:

Dr. Uwe Griebner, Tel.: 030 6392 1457

Dr. Martin Bock, Tel.: 030 6392 1442

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




First imaging of free nanoparticles in laboratory experiment using a high-intensity laser source

8 September 2017

In a joint research project, scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Technische Universität Berlin (TU) and the University of Rostock have managed for the first time to image free nanoparticles in a laboratory experiment using a high-intensity laser source. Previously, the structural analysis of these extremely small objects via single-shot diffraction was only possible at large-scale research facilities using so-called XUV and x-ray free electron lasers. Their pathbreaking results facilitate the highly-efficient characterisation of the chemical, optical and structural properties of individual nanoparticles and have just been published in "Nature Communications". The lead author of the publication is junior researcher Dr Daniela Rupp who carried out the project at TU Berlin and is now starting a junior research group at MBI.

In their experiment, the researchers expanded helium gas through a nozzle that is cooled to extremely low temperature. The helium gas turns into a superfluid state and forms a beam of freely flying miniscule nanodroplets. "We sent ultra-short XUV pulses onto these tiny droplets and captured snapshots of these objects by recording the scattered laser light on a large-area detector to reconstruct the droplet shape," explains Dr Daniela Rupp.

"Key to the successful experiment were the high-intensity XUV pulses generated in MBI's laser lab that produce detailed scattering patterns with just one single shot," explains Dr Arnaud Rouzée from MBI. "By using the so-called wide-angle mode that provides access to the three-dimensional morphology, we could identify hitherto unobserved shapes of the superfluid droplets," adds Professor Thomas Fennel from MBI and the University of Rostock. The research team's results enable a new class of metrology for analysing the structure and optical properties of small particles. Thanks to state-of-the-art laser light sources, making images of the tiniest pieces of matter is no longer exclusive to the large-scale research facilities.

Physicist Dr Daniela Rupp worked as a scientist at the Institute of Optics and Atomic Physics at TU Berlin till summer 2017. Now she is launching a Leibniz Junior Research Group at MBI where she continues her research on single particle imaging with short and intensive extreme ultraviolet light pulses. Her work has been previously awarded by the DPG's Dissertation Prize (AMOP Section), the Carl Ramsauer Prize of the Berlin Physical Society and the Physics Graduation Prize of the Wilhelm and Else Heraeus Foundation.

Streubild Heliumnanotroepfchen

Fig. 1 (click to enlarge)

Fig. 1: Pill-shaped helium nanodroplets can be detected through curved structures in the scatter image. Source: MBI

Original publication: Nature Communication 8, 493 (2017) doi:10.1038/s41467-017-00287-z
Coherent diffractive imaging of single helium nanodroplets with a high harmonic generation source
Daniela Rupp, Nils Monserud, Bruno Langbehn, Mario Sauppe, Julian Zimmermann, Yevheniy Ovcharenko, Thomas Möller, Fabio Frassetto, Luca Poletto, Andrea Trabattoni, Francesca Calegari, Mauro Nisoli, Katharina Sander, Christian Peltz, Marc J. J. Vrakking, Thomas Fennel, Arnaud Rouzée


Dr. Daniela Rupp Tel. 030 6392 1280
Dr. Arnaud Rouzée Tel. 030 6392 1440
Prof. Dr. Thomas Fennel Tel. 030 6392 1245
Prof. Dr. Marc J. Vrakking Tel. 030 6392 1200


Aspirin tablets help unravel basic physics

1 September 2017

Aspirin in form of small crystallites provides new insight into delicate motions of electrons and atomic nuclei. Set into molecular vibration by strong ultrashort far-infrared (terahertz) pulses, the nuclei oscillate much faster than for weak excitation. They gradually return to their intrinsic oscillation frequency, in parallel to the picosecond decay of electronic motions. An analysis of the terahertz waves radiated from the moving particles by in-depth theory reveals the strongly coupled character of electron and nuclear dynamics characteristic for a large class of molecular materials.

Based on its physiological activity, aspirin has found widespread pharmaceutical application in different medical areas. Looking at an individual aspirin molecule from the physics perspective, one can distinguish two types of motions: (i) molecular vibrations, i.e., oscillatory motions of the atomic nuclei in a wide frequency range, among them, e.g., the hindered rotation of the methyl group (Movie 1) at a frequency of 6 terahertz (THz) (1 THz = 1,000,000,000,000 oscillation cycles per second) and (ii) oscillatory motions of electrons in the molecule around 1000 THz (Movie 2), as induced, e.g., by ultraviolet light. While the different motions are only weakly coupled in a single aspirin molecule, they develop a very strong electric interaction in a dense molecular packaging such as in the aspirin tablets from the pharmacy. As a result, the character of particular vibrations, the so-called soft modes, changes and their oscillation frequency is substantially reduced (Movie 3). This complex coupling scheme and the resulting molecular dynamics are important for how aspirin and other molecules respond to an external stimulus. So far, this problem has remained unresolved.

In the current issue of Physical Review Letters, researchers from the Max Born Institute in Berlin and the University of Luxembourg combine top-notch experimental and theoretical methods to unravel the basic properties of soft modes. In the experiments, a sequence of two phase-locked THz pulses interacts with a 700-μm thick tablet of polycrystalline aspirin. The electric field radiated by the moving atoms serves as a probe for mapping the soft-mode oscillations in real time. Two-dimensional scans in which the time delay between the two THz pulses is varied, display a strong nonlinearity of the soft-mode response in aspirin crystals. This nonlinearity is dominated by a pronounced transient shift of the soft mode to higher frequencies (Fig. 1). The response displays a non-instantaneous character with picosecond decay times originating from the generated electric polarization of the crystallites. During the polarization decay, the soft-mode frequency returns gradually to the value it had before excitation.

The theoretical analysis shows that strong electric polarizations in the ensemble of aspirin molecules give the soft mode a hybrid character, combining nuclear and electronic degrees of freedom via dipole-dipole coupling. In the unexcited aspirin crystallites, this correlation between electrons and nuclei determines the soft-mode frequency. Strong THz excitation induces a break-up of the correlations, resulting in a transient blue-shift of the soft modes and, via the comparably slow decay (decoherence) of the polarization, a non-instantaneous response. The scenario discovered here is relevant for a large class of molecular materials, in particular for those with applications in ferroelectrics.


Fig. 1 (click to enlarge)

Fig. 1: Blue shift induced by the THz electric field acting on soft-mode transition dipole in an aspirin crystal. Depending on the electric field strength the soft-mode frequency is shifted from its initial value (red Gaussian, transmission increase) to an instantaneously blue-shifted position (ensemble of orange Gaussians, transmission decrease).
Movie 1 Movie 1: A single aspirin molecule in vacuum showing hindered rotations of the methyl group. Grey balls: carbon atoms, red balls: oxygen atoms, and white balls: hydrogen atoms. The vibrating methyl group consists of 1 carbon atom and 3 hydrogen atoms.
Movie 2 Movie 2: A single aspirin molecule in vacuum showing collective oscillations of the π electrons in the benzene ring. The latter is represented by the hexagon of carbon atoms. The oscillating yellow cloud represents the π electrons in the benzene ring.
Movie 3 Movie 3: Atomic motions of the soft mode in an aspirin crystal. In contrast to a single aspririn molecule in vacuum shown in movies 1 and 2 the hindered rotations of the methyl group are strongly coupled to the collective oscillations of the π electrons in the benzene ring.

Original publication: Physical Review Letters 119, 097404 (2017)
Strong Local-Field Enhancement of the Nonlinear Soft-Mode Response in a Molecular Crystal
Giulia Folpini, Klaus Reimann, Michael Woerner, Thomas Elsaesser, Johannes Hoja, and Alexandre Tkatchenko


Dr. Michael Woerner Tel. 030 6392 1470
Giulia Folpini Tel. 030 6392 1474
Prof. Dr. Klaus Reimann Tel. 030 6392 1476
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400


Water makes the proton shake - ultrafast motions and fleeting geometries in proton hydration

13. July 2017

Basic processes in chemistry and biology involve protons in a water environment. Water structures accommodating protons and their motions have so far remained elusive. Applying ultrafast vibrational spectroscopy, Dahms et al. map fluctuating proton transfer motions and provide direct evidence that protons in liquid water are predominantly shared by two water molecules. Femtosecond proton elongations within a hydration site are 10 to 50 times faster than proton hopping to a new site, the elementary proton transfer step in chemistry.

The proton, the positively charged nucleus H+ of a hydrogen atom and smallest chemical species, is a key player in chemistry and biology. Acids release protons into a liquid water environment where they are highly mobile and dominate the transport of electric charge. In biology, the gradient of proton concentration across cell membranes is the mechanism driving the respiration and energy storage of cells. Even after decades of research, however, the molecular geometries in which protons are accommodated in water, and the elementary steps of proton dynamics have remained highly controversial.

Protons in water are commonly described with the help of two limiting structures (Fig. 1A). In the Eigen complex (H9O4+) (left), the proton is part of the central H3O+ ion surrounded by three water molecules. In the Zundel cation (H5O2+) (right), the proton forms strong hydrogen bonds with two flanking water molecules. A description at the molecular level employs the potential energy surface of the proton (Fig. 1B) which is markedly different for the two limiting geometries. As shown in Fig. 1B, one expects an anharmonic single-minimum potential for the Eigen species and a double minimum potential for the Zundel species. In liquid water, such potentials are highly dynamic in nature and undergo very fast fluctuations due to thermal motions of surrounding water molecules and the proton.

Researchers from the Max Born Institute in Berlin, Germany, and the Ben Gurion University of the Negev in Beer-Sheva, Israel, have now elucidated the ultrafast motions and structural characteristics of protons in water under ambient conditions. They report experimental and theoretical results in Science which identify the Zundel cation as a predominant species in liquid water. The femtosecond (1 fs = 10-15 s) dynamics of proton motions were mapped via vibrational transitions between proton quantum states (red and blue arrows in Fig. 1B). The sophisticated method of two-dimensional vibrational spectroscopy provides the yellow-red and blue contours in Fig. 2A which mark the energy range covered by the two transitions. The blue contour occurs at higher detection frequencies than the red, giving the first direct evidence for the double-minimum character of the proton potential in the native aqueous environment. In contrast, for a single-minimum potential the blue contour is expected to appear at smaller detection frequencies than the red one.

The orientation of the two contours parallel to the vertical frequency axis demonstrates that the two vibrational transitions explore a huge frequency range within less than 100 fs, a hallmark of ultrafast modulations of the shape of proton potential. In other words, the proton explores all locations between the two water molecules within less than 100 fs and very quickly loses the memory of where it has been before. The modulation of the proton potential is caused by the strong electric field imposed by the water molecules in the environment. Their fast thermal motion results in strong field fluctuations and, thus, potential energy modulations on a sub-100 fs time scale. This picture is supported by benchmark experiments with Zundel cations selectively prepared in another solvent and by detailed theoretical simulations of proton dynamics (Fig. 2B).

A specific Zundel cation in water transforms into new proton accommodating geometries by the breaking and reformation of hydrogen bonds. Such processes are much slower than the dithering proton motion and occur on a time scale of a few picoseconds. This new picture of proton dynamics is highly relevant for proton transport by the infamous von Grotthuss mechanism, and for proton translocation mechanisms in biological systems.


Fig. 1 (click to enlarge)

Fig. 1 Chemical structure of hydrated protons in liquid water. A Schematic of the Eigen cation H9O4+ (left) and the Zundel cation H5O2+ (right). The arrows indicate the O-H bond coordinate r and the (O...H+...O) proton transfer coordinate z. In the Eigen cation a covalent O-H bond localizes the proton whereas in the Zundel cation the proton is delocalized between two water molecules. B Anharmonic vibrational potential (left) and double minimum potential of the Zundel cation along z (right, red). Distortions by the solvent surrounding impose a modulation of the double minimum potential (right, dotted line). Red and blue arrows indicate transitions between particular quantum states of the proton motion, i.e., the ground-state-to-first-excited-state transition (red) and the first-excited-state-to-second-excited-state transition (blue). The modulation of the potentials leads to spectral shifts of the vibrational transitions which are mapped by two-dimensional infrared spectroscopy.

Fig. 2 (click to enlarge)

Fig 2. Femtosecond dynamics of proton motions (1 fs = 10-15 s). A Two-dimensional vibrational spectra with the ground-state-to-first-excited-state transition (red) at lower detection frequency than the first-excited-state-to-second-excited-state transition (blue). The orientation of both contours parallel to the excitation frequency axis is due to ultrafast frequency fluctuations and the loss of memory in the proton position. B Simulated real-time dynamics of the proton motions in the Zundel cation. Within less than 100 fs, the proton displays large amplitude excursions along z, the coordinate linking the two water molecules in the Zundel cation. Due to the ultrafast modulation of the shape of proton potential by surrounding solvent molecules, the proton explores all locations between the two water molecules.


Fig. 3 (click to enlarge)

Fig. 3. Cartoon picture of proton hydration dynamics, visualized with the help of classical physics. The proton Smiley is sitting in the middle of a sofa with two seats. When shaking the sofa with a mechanical force, the shape of the seating changes and the proton moves forth and back on the sofa. Such motions occur on a time scale shorter than 100 fs (10-13 s). After an average time of 1 ps = 1000 fs = 10-12 s, the sofa breaks and the proton moves to a new site/sofa, including the red halve on the right.

Original publication: Science Vol. 357, Issue 6350, pp. 491-495 (2017) doi:10.1126//science.aan5144
Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy
Fabian Dahms, Benjamin P. Fingerhut, Erik T. J. Nibbering, Ehud Pines, Thomas Elsaesser


Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400
Dr. Benjamin Fingerhut Tel. 030 6392 1404
Dr. Erik T.J. Nibbering Tel. 030 6392 1477
Prof. Dr. E. Pines Tel. +972 8 6461640


A powerful laser system for driving sophisticated experiments in attosecond science

22 June 2017

Attosecond science has revolutionized the way we look into the time-dependent evolution of the microscopic world, where the behaviour of matter is governed by the rules of quantum mechanics. The technological breakthrough that made possible the development of the field is based on the generation of ultra-short laser pulses that last only a few oscillations of the electric field. These short pulses have a focused intensity where the electric field is comparable to the one electrons experience inside atoms and molecules. It is possible to control both the exact temporal shape and the waveform of these ultra-short pulses. While ultra-short laser pulses have been used in a few laboratories worldwide to study light-induced dynamics in atoms and molecules, many questions remain unanswered, due to the low data rates and inherently low SNR achievable with current state-of-the-art laser systems.

Quelle MBI
Quelle: MBI

At the Max Born Institute, a powerful laser system has now been completed, capable of reproducing the parameters of laser systems typically used in attosecond science experiments, but with a 100 times higher pulse repetition rate. This new laser system enables an entirely new class of experiments in simple atomic and small molecular systems, as well as high fidelity investigations of more complex molecules.


In the last 15-20 years, the availability of light pulses in the extreme ultraviolet (XUV) region of the electromagnetic spectrum, with durations in the order of 100s of attoseconds (1 as = 10-18 s) has enabled the emergence of the field of attosecond science. Utilizing these extremely short pulses scientists have gained unprecedented insight into the time evolution of electrons in atoms, molecules and solids, by taking advantage of the pump-probe technique: The system under investigation is excited by one "pump" laser pulse und after some time delay a second "probe" pulse interrogates the system (e.g. through ionization). The dynamics induced by the pump pulse can be retrieved by repeating the experiment at different delay times. Using the pump-probe technique a number of impressive results have been obtained in the last years addressing topics like light-induced charge migration, multi-electron correlations, and the coupling between electronic and nuclear degrees of freedom. Typically the velocity distributions of ions or electrons generated during the pump-probe sequence is determined experimentally or the transient absorption spectrum of the XUV pulse as a function of the pump-probe delay is detected. Often the light-induced processes are complex and measuring only one observable is not sufficient to fully understand the experimental results. Already several years ago, thanks to the development of the so-called "reaction microscope", a great improvement was achieved. This apparatus enables a measurement of the three-dimensional velocity distribution of all electrons and ions created in the pump-probe process The drawback of this technique is that very low signal rates are necessary, i.e. only 10% to 20% of all laser shots should induce the formation of an electron-ion pair. This leads to very long measurement times using current state of the art laser systems.

Pulses in the XUV with attosecond duration are produced when a strong laser pulse in the VIS-NIR interacts with a gas of atoms in a process called high-order harmonic generation (HHG). In order for a single XUV pulse with attosecond duration to be formed during the HHG process, the laser pulses interacting with the gas should last only a few oscillations of the electromagnetic field, which typically means less than 10 fs (1 fs = 10-15 s), and the exact temporal shape of the pulse must be controlled. The most widely spread way of producing such laser pulses consists in amplifying short pulses with a controlled waveform (Carrier-Envelope Phase- or CEP-controlled) in a Ti:Sapphire laser amplifier and shortening the duration of the pulses via non-linear pulse compression, using e.g. a gas-filled hollow-core capillary. However, the pulse repetition rate of these systems is typically limited to a few (1-3) kHz, and a maximum reported frequency of 10 kHz, due to detrimental thermal effects intrinsic to the laser amplifiers.

Now, researchers at the Max Born Institute in Germany, in collaboration with colleagues at the Norwegian Defence Research Establishment, have designed and built a laser system capable of operating at much higher pulse repetition rates than the typical Ti:Sapphire amplifiers. The newly developed system is perfectly suited for performing pump-probe experiments in attosecond science implementing electron-ion coincidence detection in a reaction microscope.

The system is based on a noncollinear optical parametric amplifier (NOPA). In a parametric amplifier, the energy from a strong pump pulse is transferred to a weak signal pulse in an instantaneous nonlinear interaction in a crystal. The gain and the bandwidth of the process are determined by conditions of phase-matching, that is, by ensuring that all the photons at the signal frequency are emitted in phase and add up coherently as the signal pulse propagates in the crystal. When the pump and the seed pulses enter the crystal subtending a small angle (noncollinear geometry), the bandwidth of the process is maximized and it is possible to amplify ultrashort pulses lasting only a few cycles. Moreover, since the process is instantaneous and there is no absorption of light in the crystal, there is no heat accumulation and thermal problems are almost negligible. Therefore, NOPA amplifiers are well suited for high repetition rates.

In the laser system presented in a recently published article in Optics Letters (https://doi.org/10.1364/OL.42.002495), the researchers amplified ultrashort CEP-stable pulses from a Ti:Sapphire laser oscillator in a NOPA amplifier pumped by a high repetition rate commercial Yb:YAG thin-disk laser. In the parametric amplifier a large fraction (about 20%) of the energy of the pulses from the Yb:YAG system is efficiently transferred to the ultrashort CEP-stable pulses from the Ti:Sapphire laser oscillator. The NOPA system is thus capable of delivering pulses with 0.24 mJ of energy at a repetition rate of 100 kHz, resulting in an average power of 24 W at an approximate central wavelength of 800 nm. After compression, filtering of parasitic second harmonic and a broadband variable attenuator for controlling the power incident into the experiments, CEP-stable pulses with 0.19 mJ (19 W) and 7 fs duration (i.e. 2.6 cycles) are available for experiments. The system will be employed for HHG and isolated attosecond pulse production, and will be the basis of an attosecond pump-probe beamline with coincidence detection capabilities.

Furch NOPA Fig. 1: High power NOPA at the MBI
Fig. (click to enlarge)  

Original publication: Optic Letters 2017, Vol. 42, Issue 13
"CEP-stable few-cycle pulses with more than 190 μJ of energy at 100 kHz from a noncollinear optical parametric amplifier"
Federico J. Furch, Tobias Witting, Achut Giree, Chao Luan, Felix Schell, Gunnar Arisholm, Claus P. Schulz, and Marc J. J. Vrakking


Dr. Federico Furch Tel. (030) 6392 1277
Dr. Tobias Witting Tel. (030) 6392 1228
Dr. Claus-Peter Schulz Tel. (030) 6392 1252
Prof. Dr. Marc Vrakking Tel. (030) 6392 1200


A perfect attosecond experiment

16 June 2017

Attosecond science techniques are currently revolutionizing ultrafast laser physics research, and enable experiments that provide unprecedented insights into the structure and time-dependent dynamics of electrons in atoms, molecules and condensed phase systems. In a new experiment, physicists from Waseda University (Japan), the National Research Council (Canada) and the Max Born Institute (Germany) have used attosecond science techniques to fully characterize the quantum mechanical wave function of an electron that is formed by photoionization. The work, reported in Science , is the first example of a "perfect" experiment using attosecond technology.

Quelle:NRC Ottawa
Detection of the shape of an electronic wave function with a six-fold symmetry.
Source: NRC Ottawa
The development of quantum mechanics in the early part of the last century forced scientists to accept that at the microscopic level matter behaves according to physical laws that are altogether different from the physical laws that apply in our macroscopic world. In the microscopic world concepts like the uncertainty principle play a role, posing limits on the precision with which certain properties of tiny particles, such as their position and speed, can simultaneously be measured. Quantum mechanics furthermore introduced wave-particle duality, meaning that the behavior of tiny particles can sometimes better be understood by considering the particles as waves.

These counterintuitive manifestations of quantum mechanics are due to the fact that every measurement that is performed on a quantum mechanical system only gives one out of a huge range of possible outcomes. The likelihood to measure a certain outcome is determined by a probability distribution that derives from the fundamental entity in quantum mechanics, the wave function. The wave function itself is not directly measurable, although strategies can be devised whereby multiple measurements performed on a quantum system lead to a complete characterization of the wave function.

In a paper published in Science (Villeneuve et al., "Coherent Imaging of an Attosecond Electron Wave Packet"), a novel approach is presented for the complete characterization of an atomic wave function using novel ultrafast lasers that have only been developed in the last few years. In the measurement, the scientists characterize the wave function of an electron that is released from a Neon atom as a result of the interaction of the atom with a series of laser pulses.

Electrons are elementary particles that are responsible for everyday things like electricity. They are characterized by several properties, such as one unit of (negative) charge, and an angular momentum, which is a vector that characterizes the rotation of the electron around the center of the atom. A slow rotation or a rotation close to the positive core of the atom, correspond to a low angular momentum, whereas a fast rotation or a rotation far away from the core imply a high angular momentum. The laws of quantum mechanics dictate that the angular momentum can only have certain distinct magnitudes. Accordingly, angular momentum states are called "s", "p", "d" and "f" for angular momentum quantum numbers l=0-3. In addition to the magnitude of the angular momentum, the length of the projection of the angular momentum vector onto a chosen laboratory frame axis (e.g. the polarization axis of the laser used in the experiment), characterized by the magnetic quantum number m, affects the outcome and interpretation of laboratory experiments.

In their paper, the scientists managed to accomplish a complete characterization of the wave function of the ionized electron, which contains contributions from angular momenta up to a value l=3 (i.e. s, p, d and f-contributions). Each of these angular momentum states is contained in the wave function with a specific amplitude, meaning a magnitude and a phase. In the experiment, these magnitudes and phases are determined by carrying out a series of interference experiments. Interference experiments exploit the wave-like character of quantum mechanical particles. Just like two water waves that cross each other can extinguish or enhance each other, so too can interference between different parts of a quantum mechanical wave function lead to an enhanced or a reduced probability to detect the particle at a particular place or with a particular speed. By performing a series of interference experiments under different conditions, pairwise interferences could be observed between the s- and d-part of the wave function, between the p- and the f-part, and finally, between all four components combined (see Figure 1). Accordingly, an exact and complete mathematical expression was obtained for the wave function of the ionized electron.

A crucial component in the accomplishment of this unique feat was the use of attosecond laser pulses (1 as = 10-18 s). Attosecond pulses are the shortest laser pulses that can be produced in state-of-the-art laser laboratories. They are produced in a process called "high-harmonic generation". Here, an atomic gas is exposed to an intense infrared laser that typically has a duration in the femtosecond (1 fs = 10-15 s) range. If the intensity of the infrared laser is high enough, the laser can pull electrons out of the atoms, which are subsequently accelerated by the oscillatory electric field of the infrared laser. Some of the accelerated electrons collide with the atoms from which they were previously removed. When this happens, the electron may be re-absorbed by the atom. All the energy that has been invested in the ionization and acceleration of the electron is then released in the form of a very energetic light particle (i.e. photon in the extreme ultra-violet (XUV) or soft X-ray part of the wavelength spectrum). Since the different steps in the high-harmonic generation process all occur on a timescale that is short compared to the duration of one optical cycle of the infrared laser (typically, just a few femtoseconds), this XUV/X-ray light appears in the form of a short - i.e. attosecond - pulse.

In the experiment the researchers used attosecond XUV pulses to ionize the Neon atoms. When only the attosecond pulse was fired in the experiment, a combination of s- and d-type electrons were formed, whose amplitude and relative phase could be determined from their angular distribution (see Figure 1A). When the ionization by the attosecond pulse was performed under conditions where beside the attosecond laser a replica of the infrared laser was present in the experiment, the amplitude and relative phase of the p- and f-components could be extracted (see Figure 1B). Finally, when the attosecond pulses were generated using a two-color laser field (both the afore-mentioned infrared laser and a copy of this laser with half the wavelength) the amplitude and relative phase of all four components (s, p, d and f) could be determined. The results of the experiment and the determination of the amplitude and phase of all angular momentum components are shown in Figures 1C and 1D. The clearly visible six-fold structure is caused by the dominant contribution of the f-orbital with m=0, which is produced by XUV+IR ionization. By the coherent addition of a contribution from the totally symmetric s-orbital (produced by XUV-only ionization), and changing the delay between the XUV and the IR pulse, an oscillation up and down along the vertical laser polarization axis is induced, revealing the phase of the f-orbital contribution.

The experiment is what atomic physicists consider a "complete" experiment, yielding a complete mathematical description of the wave function of the ionized electron, and is the latest example of how attosecond science techniques are currently revolutionizing ultrafast laser physics research. With the present work, this research has for the first time reached a state of perfection. (German Translation: Dr. Claus-Peter Schulz)

VRAttosecondExperiement Figure 1: A) XUV-only ionization produces an electron that is in a state characterized by "s" and "d" angular momentum, where a measurement of the electron angular distribution yields a determination of their relative amplitude and phase; B) XUV+IR ionization produces an electron that is in a state characterized by "p" and "f" angular momentum, where a measurement of the electron angular distribution once more yields a determination of their relative amplitude and phase; C) combined XUV-only and XUV+IR ionization produces an electron wave function containing both "s", "p", "d" and "f" contributions. The interference between these angular momentum components evolves with the delay between the XUV pulse and the co-propagating IR pulse. The large contribution of the "f" component is clearly visible in the first and last image; D) Measured electron momentum images at two time delays between the XUV pulse and the co-propagating IR pulse (corresponding to the first and last image in C). The reported experiment provides a complete determination of the relative amplitude and phases of all angular momentum components and thus represents a "perfect" experiment.

Original publication: Science. 2017, Vol. 356, Issue 6343
"Coherent imaging of an attosecond electron wave packet"
D. M. Villeneuve, Paul Hockett, M. J. J. Vrakking, Hiromichi Niikura


Prof. Dr. Marc Vrakking Tel. (030) 6392 1200


Turmoil in sluggish electrons' existence

22 May 2017

An international team of physicists has monitored the scattering behavior of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.
We can refer to electrons in non-conducting materials as 'sluggish'. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence relatively still in a dielectric crystal lattice. This idyll has now been heavily shaken up by a team of physicists from various research institutions, including the Laboratory of Attosecond Physics (LAP) at the Ludwig-Maximilians-Universität Munich (LMU) and the Max Planck Institute of Quantum Optics (MPQ), the Institute of Photonics and Nanotechnologies (IFN-CNR) in Milan, the Institute of Physics at the University of Rostock, the Max Born Institute (MBI), the Center for Free-Electron Laser Science (CFEL) and the University of Hamburg. For the first time, these researchers managed to directly observe the interaction of light and electrons in a dielectric, a non-conducting material, on timescales of attoseconds (billionths of a billionth of a second).

The scientists beamed light flashes lasting only a few hundred attoseconds onto 50 nanometer thick glass particles, which released electrons inside the material. Simultaneously, they irradiated the glass particles with an intense light field, which interacted with the electrons for a few femtoseconds (millionths of a billionth of a second), causing them to oscillate. This resulted, generally, in two different reactions by the electrons. First, they started to move, then collided with atoms within the particle, either elastically or inelastically. Because of the dense crystal lattice, the electrons could move freely between each of the interactions for only a few ångstrom (10-10 meter). "Analogous to billiard, the energy of electrons is conserved in an elastic collision, while their direction can change. For inelastic collisions, atoms are excited and part of the kinetic energy is lost. In our experiments, this energy loss leads to a depletion of the electron signal that we can measure," explains Prof. Francesca Calegari (CNR-IFN Milan and CFEL/University of Hamburg).

Since chance decides whether a collision occurs elastically or inelastically, with time inelastic collisions will eventually take place, reducing the number of electrons that scattered only elastically. Employing precise measurements of the electrons' oscillations within the intense light field, the researchers managed to find out that it takes about 150 attoseconds on average until elastically colliding electrons leave the nanoparticle. "Based on our newly developed theoretical model we could extract an inelastic collision time of 370 attoseconds from the measured time delay. This enabled us to clock this process for the first time," describes Prof. Thomas Fennel from the University of Rostock and Berlin's Max Born Institute in his analysis of the data.

The researchers' findings could benefit medical applications. With these worldwide first ultrafast measurements of electron motions inside non-conducting materials, they have obtained important insight into the interaction of radiation with matter, which shares similarities with human tissue. The energy of released electrons is controlled with the incident light, such that the process can be investigated for a broad range of energies and for various dielectrics. "Every interaction of high-energy radiation with tissue results in the generation of electrons. These in turn transfer their energy via inelastic collisions onto atoms and molecules of the tissue, which can destroy it. Detailed insight about electron scattering is therefore relevant for the treatment of tumors. It can be used in computer simulations to optimize the destruction of tumors in radiotherapy while sparing healthy tissue," highlights Prof. Matthias Kling of the impact of the work. As a next step, the scientists plan to replace the glass nanoparticles with water droplets to study the interaction of electrons with the very substance which makes up the largest part of living tissue.
(Text: Thorsten Naeser)


Fig. (click to enlarge)

Figure: A team of physicists clocked the time it takes electrons to leave a dielectric after their generation with extreme ultraviolet light. The measurement (false color plot) was the first of its kind in a dielectric material and yielded a time of 150 attoseconds (as), from which the physicists determined that inelastic scattering in the dielectric takes about 370 as.

Originalpublication: Nature Physics (2017) doi:10.1038/nphys4129
Attosecond Chronoscopy of Electron Scattering in Dielectric Nanoparticles
L. Seiffert, Q. Liu, S. Zherebtsov, A. Trabattoni, P. Rupp, M. C. Castrovilli, M. Galli, F. Süßmann, K. Wintersperger, J. Stierle, G. Sansone, L. Poletto, F. Frassetto, I. Halfpap, V. Mondes, C. Graf, E. Rühl, F. Krausz, M. Nisoli, T. Fennel, F. Calegari, M. F. Kling.


Prof. Dr. Thomas Fennel Tel. 030 6392 1245


Thomas Fennel started as a Heisenberg fellow at the MBI

12 April 2017


Prof. Thomas Fennel, group leader at the Institute of Physics at the University of Rostock, has been awarded a prestigious Heisenberg Fellowship funded by the Deutsche Forschungsgemeinschaft (DFG).

Prof. Dr. Thomas Fennel - Photo: Julia Tetzke, Uni Rostock  

Prof. Thomas Fennel, group leader at the Institute of Physics at the University of Rostock, has been awarded a prestigious Heisenberg Fellowship funded by the Deutsche Forschungsgemeinschaft (DFG). With the Heisenberg fellowship, which officially started on January 1st 2017, the DFG is supporting a research project to explore new routes for imaging and controlling ultrafast electronic motion in nanostructures. The underlying research will be carried out in a joint effort between Prof. Fennel's team at the University of Rostock and researchers in division A of the Max Born Institute, which is led by Prof. Marc Vrakking and to which Prof. Fennel is affiliated as an associated researcher.

The research activities are devoted to the active manipulation and visualization of ultrafast correlated and collective electron motion in finite systems. On the one hand, routes to the control of electronic processes in clusters, nanoparticles, and jets on the timescale of a single optical cycle of light via its detailed electric waveform or with multi-color fields will be explored, theoretically and experimentally. On the other hand, the technology for characterizing the attosecond electron motion in nanostructures via coherent diffractive imaging experiments using ultrashort intense XUV and x-ray laser pulses from free electrons lasers and lab-based high-harmonic sources will be developed. Finally, both approaches should be combined to trace light-induced electron dynamics with unprecedented spatial and temporal resolution and to reveal its classical and quantum aspects. Prof. Fennel is an expert in numerical many-particle physics and nanophotonics. He aims at the further development of atomistic electromagnetic plasma simulations and the efficient inclusion of the relevant quantum dynamics to tackle the challenging scientific questions of the project. The Max Born Institute is happy to welcome Prof. Fennel and is looking forward to a fruitful collaboration with the local experimental and theoretical groups.


Prof. Dr. Marc Vrakking Tel. (030) 6392 1200
Prof. Dr. Thomas Fennel Tel. (030) 6392 1295


Nanostructures give directions to efficient laser-proton accelerators

14 March 2017

Nanostructured surfaces have manifold applications. Among others they are used to selectively increase aborption of light. You can find them everywhere where light harvesting is the key point, e.g. in photovoltaics. But also in laser proton acceleration this approach attracts a lot of attention as nanostructured targets hold the promise to significantly increase maximum proton energies and proton numbers at a given laser energy. As for any other new technology, a high efficiency is a key for a potential future use. Scientists at the Max-Born-Institute (MBI) in Berlin have now investigated under which conditions the use of nanostructures in laser ion acceleration is beneficial.

If an ultrashort laser pulse (˜30 fs, >1 J) is focused onto a solid target foil, such that relativistic intensities (>1018 W/cm2) are reached, matter is transformed immediately into a plasma by field ionization. Electrons are accelerated to relativistic energies in the laser field. While fastest electrons can leave the target, those with less (but still relativistic) energy are trapped in the coulomb field of the (now) positively charged target and start to oscillate in this field. They form a dynamic sheath that, together with the target surface, generates an electric field of several megavolts per micrometer, in which positive ions (e. g. protons and carbon ions from the surface contamination layer) experience extreme acceleration. This process is called target normal sheath acceleration (TNSA). Fig. 1 shows an image of such a proton bunch.

The idea behind using nanostructured surfaces is now straight forward: Nanostructures increase laser absorption, i. e. more and more energetic electrons are generated which, in turn, can accelerate protons to higher energies.

But there are also alternatives for optimizing the TNSA mechanism - particularly important is the optimization of the plasma gradient, i. e. the density profile of the target. The laser intensities applied are so huge, that ionization of the target does not only happen when the peak of the laser pulse interacts with the target, but already starts during the rising edge of the pulse. The pre-ionized plasma expands, the plasma density decreases. The plasma gradient is therefore essentially determined by the exact temporal pulse structure.

The team of Dr. Matthias Schnuerer from Max-Born-Institute in Berlin has investigated, under which conditions the use of nanostructured targets is beneficial. For this purpose, the physicists have laser structured their targets in-situ. This method of generation of periodic surface structures via a laser (LIPSS) is particularly simple and in principle allows the development of a high repetition rate target system. In a first step, the target surface is nanostructured by applying about 20 strongly attenuated laser pulses. A representative scanning electron microscopy image of such a surface is shown in fig. 2. The structural parameters are similar to those that maximize laser absorption. Structural analysis and simulations show that these structures possess nearly optimal parameters for maximum laser absorption. In the following step, a single fully amplified pulse is focused onto this nanostructured area. Dr. Andrea Lübcke and her co-workers have investigated the influence of those nanostructures on the proton spectrum for different laser intensities. They chose a laser contrast that is optimal at highest intensities. First of all, the scientists could show that nanostructures remain functional even at highest intensities at the present contrast conditions in the sense that they increase the laser absorption as evident from an increase of Kα yield (see. Fig. 2a). For relatively low intensities, nanostructures significantly enhance both the conversion efficiency and proton energies. For example, at 5x1017 W/cm2 the maximum proton energies were increased by a factor of four, the conversion efficiency from laser to proton energy was even enhanced by two orders of magnitude. However, at highest laser intensities with optimal laser plasma parameters no significant benefits from the nanostructures for ion acceleration were measured (Fig. 2b,c). The researchers speculate about fundamental limitations in the energy transfer processes. The scientists were, however, not fully surprised by these results: As in many optimization problems, there are different paths to the optimum and combining them usually does not lead to an even better result. So far, these experiments performed at extreme conditions cannot be theoretically simulated in every respect. It is therefore the merit of this work to have clarified under which conditions the use of nanostructures is beneficial and in which direction new theoretical investigations can be initiiated.


Abb. 1 (click to enlarge)

Fig. 1: Laser accelerated ions, becoming visible in a Wilson chamber.

Abb. 1 (click to enlarge)

Fig. 2: Typical scanning electron microscopy image of nanostructured titanium surface (top). The Kα yield (a) of the nanostructured target is enhanced compared to the plane target over the entire investigated intensity range and indicates that nanostructures are functional even at highest intensity. In contrast, the conversion efficiency (energy transfer into fast protons) (b, logarithmic scale) and the maximum proton energy (c) of the two different targets approach each other at highest intensities.

Original publication: Scientific Reports 7, 44030 (2017) doi:10.1038/srep44030
Prospects of target nanostructuring for laser proton acceleration
Andrea Luebcke, Alexander A. Andreev, Sandra Hoehm, Ruediger Grunwald, Lutz Ehrentraut, Matthias Schnuerer


Dr. Andrea Luebcke Tel. 030 6392 1247
Dr. Matthias Schnuerer Tel. 030 6392 1350


Lattice of nanotraps and line narrowing in Raman gas

8 February 2017

Decreasing the emission linewidth from a molecule is one of the key aims in precision spectroscopy. One approach is based on cooling molecules to near absolute zero. An alternative way is to localize the molecules on subwavelength scale. A novel approach in this direction uses a standing wave in a gas-filled hollow fibre. It creates an array of deep, nanometer-scale traps for Raman-active molecules, resulting inlinewidth narrowing by a factor of 10 000.

The radiation emitted by atoms and molecules is usually spectrally broadened due to the motion of the emitters, which results in the Doppler effect. Overcoming this broadening is a difficult task, in particular for molecules. One possibility to overcome the molecular motion is by building deep potential traps with small dimensions. Previously, this was done e.g. by arranging several counterpropagating beams in a complicated setup, with limited success.

In a cooperation effort of the Max Born Institute (A. Husakou) and Xlim Institute in Limoges, researchers show that subwavelength localization and line narrowing is possible in a very simple arrangement due to self-organization of Raman gas (molecular hydrogen) in a hollow photonic crystal fibre. Due to Raman scattering, the continuous-wave pump light transforms into the so-called Stokes sideband, which travels back and forth in the fibre due to reflections from fibre ends and forms a stationary interference pattern - a standing wave with interchanging regions of high and low field [Fig. 1]. In the high-field regions, the Raman transition is saturated and is not active, and the molecules have high potential energy since they are partially in the excited state. In the low-field region, the molecules are Raman-active, and they have low potential energy since they are close to the ground state. These low-field regions form an array of roughly 40 000 narrow, strong traps, which contain localized Raman-active molecules. The size of these traps is around 100 nm (1 nm = 10-9m), which is much smaller than the light wavelength of 1130 nm. Therefore the emitted Stokes sidebands have a very narrow spectral width of only 15 kHz - this is 10 000 times narrower than the Doppler-broadened sidebands for the same conditions!

The self-organization of the gas manifests also on the macroscopic scale. First, the calculations show that the Raman process mainly happens exactly in the fibre section where the standing wave is formed, as shown in the top panel of Fig. 1. Second, the macroscopic gradient of the potential leads to the gas flow towards the fibre end, which is observed by eye in the experiment. This strong localization and the linewidth narrowing can find various uses, e.g. in spectroscopy. However, it can also be used as well as a method to periodically modulate the density of the gas, which is naturally suited for developing quasi-phase-matching schemes for other nonlinear processes, such as effective generation of high harmonics.


Fig. 1 (click to enlarge)

Fig. 1: On the macroscopic scale, the pump light transforms into forward-propagating Stokes (FS) radiation, which is partially reflected from the fibre end and becomes backward-propagating Stokes radiation (BS) which is also amplified by the pump. In the region where both FS and BS are strong, they form interference pattern of standing wave, which is shown on the microscopic scale. In the low-field regions (denoted by red-color molecules) the molecules are in the ground state and strongly trapped, as shown by the potential in the bottom panel. Exactly these trapped molecules are Raman-active, leading to line narrowing.

Original publication: Nature Communications 7, 12779 (2016) doi:10.1038/ncomms12779
"Raman gas self-organizing into deep nano-trap lattice"
M. Alharbi, A. Husakou, M. Chafer, B. Debord, F. Gérôme and F. Benabid


Dr. A. Husakou Tel. 030 6392 1280


Ultrasmall atom motions recorded with ultrashort x-ray pulses

1st Februar 2017

Periodic motions of atoms over a length of a billionth of a millionth of a meter (10-15 m) are mapped by ultrashort x-ray pulses. In a novel type of experiment, regularly arranged atoms in a crystal are set into vibration by a laser pulse and a sequence of snapshots is generated via changes of x-ray absorption.

A crystal represents a regular and periodic spatial arrangement of atoms or ions which is held together by forces between their electrons. The atomic nuclei in this array can undergo different types of oscillations around their equilibrium positions, the so-called lattice vibrations or phonons. The spatial elongation of nuclei in a vibration is much smaller than the distance between atoms, the latter being determined by the distribution of electrons. Nevertheless, the vibrational motions act back on the electrons, modulate their spatial distribution and change the electric and optical properties of the crystal on a time scale which is shorter than 1 ps (10-12 s). To understand these effects and exploit them for novel, e.g., acoustooptical, devices, one needs to image the delicate interplay of nuclear and electronic motions on a time scale much shorter than 1 ps.

In a recent Rapid Communication in Physical Review B, researchers from the Max Born Institute in Berlin (Germany), the Swiss Federal Laboratories for Materials Science and Technology in Dübendorf (Switzerland), and the National Institute of Standards and Technology, Gaithersburg (USA) apply a novel method of optical pump - soft x-ray probe spectroscopy for generating coherent atomic vibrations in small LiBH4 crystals, and reading them out via changes of x-ray absorption. In their experiments, an optical pump pulse centered at 800 nm excites via impulsive Raman scattering a coherent optical phonon with Ag symmetry [movie]. The atomic motions change the distances between the Li+ und (BH4)- ions. The change in distance modulates the electron distribution in the crystal and, thus, the x-ray absorption spectrum of the Li+ ions. In this way, the atomic motions a mapped into a modulation of soft x-ray absorption on the so-called Li K-edge around 60 eV. Ultrashort x-ray pulses measure the x-ray absorption change at different times. From this series of snapshots the atomic motions are reconstructed.

This novel experimental scheme is highly sensitive and allows for the first time to kick off and detect extremely small amplitudes of atomic vibrations. In our case, the Li+ ions move over a distance of only 3 femtometers = 3 x 10-15 m which is comparable to the diameter of the Li+ nucleus and 100000 times smaller than a distance between the ions in the crystal. The experimental observations are in excellent agreement with in-depth theoretical calculations of transient x-ray absorption. This new type of optical pump-soft x-ray probe spectroscopy on a femtosecond time scale holds strong potential for measuring and understanding the interplay of nuclear and electronic motions in liquid and solid matter, a major prerequisite for theoretical simulations and applications in technology.


Abb. 1 (click to enlarge)

Abb. 1: In an x-ray absorption experiment light excites a strongly bound core electron into a conduction band state. On the left of the figure such a transition is shown. An electron which is strongly bound to a Lithium nucleus (green) is excited into a conduction band state (red) that interacts with both the Lithium nucleus and Borohydride group. This conduction band state is therefore sensitive to a modulation of the distance Q between Lithium nucleus and Borohydride group and as a result the x-ray absorption process is sensitive to such a modulation (cf. Figs. 2(b) and 3(d) in the main article). On the right side of the figure the Lithium K-edge x-ray absorption spectrum for different strongly exaggerated displacements is shown.
Movie Movie: What happens in the unit cell of crystalline LiBH4 after impulsive Raman excitation with a femtosecond laser pulse? Upper panel: measured transient absorption change Δ A(t) (symbols) as we vary the time delay between infrared pump pulses and soft x-ray probe pulses at photon energy of ħω = 61.5 eV [cf. Fig. 3(a) in the main article. The lower box shows the atoms in the unit cell of LiBH4 with red boron atoms, gray hydrogen atoms, and green Li atoms. The moving blue circle in the upper panel is synchronized with the moving atoms in the lower panel. The amplitude of the motion is strongly exaggerated (i.e. times 30000) to visualize the pattern of the motion. The reddish color of the unit cell indicates the intensity of the infrared pump pulse.

Original publication: Physical Review B 95, 081101 (R) (2017)
Ultrafast modulation of electronic structure by coherent phonon excitations
J. Weisshaupt, A. Rouzée, M. Woerner, M. J. J. Vrakking, T. Elsaesser, E. L. Shirley, and A. Borgschulte


Dr. Michael Woerner Tel. 030 6392 1470
Jannick Weisshaupt Tel. 030 6392 1471
Dr. Arnaud Rouzée Tel. 030 6392 1240
Prof. Dr. Marc Vrakking Tel. 030 6392 1200
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400


Unified time and frequency picture of ultrafast atomic excitation in strong fields

5 January 2017

The insight that light sometimes needs to be treated as an electromagnetic wave and sometimes as a stream of energy quanta called photons is as old as quantum physics. In the case of interaction of strong laser fields with atoms the dualism finds its analogue in the intuitive pictures used to explain ionization and excitation: The multiphoton picture and the tunneling picture. In a combined experimental and theoretical study on ultrafast excitation of atoms in intense short pulse laser fields scientists of the Max Born Institute succeeded to show that the prevailing and seemingly disparate intuitive pictures usually used to describe interaction of atoms with intense laser fields can be ascribed to a single nonlinear process. Moreover, they show how the two pictures can be united. The work appeared in the journal Physical Review Letters and has been chosen to be an Editors' suggestion for its particular importance, innovation and broad appeal. Beside the fundamental aspects the work opens new pathways to determine laser intensities with high precision and to control coherent Rydberg population by the laser intensity.

Although the Keldysh parameter, introduced in the 1960's by the eponymous Russian physicist, clearly distinguishes the multiphoton picture and the tunneling picture, it has remained an open question, particularly in the field of strong field excitation, how to reconcile the two seemingly opposing approaches.

In the multiphoton picture the photon character shines through as resonant enhancement in the excitation yield whenever an integer multiple of the photon energy matches the excitation energy of atomic states. However, the energy of atomic states is shifted upwards with increasing laser intensity. This results in resonant-like enhancements in the excitation yield, even at fixed laser frequency (photon energy). In fact, the enhancement occurs periodically, whenever the energy shift corresponds to an additional photon energy (channel closing).

In the tunneling picture the laser field is considered as an electromagnetic wave, where only the oscillating electric field is retained. Excitation can be viewed as a process, where initially the bound electron is liberated by a tunneling process, when the laser field reaches a cycle maximum. In many cases the electron does not gain enough drift energy from the laser field to escape the Coulomb potential of the parent ion by the end of the laser pulse, which would lead to ionization of the atom. Instead, it remains bound in an excited Rydberg state. In the tunneling picture there is no room for resonances in the excitation since tunneling proceeds in a quasi-static electric field, where the laser frequency is irrelevant.

In the study the excitation yield of Ar and Ne atoms as a function of the laser intensity has been directly measured for the first time, covering both the multiphoton and tunneling regimes. In the multiphoton regime pronounced resonant enhancements in the yield have been observed, particularly in the vicinity of the channel closings, while in the tunneling regime no such resonances appeared. However, here excitation has been observed even in an intensity regime which lies above the threshold for expected complete ionization.

The numerical solution of the time dependent Schrödinger equation for the investigated atoms in a strong laser field provided excellent agreement of the theory with the experimental data in both regimes. A more detailed analysis revealed that both pictures represent a complementary description in the time and frequency domain of the same nonlinear process. If one considers excitation in the time domain one can assume that electron wave packets are created periodically at the field cycle maxima. In the multiphoton regime it can be shown that the wave packets are created predominantly close to the maximum intensity of the pulse and thus interfere constructively only if the intensity is close to a channel closing. With this, regular enhancement in the excitation spectrum results effectively only at the photon energy separation. In the tunneling regime the wavepackets are also created periodically at the field cycle maxima, however, predominantly at the rising edge of the laser pulse which, in turn, leads to an irregular interference pattern and consequently, to irregular variations in the excitation spectrum. These rapid variations are not resolved in the experiment and the detected excitation spectrum is smooth.


Fig. 1 (click to enlarge)

Fig. 1: Yield of excited atoms as a function of the laser intensity. At a laser intensity of 200TW/cm2, in the vicinity of a 6 photon channel closing, a strong resonant enhancement of a factor 100 is visible. For the argon data, the theoretical curve is also displayed (red dashed curve), which is in excellent agreement with the experimental data.

Original publication: Phys. Rev. Lett. 118, 013003 (2017) doi:10.1103/PhysRevLett.118.013003
"Unified Time and Frequency Picture of Ultrafast Atomic Excitation in Strong Laser Fields"
H. Zimmermann, S. Patchkovskii, M. Ivanov, and U. Eichmann


Dr. S. Patchkovskii Tel. 030 6392 1241
Prof. Dr. U. Eichmann Tel. 030 6392 1371


Amplification of relativistic Electron Pulses by Direct Laser Field Acceleration

5 January 2017

Controlled direct acceleration of electrons in very strong laser fields can offer a path towards ultra-compact accelerators. Such a direct acceleration requires rectification and decoupling of the oscillating electromagnetic laser field from the electrons in a suitable way. Researchers worldwide try to tackle this challenge. In experiments at the Max Born Institute, direct laser acceleration of electrons could now be demonstrated and understood in detail theoretically. This concept is an important step towards the creation of relativistic and ultra-short electron pulses within very short acceleration distances below one millimeter. Resulting compact electron and related x-ray sources have a broad spectrum of applications in spectroscopy, structural analysis, biomedical sciences and for nanotechnology.

The way electrons can be accelerated up to relativistic kinetic energies in strong laser fields is a fundamental issue in the physics of light-matter interaction. Although the electromagnetic fields of a laser pulse force a free electron previously at rest to oscillations with extremely high velocities, these oscillations cease again when the light pulse has passed by. A net energy transfer by such a direct acceleration of a charged particle in the laser field cannot take place. This fundamental principle - often discussed in physics exams - is valid for certain boundary conditions of the spatial extent and intensity of the laser pulse. Only for particular, different boundary conditions, electrons can indeed receive a net energy transfer via acceleration from the strong laser field. These conditions can be set e.g. by focusing of the laser pulse or the presence of strong electrostatic fields in a plasma.

Worldwide, scientists are looking for solutions how fast electrons can be extracted from extremely strong laser fields and how one can obtain short electron pulses with a high charge density via ultra-short laser pulses.

In light fields of relativistic intensity (I> 1018 W/cm2) electrons oscillate with velocities close to the speed of light. The corresponding kinetic energy reaches values from MeV to GeV (at I> 1022 W/cm2. Strong light fields are realized by focusing ultra-short laser pulses with high energy down to areas of few micrometers. The resulting spatial intensity distribution does already enable the acceleration of the electrons up to high kinetic energies. This process is known as ponderomotive acceleration. It is an essential process for the interaction between strong light fields and matter. Various theoretical studies, however, have predicted that the number of electrons and their kinetic energy can be further significantly increased by a direct acceleration in the laser field, but only if the electron-light interaction is interrupted in a properly tailored way. These considerations were the starting point for the experiments by Julia Braenzel and her colleagues at the Max Born Institute.

In the experiments at MBI, the electrons were decoupled from the light pulse at a particular moment in time, using a separator foil that is opaque for the laser light but can transmit fast electrons. We could show that this method leads to an increase of the number of electrons with high velocities. At first, a 70 TW Ti:Sapphire laser pulse (2 J @ 35 fs) irradiates an 30 - 100 nm thin target foil consisting of a PVF-polymer. In the laser propagation direction, about 109electrons are accelerated up to several MeV energy via the ponderomotive force. During this interaction the foil is almost fully ionized and transformed into plasma.

For sufficiently thin target foil thicknesses below 100nm a fraction of the incident laser light can be transmitted through the plasma. The transmitted light starts to overtake the electrons already emitted in this direction. This corresponds to a quasi-intrinsically synchronized injection of slow electrons into the transmitted, but still relativistic laser field (<8 x 1018W/cm2). If a second thin separator foil is placed at the correct distance behind the first one, amplification in the electron signal for a particular energy interval is observed. Fig. 1a) shows a schematic of the temporal evolution in the experiment and Fig. 1b) presents a direct comparison of the detected electron spectral distribution for a single foil and a double foil configuration, where the second foil acts as a separator. This foil is opaque for the laser light but is transparent for the fast electrons and hence enables a decoupling of both. The time at which the interaction between electrons and transmitted light is interrupted depends on the distance between the two foils.

The experiments carried out in the group of Matthias Schnürer demonstrate that an amplification of the electron signal can obtained and is maximized for a particular distance. The amplification vanishes for very big distances. Numerous measurements as well as numeric simulations confirmed the hypothesis that electrons with high kinetic energy can indeed be extracted out of the light field if they are decoupled appropriately. If the separator foils is located at an optimized position, slow electrons with kinetic energies below 100keV are accelerated to about ten times higher kinetic energies. This effect leads to a concentration of electrons in a narrow energy interval. In contrast to experiments using the different mechanism of laser wake field acceleration, where the production of GeV electrons has already been demonstrated, the direct laser acceleration demonstrated here can be scaled up to high laser intensities and high plasma densities. Beyond the fundamental insight in laser-matter interactions, the direct laser acceleration demonstrated in this work holds promise for the future realization of compact sources of relativistic electrons.


Fig. 1a (click to enlarge)

Fig. 1a: Schematic of the direct electron acceleration in a laser field and its realization in the experiment.
Steinmeyer Fig. 1b: Detected electrons in the laser propagation direction from a single (F1) and double foil (F1F2) target configuration, where the soncond foils acts as a speparator. The plastic foils used were about F1=35nm and F2=85 nm thick. N e values represent the integrated electron numbers for the whole detection range (0,2-7,5 MeV) with respect to the spectrometer aperture.
Fig. 1b (click to enlarge)  

Original Publication: Phys. Rev. Lett. 118, 014801 (2017) doi:10.1103/PhysRevLett.118.014801
"Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields"
J. Braenzel, A.A. Andreev, F. Abicht, L. Ehrentraut, K. Platonov, and M. Schnürer


Julia Bränzel Tel. 030 6392 1338
Dr. Matthias Schnuerer Tel. 030 6392 1315