The onset of electrical resistance

16th December 2011

Researchers at the Max-Born-Institute, Berlin, Germany, observed the extremely fast onset of electrical resistance in a semiconductor by following electron motions in real-time.

When you first learned about electric currents, you may have asked how the electrons in a solid material move from the negative to the positive terminal. In principle, they could move ballistically or ‘fly’ through the solid, without being affected by the atoms or other charges of the material. But this actually never happens under normal conditions because the electrons interact with the vibrating atoms or with impurities. These collisions typically occur within an extremely short time, usually about 100 femtoseconds (10-13 seconds, or a tenth of a trillionth of a second). So the electron motion along the material, rather than being like running down an empty street, is more like trying to walk through a very dense crowd. Typically, electrons move only with a speed of 1m per hour, they are slower than snails.

Though the electrons collide with something very frequently in the material, these collisions do take a finite time to occur. Just like if you are walking through a crowd, sometimes there are small empty spaces where you can walk a little faster for a short distance. If it were possible to follow the electrons on an extremely fast (femtosecond) time scale, then you would expect to see that when the battery is first turned on, for a very short time, the electrons really do fly unperturbed through the material before they bump into anything. This is exactly what scientists at the Max-Born-Institute in Berlin recently did in a semiconductor material and report in the current issue of the journal Physical Review Letters [volume 107, 256602 (2011)]. Extremely short bursts of terahertz light (1 terahertz = 1012 Hz, 1 trillion oscillations per second) were used instead of the battery (light has an electric field, just like a battery) to accelerate optically generated free electrons in a piece of gallium arsenide. The accelerated electrons generate another electric field, which, if measured with femtosecond time resolution, indicates exactly what they are doing. The researchers saw that the electrons travelled unperturbed in the direction of the electric field when the battery was first turned on. About 300 femtoseconds later, their velocity slowed down due to collisions.

In the attached movie, we show a cartoon of what is happening in the gallium arsenide crystal. Electrons (blue balls) and holes (red balls) show random thermal motion before the terahertz pulse hits the sample. The electric field (green arrow) accelerates electrons and holes in opposite directions. After onset of scattering this motion is slowed down and results in a heated electron-hole gas, i.e., in faster thermal motion.
 
The present experiments allowed the researchers to determine which type of collision is mainly responsible for the velocity loss. Interestingly, they found that the main collision partners were not atomic vibrations but positively charged particles called holes. A hole is just a missing electron in the valence band of the semiconductor, which can itself be viewed as a positively charged particle with a mass 6 times higher than the electron. Optical excitation of the semiconductor generates both free electrons and holes which the terahertz bursts, our battery, move in opposite directions. Because the holes have such a large mass, they do not move very fast, but they do get in the way of the electrons, making them slower.
Such a direct understanding of electric friction will be useful in the future for designing more efficient and faster electronics, and perhaps for finding new tricks to reduce electrical resistance.

Contact:

Dr. Michael Wörner, Tel.: +49-30 6392 1470

Prof. Dr. Klaus Reimann, Tel. +49-30 6392 1476

Prof. Dr. Thomas Elsässer, Tel. +49-30 6392 1400

Two former PhD students of the Max-Born-Institute receive the Carl Ramsauer Prize 2011

14th November 2011

Dr. Christian Eickhoff and Dr. Wilhelm Kühn are among this year's winners of the Carl Ramsauer Prize of the Physikalische Gesellschaft zu Berlin (PGzB), an award for outstanding PhD work. 

Dr. Christian Eickhoff's dissertation was devoted to “Time-resolved two-photon photoemission at the Si(001)-surface: Hot electron dynamics and two-dimensional Fano resonance.” He investigated the (100) surface of silicon, the basic material of the present semiconductor technology. The ongoing miniaturization and increasing efficiency of devices require a basic understanding of the electronic and and dynamic properties of carriers at the Si surface. Eickhoff designed a complex experiment consisting of a laser system which provides widely tunable photon energies, and a ultrahigh vacuum setup including a two-dimensional detector for photoelectrons. He observed the dynamics of optically excited electrons in surface states and in the conduction band of silicon with a time resolution of several tens of femtoseconds (1 fs = 10¹⁵ sec). He also investigated the influence of elastic and inelastic scattering processes on energy relaxation and surface recombination. He showed how optical excitation of carriers leads to complex two-dimensional interference phenomena which are accounted for by Fano resonances in the initial and intermediate state. His work explained why the elevated temperature of hot electrons in the conduction band is maintained over an unexpectedly long period of many picoseconds (1 ps = 10¹² sec). The extraction of hot electrons may find application for increasing the efficiency of third-generation solar cells.

Dr. Wilhelm Kühn has developed a novel method of nonlinear spectroscopy in the THz frequency domain (1 THz = 1012 Hz) and applied it in basic solid state research. The nonlinear interaction of light and matter is measured in two independent dimensions in time to derive two-dimensional (2D) spectra in the frequency domain. Such spectra give insight into the couplings between different excitations of the system under study and their time evolution. THz oscillations are extremely slow compared to visible light, an oscillation period lasts approximately 250 fs. Focusing the THz beam allows for generating high electric fields (ca. 300 kV/cm) and applying them for the acceleration of charge carriers in solids. With this novel method, Kühn studied the transport properties of electrons in the semiconductor gallium arsenide (GaAs).  He observed that strongly accelerated electrons move without appreciable friction ('ballistic motion'). At very high kinetic energies, however, they slow down and are accelerated into the opposite direction, due to their negative effective mass. In this way, the resulting circular motion of electrons, the so-called Bloch oscillations, was observed for the first time in a bulk crystal.
In a second experiment, Kühn investigated a semiconductor model system displaying an extremely strong coupling of electrons and the crystal lattice. He was able to show the formation of a new particle, the polaron which consists of an electron and a cloud of phonons. His results which were confirmed by theoretical calculations, also demonstrate how and into which channels electron energy is transferred to the crystal lattice.

More Information:

Dr. Christian Eickhoff, Freie Universität Berlin, Tel.: 030 838-56047
Tutor: Prof. Martin Weinelt, Freie Universität Berlin, Tel.: 030 838-56060

Dr. Wilhelm Kühn, Tel.: 0163-7356544
Tutor: Prof. Thomas Elsässer, Max-Born-Institute, Tel.: 030 6392 1401

Physikalische Gesellschaft zu Berlin, Managing Director Prof. Dr. Holger Grahn
Tel.: 030 20377-318

Imaging the Kramers-Henneberger atom

15th September 2011

Today laser pulses with electric fields comparable to or higher than the electrostatic forces binding valence electrons in atoms and  molecules have become a routine tool with applications in laser acceleration of electrons  and ions, generation of short wavelength emission from plasmas  and clusters, laser fusion, etc. Intense fields are also naturally created during laser filamentation in the air  or due to local field enhancements in the vicinity of metal nanoparticles. One would expect that very intense fields would always lead to fast ionization of atoms or molecules. However, recently the experimental team at the MBI division B observed acceleration of neutral atoms at the rate of 1015 m/sec2 when exposing these atoms to very intense infrared (IR) laser pulses [1]. Thus, substantial fraction of atoms remained stable during the pulse. What is the structure of these exotic laser-dressed atoms surviving superatomic fields? Can it be directly imaged using modern experimental tools? Using ab-initio calculations for potassium atom, we show [2] how the electronic structure of these stable "laser-dressed" atoms can be unambiguously identified and imaged in the angle resolved photoelectron spectra obtained with standard femtosecond laser pulses and velocity map imaging techniques, see e.g. recent experiments [3,4]. We find that the electronic structure of these atoms follows the theoretical predictions made over 40 years ago by W. Henneberger [5], that have so far remained unconfirmed experimentally and thus not generally accepted. We also show that the so-called Kramers-Henneberger (KH) atom is formed and can be detected even before the onset of stabilization regime. Our findings open the way to visualizing and controlling bound electron dynamics in strong laser fields and reexamining its role in various strong field processes, including microscopic description of high order Kerr non linearities and their role in laser filamentation [6].

Fig. 1 Direct visualization of the exotic  Kramers-Henneberger  atom in the photoelectron spectra: Angle and energy-resolved photoelectron spectra for potassium interacting with a 800 nm, 1.4·1013 W/cm2, 65 fs laser pulse (pz and pρ are electron momenta along and perpendicular to the laser polarization).Click to enlarge.

1. Eichmann et al., “Acceleration of neutral atoms in strong short-pulse laser fields”, Nature, 461, 1261-1264 (2009).
2. Felipe Morales, Maria Richter, Serguei Patchkovskii and Olga Smirnova,
“Imaging the Kramers-Henneberger atom”, PNAS, doi:10.1073/pnas.1105916108.
3. Wollenhaupt M., Krug M., Köhler J., Bayer T., Sarpe-Tudoran C. & Baumert T., “Photoelectron angular distributions from strong-field coherent electronic excitation”, Appl. Phys. B, 95, 245  (2009).
4. Schuricke M., Zhu G., Steinmann J., Simeonidis K., Ivanov I., Kheifets A., Grum-Grzhimailo A. N., Bartschat K., Dorn A. & Ullrich J., “ Strong-field ionization of lithium”, Phys. Rev. A, 83, 023413 (2011).
5. W. Henneberger, “Perturbation method for atoms in intense laser fields”, Phys. Rev. Lett., 21,838 (1968).
6. Béjot et al., “Higher-Order Kerr Terms Allow Ionization-Free Filamentation in Gases”, Phys. Rev. Lett., 104, 103903 (2010).

Contact: F. Morales, M. Richter, O. Smirnova, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie

Electrons and Lattice Vibrations—A Strong Team in the Nano World

4th August 2011

Semiconductor electronics generates, controls, and amplifies electrical current in devices like the transistor. The carriers of the electric current are mobile electrons, which move with high velocities through the crystal lattice of the semiconductor. Doing this, they lose part of their kinetic energy by causing atoms in the lattice to vibrate. In semiconductors like gallium arsenide the positively and negatively charged ions of the crystal lattice vibrate with an extremely short period of 100 fs (1 fs = 10^-15 s = 1 billionth part of one millionth of a second). In the microcosm of electrons and ions such vibrations are quantized. This means that the vibrational energy can only be an integer multiple of a vibrational quantum, also known as a phonon. When an electron interacts with the crystal lattice (the so called electron-phonon interaction), energy is transferred from the electron to the lattice in the form of such vibrational quanta.

Berlin researchers report in the latest issue of the scientific journal Physical Review Letters that the strength of the electron-phonon interaction depends sensitively on the electron size, i.e., on the spatial extent of its charge cloud. Experiments in the time range of the lattice vibration show that reducing the electron size leads to an increase of the interaction by up to a factor of 50. This results in a strong coupling of the movements of electrons and ions. Electron and phonon together form a new quasi particle, a polaron.

To visualize this phenomenon, the researchers used a nanostructure made from gallium arsenide and gallium aluminum arsenide, in which the energies of the movements of electrons and ions were tuned to each other. The coupling of both movements was shown by a new optical technique. Several ultrashort light pulses in the infrared excite the system under study. The subsequent emission of light by the moving charge carriers is measured in real time. In this way two-dimensional nonlinear spectra (see Fig.) are generated, which allow the detailed investigation of coupled transitions and the determination of the electron-phonon coupling strength. From the coupling strength one finds the size of the electron cloud, which is just 3-4 nanometers (1 nanometer = 10^-9 m = 1 billionth of one meter). Furthermore, this new method shows for the first time the importance of electron-phonon coupling for optical spectra of semiconductors. This is of interest for the development of optoelectronic devices with custom-tailored optical and electric properties.

More information:

Publications: W. Kuehn et al., Phys. Rev. Lett. 107 (6), 067401(5),(2011); J. Phys. Chem. B 115, 5448 (2011).

Contact: Klaus Reimann, M. Wörner, T. Elsässer, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Tel.: +49 30 6392 1470

CRASY Spectroscopy Plays with Quantum Physics

Scientists at the Max-Born-Institute developed a novel spectroscopic method for the simultaneous measurement of molecular structure and composition. They reported their work in Science.

 
In our daily experience, the observation of multiple material properties is a trivial task: Even a small kid will have no trouble to sort his building blocks according to shape and color simultaneously. But in the world of atoms and molecules, every observation must conform to the laws of quantum physics, which state that an observation always changes the observed system. Therefore, simultaneous observation of multiple molecular properties is a tricky proposition.

Scientists can play with a large set of spectroscopic tools when they wish to analyze specific properties of molecules. Rotational spectroscopy, for example, can resolve different molecular structures, because each molecule rotates with a characteristic set of frequencies. Mass spectrometry can determine the mass of a molecule and its fragments, and therefore offers information about the atomic composition of the sample. To date, experiments such as rotational spectroscopy or mass spectrometry were only performed separately. The method of Correlated Rotational Alignment Spectroscopy (CRASY) now allows the simultaneous (“correlated”) measurement of both, atomic composition and molecular structure.


CRASY experiments resolve mass and structure
of inseparable compounds, such as the depicted CS2 isotopes.

To perform this experiment, the scientists used an experimental trick: They first used an ultra-short laser pulse to initiate a rotational motion in each molecule of a molecular ensemble. After a short time, a second laser pulse was used to remove an electron, i.e., to ionize the molecules. The mass of all molecular ions was then determined in a mass spectrometer. The rotational motion turns the molecules in space (“rotational alignment”), and thereby affects the probability to ionize. When the molecules are allowed to rotate for different amounts of time, the rotational motion is directly reflected in the number of detected molecular ions and the rotational frequencies can be calculated. With the simultaneous determination of rotational frequencies and masses, the researchers overcame the limits of the individual spectroscopic methods and obtained correlated information on molecular structure and atomic composition.

„CRASY experiments contain much more information than conventional spectroscopic experiments, because the information content scales as the product of that in individual experiments”, claims Thomas Schultz from the MBI. This should permit the investigation of increasingly complex systems. The researchers first demonstrated their technique with the analysis of rotational constants for ten isotopes in a natural carbon disulfide sample. In a single experiment, they were able to reproduce all rotational constants in the literature and to determine three additional constants, which were previously inaccessible by spectroscopic measurements. “As compared to conventional rotational spectroscopy, we only require minute amounts of sample and the sample can be highly impure”, continues Schultz. In the future, the researchers plan to use CRASY experiments for the analysis of photochemical reactions in DNA bases.

More information:

Contact: Dr. Thomas Schultz, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy

Electron Ping Pong in the Nano-world

5th May 2011

An international team of researchers succeeded to control and monitor strongly accelerated electrons from nano-spheres with extremely short and intense laser pulses.
When intense laser light interacts with electrons in nanoparticles that consist of many million individual atoms, these electrons can be released and strongly accelerated. Such an effect in nano-spheres of silica was recently observed by an international team of researchers in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics. The researchers report how strong electrical fields (near-fields) build up in the vicinity of the nanoparticles and release electrons. Driven by the near-fields and collective interactions of the charges resulting from ionization by the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that were observed so far for single atoms. The exact movement of the electrons can be precisely controlled via the electric field of the laser light. The new insights into this light-controlled process can help to generate energetic extreme ultraviolet (XUV) radiation. The experiments and their theoretical modeling, which are described by the scientists in the journal “Nature Physics,” open up new perspectives for the development of ultrafast, light-controlled nano-electronics, which could potentially operate up to one million times faster than current electronics.

Electron acceleration in a laser field is similar to a short rally in a ping pongmatch: a serve, a return and a smash securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. An international team, in which Prof. Marc Vrakking from the Max-Born-Instiute in Berlin participates, was now successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light-fields.
The researchers illuminated silica nanoparticles with a size of around 100 nm with very intense light pulses, lasting around five femtoseconds (one femtosecond is a millionth of a billionth of a second). Such short laser pulses consist of only a few wave cycles. The nanoparticles contained around 50 million atoms each. The electrons are ionized within a fraction of a femtosecond and accelerated by the electric field of the remaining laser pulse. After travelling less than one nanometer away from the surface of the nano-spheres, some of the electrons can be returned to the surface by the laser field to the surface, where they were smashed right back (such as the ping pong ball being hit by the paddle). The resulting energy gain of the electrons can reach very high values. In the experiment electron energies of ca. 60 times the energy of a 700 nm wavelength laser photon (in the red spectral region of light) have been found.
For the first time, the researchers could observe and record the direct elastic recollision phenomenon from a nanosystem in detail. The scientists used polarized light for their experiments. With polarized light, the light waves are oscillating only along one axis and not, as with normal light, in all directions. “Intense radiation pulses can deform or destroy nanoparticles. We have thus prepared the nanoparticles in a beam, such that fresh nanoparticles were used for every laser pulse. This was of paramount importance for the observation of the highly energetic electrons”, explains Prof. Eckart Rühl from the Free University of Berlin. The accelerated electrons left the atoms with different directions and different energies. The flight trajectories were recorded by the scientists in a three-dimensional picture, which they used to determine the energies and emission directions of the electrons. “The electrons were not only accelerated by the laser-induced near-field, which by itself was already stronger than the laser field, but also by the interactions with other electrons, which were released from the nanoparticles,” describes Prof. Matthias Kling from the Max Planck Institute of Quantum Optics in Garching about the experiment. Finally, the positive charging of the nanoparticle-surface also plays a role. Since all contributions add up, the energy of the electrons can be very high. “The process is complex, but shows that there is much to explore in the interaction of nanoparticles with strong laser fields,” adds Kling. The electron movements can also produce pulses of extreme ultraviolet light when electrons that hit the surface do not bounce back, but are absorbed releasing photons with wavelengths in the XUV. XUV light is of particular interest for biological and medical research. “According to our findings, the recombination of electrons on the nanoparticles can lead to energies of the generated photons, which are up to seven-times higher than the limit that was so far observed for single atoms,” explains Prof. Thomas Fennel from the University of Rostock. The evidence of collective acceleration of electrons with nanoparticles offers great potential. Matthias Kling believes that “From this may arise new, promising applications in future, light-controlled ultrafast electronics, which may work up to one million times faster than conventional electronics.”

More information:

Contact: Prof. Dr. Marc Vrakking, Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy

Controlled near-field enhanced electron acceleration from dielectric nanospheres with intense few-cycle laser fields
Sergey Zherebtsov, Thomas Fennel, Jürgen Plenge, Egill Antonsson, Irina Znakovskaya, Adrian Wirth, Oliver Herrwerth, Frederik Süßmann, Christian Peltz, Izhar Ahmad, Sergei A. Trushin, Vladimir Pervak, Stefan Karsch1, Marc J.J. Vrakking, Burkhard Langer, Christina Graf, Mark I. Stockman, Ferenc Krausz, Eckart Rühl, Matthias F. Kling
Nature Physics, 24. April, doi: 10.1038/NPHYS1983

Figure 1:
Mechanism of the acceleration of electrons near silica nanospheres. Electrons (depicted as green particles) are released by the laser field (red wave). These electrons are first accelerated away from the particle surface and then driven back to it by the laser field. After an elastic collision with the surface, they are accelerated away again and reach very high kinetic energies. The figure shows three fsnapshots of the acceleration (from left to right): 1) the electrons are stopped and forced to return to the surface , 2) when reaching the surface, they elastically bounce right back 3) the electrons are accelerated away from the surface of the particle reaching high kinetic energies.

Figure 2:
Amplified near-fields at the poles of a silica nanosphere. The local field on the polar axis is plotted as function of time, where time within the few-cycle wave runs from the lower right to the upper left. The fields show a pronounced asymmetry along the polarization axis of the laser (i.e. along the rims and valleys of the wave). This asymmetry leads to higher energies gained by electrons on one side of the nanoparticle as compared to the other side. For the given example the most energetic electrons are emitted from the backside, where the highest peak field is reached. The energies of the electrons and their emission directions are determined from the experiment.
Courtesy of Christian Hackenberger/LMU

Wilhelm-Ostwald-Prize for Young Scientists 2010

16th February 2011

The Wilhelm-Ostwald-Gesellschaft Grossbothen e.V., the German Bunsen Society for Physical Chemistry, and the German Chemical Society have awarded the Wilhelm-Ostwald-Prize for Young Scientists 2010 to Dr. Ingo Barth (Max-Born-Institut Berlin) for his PhD thesis "Quantum control of electron and nuclear circulation, ring currents, and induced magnetic fields in atoms, ions and molecules by circularly polarized laser pulses".
Ingo Barth did his Ph.D. at the Free University of Berlin in the theoretical chemistry group of Prof. Dr. Jörn Manz. His thesis develops the quantum theoretical basis for excitations of ring currents and induced magnetic fields in molecules or molecular or atomic ions, by means of circularly polarized laser pulses. The main goal is to generate stationary electronic and nuclear ring currents in degenerate excited electronic or vibrational states. The advantage of Barth's new approach is that circularly polarized laser pulses are typically two orders of magnitude more efficient than traditional external magnetic fields. In addition, they allow to control the ring currents, e.g. to drive them along selective bonds. Barth also predicts that the magnetic fields in the center of the ring currents should be huge.
Following the example of the fundamental interdisciplinary work of Ostwald, Barth's PhD thesis provides new links between different fields, including theoretical chemistry, physical chemistry, theoretical physics, experimental physics, and mathematics. The results are also relevant for applications to special molecules of organic chemistry. Barth's thesis has already initiated new directions of research in organic chemistry, physical chemistry, and experimental physics.
It is worth noting that Ingo Barth was born deaf. In fact, among the community of the deaf, he is the first with a Ph.D. in chemistry in Germany. As a teacher of the German Sign Language, he has also built a new bridge to the humanities. Specifically, he has expanded the vocabulary of the German Sign Language by around 500 technical terms of theoretical and physical chemistry.
The prize is endowed with 2,500 Euros and will be awarded during a meeting of the Wilhelm-Ostwald-Gesellschaft in Grossbothen near Leipzig, the workplace of the Nobel Prize laureate for chemistry in 1909, Wilhelm Ostwald, on March 12, 2011. (Location: Wilhelm-Ostwald-Park, 04668 Grossbothen, Grimmaer Str. 25, Haus Werk)

More information:

Contact: Dr. Ingo Barth, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie

BLiX – the Leibniz-Application Lab of the Max Born Institute at TU Berlin

4th February 2011

On February, 4 the official grand opening of BLiX - the "Berlin Laboratory for innovative X-ray Technologies" www.blix.tu-berlin.de - was celebrated at TU (Technische Universität) Berlin. BLiX represents an Application Lab in the knowledge triangle university, research and enterprises and is jointly operated by the Institut für Optik und Atomare Physik of the TU Berlin and the Max-Born Institute. BLiX is a "Leibniz-Applikationslabor" of MBI.
BLiX is attached to the endowed professorship "Analytical X-ray Physics" held by Birgit Kanngießer. The laboratories of BLiX, completely renovated by TU cover an area of about 250 sqm and provide state of the art equipment for users from science and industry. A novel thin disk laser system as a driver laser for a plasma based highly brilliant x-ray source has been developed at MBI and transferred to BLiX. As a result of a BMBF joint research project a laboratory based x-ray microscope will be transferred from MBI to BLiX in the next months. The IOAP will provide a 3D micro XRF system for sensitive samples like cultural heritage objects as well as a novel x-ray spectrometer. In addition BLiX will provide lecture rooms for training and education in the field of x-ray physics. The current BLiX staff consists of 10 employee. It is managed by Dr. W. Malzer (TU) und Dr. H. Stiel (MBI).

More information:

Contact: Dr. Wolfgang Malzer, Technische Universität Berlin
Dr. Holger Stiel, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie

Holography with electrons

16th December 2010

The principle of holography was discovered in 1947 by the Hungarian scientist Dennis Gábor, in connection with attempts to improve the resolution of electron microscopes. The experimental realization of the concept of holography had to wait, however, until the mid-60s. Holograms were then made using newly-discovered laser light sources, rather than with electrons. Physicists from the Max Born Institute in Berlin have now returned to the use of electrons in holography. A special element in their approach is that the electrons that image the object are made from the object itself using a strong laser. A report is published in this week´s issue of Science.
Holography, as it is encountered in everyday life, uses coherent light, that is, a source of light where all the emitted light waves march in step. This light wave is divided into two parts, a reference wave and an object wave. The reference wave directly falls onto a two-dimensional detector, for example a photographic plate. The object wave interacts with and scatters off the object, and is then also detected. The superposition of both waves on the detector creates interference patterns, in which the shape of the object is encoded.
What Gábor couldn´t do, to construct a source of coherent electrons, is commonplace in experiments with intense laser fields. With intense, ultra-short laser fields, coherent electrons can readily be extracted from atoms and molecules. These electrons are the basis for the new holography experiment, which was carried out using Xe atoms. Marc Vrakking describes what happens: ´In our experiment, the strong laser field rips electrons from the Xe atoms and accelerates them, before turning them around. It is then as if one takes a catapult and shoots an electron at the ion that was left behind. The laser creates the perfect electron source for a holographic experiment´.
Some of the electrons re-combine with the ion, and produce extreme ultra-violet (XUV) light, thereby producing the attosecond pulses that are the basis for the new attosecond science program that is under development at MBI. Most electrons pass the ion and form the reference wave in the holographic experiment. Yet other electrons scatter off the ion, and form the object wave. On a two-dimensional detector the scientists could observe holographic interference patterns caused by the interaction of the object wave with the Coulomb potential of the ion.
In order to successfully carry out the experiments, certain conditions had to be met. In order to create the conditions for holography, the electron source had to be put as far away as possible from the ion, ensuring that the reference wave was only minimally influenced by the ion. The experiments were therefore carried out in the Netherlands, making use of the mid-infrared free electron laser FELICE, in a collaboration that encompassed – among others – the FOM Institutes AMOLF and Rijnhuizen. At FELICE, the Xe atoms where ionized using laser light with a 7 ?m wavelength, creating ideal conditions for the observation of a hologram.
The ionization process produces the electrons over a finite time interval of a few femtoseconds. Theoretical calculations under the guidance of MBI Junior Groupleader Olga Smirnova show that the time dependence of the ionization process is encoded in the holograms, as well as possible changes in the ion between the time that the ionization occurs and the time that the object wave interacts with the ion. This suggests a big future promise for the new technique. As Vrakking states: "So far, we have demonstrated that holograms can be produced in experiments with intense lasers. In the future we have to learn how to extract all the information that is contained in the holograms. This may lead to novel methods to study attosecond time-scale electron dynamics, as well as novel methods to study time-dependent structural changes in molecules".
The following highlight (also from 16th December 2010) stands in thematical connection with this.

More information:

DOI:10.1126/science.1198450
Original article on demand

Contact: Prof. Marc Vrakking, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Tel.: 030-6392 1200, Mobil: 0151-57153446, further e-mail

Understanding the "Ionization Surprise"

16th December 2010

In 2009 the Journal Nature Physics called it the "Ionization Surprise". Where it had been commonly thought that the ionization of atoms by strong laser fields was meanwhile well-understood, novel experiments where rare gas atoms were ionized using relatively long (few-µm) wavelength laser light suddenly revealed an unexpected and universal low-energy feature that defied explanation. In this week´s issue of Physical Review Letters, scientists from the University of Rostock, the Max-Planck Institut für Kernphysik in Heidelberg and the Max-Born Institute provide an explanation.
Ionization of atoms by strong laser fields plays an important role in today´s ultrafast laser laboratories. It is at the basis of important techniques such as high-harmonic generation, which allows the generation of attosecond (1 as = 10^-18 s) laser pulses, and furthermore allows the development of tomographic methods that make it possible to observe ultrafast electronic and atomic movements on the attosecond to few femtosecond (1 fs = 10^-15 s) timescale. Theoretical methods for describing strong laser field ionization were already developed a few decades ago. They are commonly based on the so-called “strong-field approximation” (SFA), which argues that after ionization the motion of the ionized electrons is largely determined by the electric field of the ionizing laser, and hardly by the Coulomb force that the electron and the ion left behind exert on each other.
For several decades the strong-field approximation has served scientists well and has allowed to understand many observations that were experimentally made in connection of with the strong field laser ionization. That is to say, until now. In a remarkable paper last year, scientists from the US and Germany reported the observation of a new phenomenon in strong-field laser ionization, namely a very pronounced peak at low energies in the photoelectron kinetic energy distribution, that contained as many as 50% of the emitted photoelectrons. Remarkably, its physical origin could not be identified.
In the new paper, the Rostock, Heidelberg and MBI scientists argue that it is the failure to include the Coulomb attraction between the departing electron and the ion left behind that is at the root of the low energy feature. They developed a novel theoretical description of strong-field ionization process, which in its initial stages mimics the traditional SFA approach, but then switches to calculating trajectories that the electrons follow in the combined Coulomb + laser field. This approach convincingly reproduces the low energy feature, and shows that it is caused by electrons that are pushed back-and-forth by the oscillatory laser field. In this process the electrons are brought into close proximity to the ion, which strongly disturbs the electron orbit, leading to a situation where the electron can just barely escape the attraction of the ion.
The 'Coulomb-corrected' SFA formalism based on interfering quantum orbits described above, not only solves the mystery of the “Ionization Surprise” but was also instrumental in related work on the appearance of holographic structures in strong-field ionization, which appears in Science Express this week.

More information:

10.1103/PhysRevLett.105.253002

Contact: Prof. Marc Vrakking, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Tel.: 030-6392 1200, Mobil: 0151-57153446, further e-mail
Prof. Dieter Bauer, Universität Rostock, Institut für Physik, Tel.: 0381 498- 6940

Innovation Prize Berlin Brandenburg for femtosecond x-ray plasma source mapping molecular structures in real-time

10th December 2010

The company IfG – Institute for Scientific Instruments GmbH - and the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI) jointly received the Innovation Prize 2010 for their development of a laser-driven hard x-ray plasma source. The compact new device generates – driven by an amplified femtosecond laser – extremely short x-ray pulses of 100 fs duration (1 fs = 1010^-15s = 1 billionth of a millionth of a second) at kilohertz repetition rates. The x-ray pulses are synchronized with optical pulses from the laser system and allow for x-ray diffraction experiments with very high time resolution. During the award ceremony, Prof. Norbert Langhoff, IfG, and Prof. Thomas Elsaesser emphasized that 'this success is the result of an intense exchange and a close cooperation of scientists and engineers on the campus Berlin-Adlershof'. video
Basic properties and function of systems in molecular biology, nano- and/or sensor technology are connected with motions of atoms and molecules. Such motions occur on extremely short length and time scales. Understanding such processes requires their visualization in a time-resolved way by taking a sequence of 'snapshots' with an ultrafast 'camera'. This dream becomes reality with the new femtosecond x-ray plasma source enabling the direct imaging of ultrafast atomic and molecular motions with a 100 fs time resolution.
In the x-ray source, a hot plasma is generated by exciting a thin metal tape, e.g., made from copper, with intense femtosecond pulses from a commercial laser system. Electrons in the plasma are strongly accelerated by the high electric field of the laser pulses. They penetrate into the metal target and generate characteristic hard x-rays, similar to electrons in a conventional x-ray tube. The acceleration of electrons and, thus, the x-ray generation are limited in time to the 50 fs duration of the driving laser pulse, resulting in an extremely short x-ray flash. X-ray snapshots are produced by initiating the structure changing process in the sample with an optical pulse and diffracting the x-ray pulse from the excited sample. Recording such images for different delays of the x-ray pulse after the optical pulse gives a sequence of snapshots, the 'x-ray movie'.
The new x-ray source is unrivaled as a commercial device and represents a key prerequisite for the leading role of MBI in time-resolved structure research. Recently, MBI demonstrated the first x-ray powder diffraction study with femtosecond time resolution, a result that has been recognized internationally as a major breakthrough in the field. The present femtosecond x-ray research at MBI is supported through an Advanced Grant for Pioneering Frontier Research of the European Research Council. New areas of application of this technology will be materials research, nanotechnology, chemistry and pharmacy. IfG has built and sold 4 x-ray sources and is in negotiations with another 4 customers. The generated turnover has a net amount of 2 Million Euros.

Photo: PUBLICIS Berlin GmbH More information:

Contact: Prof. Dr. Thomas Elsässer, Tel +4930 6392 1401
Prof. Dr. Norbert Langhoff, IfG, Tel +4930 6392 6500

Even higher performance for high-power lasers

14th October 2010

Either with enormous pulse energy or with high repetition rate – this might describe the present state of the art of laser technology. Scientists of the Max Born Institute (MBI) in Berlin, in close cooperation with diode laser specialists of the Ferdinand-Braun-Institute (FBH), plan to combine the two properties. The goal are lasers which reach high single-pulse power at 100 shots per second or more. For the development of such novel lasers the scientists have obtained a grant of 3 million Euros from the European Union (European Fund for Regional Development - EFRD), the total volume of the project being 6 million Euros.
High-intensity lasers are a relatively young product of physics research. They are able to emit single light pulses of inconceivable power – much more than the worldwide capacity of all power plants together. For this purpose, the energy of a single pulse is concentrated in a period much shorter than a millionth of a millionth of a second. Due to their high single-pulse power these lasers are revolutionizing many fields of science, technology and medicine. They are being used for producing new states of matter, for ultra-precise materials processing or for the generation of particle or photon radiation with unprecedented properties. Within the next year even nuclear fusion by high-intensity lasers is expected to be demonstrated – maybe one step towards a relatively clean and virtually inexhaustible source of energy.
A technological void, however, exists in almost all of these lasers: the repetition rate of their light pulses is limited to ten shots per second (10 Hertz), often even considerably less. "Even though the single pulses are enormously powerful, the total power, i.e. the average power of conventional high-intensity lasers is little more than 10 watts. This corresponds to an energy-saving lamp", says MBI Director Professor Wolfgang Sandner, whose division is leading the new project.
The new MBI laser development leaps into this gap. For several years now, MBI has acquired a world leading position in the field of picosecond lasers of high pulse energy and repetition rate, i.e. high average power. Due to an innovative highly efficient laser cooling, these systems provide more than 100 shots per second. "In order to further boost the average power of these lasers we intend to increase the energy of the single pulses, first to a few joules, then probably to significantly more", announces MBI project coordinator Dr. Ingo Will. This kind of lasers would have an average power in the kilowatt range with a picosecond pulse duration and very high single-pulse energy which has not yet been achieved by any other laser.
High-power lasers of this kind are urgently needed as a technological basis for European large-scale projects such as ELI. The acronym stands for Extreme Light Infrastructure, the future most powerful laser for basic science in the world. First studies for the front-end of a 10 petawatt laser as demonstrator for the ELI project are already completed. The pump laser for this front-end is now being built at MBI and will be delivered to the Institut d'Optique in Palaiseau, France shortly.
The underlying concept of the new laser is a diode-pumped solid-state laser. The so-called thin disk laser architecture is one of the most promising laser architectures which has been adopted and further developed by MBI’s cooperating partners IfSW Stuttgart, DLR Stuttgart und TRUMPF Lasertechnik GmbH with financial support from the Berlin technology promotion action PROFIT and from the Leibniz programme SAW. The project partner FBH will provide novel pump diodes for the disk laser. Funding agency for the EFRD project is the Senatsverwaltung für Bildung, Wissenschaft und Forschung of the state of Berlin.
As a first milestone, the MBI researchers plan to pump a worldwide unique compact X-ray laser in their own institute. With a wavelength of 13 nanometres and a repetition rate of 100 Hertz this laser will generate coherent X-radiation in the laboratory which so far is possible only with large free electron lasers such as FLASH in Hamburg. In the long run, thin disk lasers can be used as an energy source for the next generation of high-intensity lasers. In addition MBI plans to set up a high-power attosecond source in cooperation with Prof. Marc Vrakking, the recently appointed director of MBI.

More information:

Contact: Dr. Ingo Will, Tel +4930 6392 1320
see also Pressemitteilung Forschungsverbund

Elementary particles starring a dance movie

16th August 2010

Scientists of the Max-Born-Institute in Berlin (Germany) measure directly the spatial positions of electrons and protons during a chemical reaction using ultrashort x-ray flashes
A chemical reaction generates new compounds out of one or more initial species. On a molecular level, the spatial arrangement of electrons and nuclei changes. While the structure of the initial and the product molecules can be measured routinely, the transient structures and molecular motions during a reaction have remained unknown in most cases. This knowledge, however, is a key element for the exact understanding of the reaction.
The ultimate dream is a "reaction microscope" which allows for an in situ imaging of the molecules during a reaction. The technological challenges of such an ultrafast "cinema" were mastered only recently. Using ultrashort x-ray pulses, researchers at the Max-Born-Institute in Berlin, Germany, now made a "molecular movie" of a chemical reaction with a resolution on atomic length (10^-10 meters) and time scales (10^-13 seconds).
Michael Woerner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Premont-Schwarz und Thomas Elsaesser report on a direct time-resolved observation of a chemical reaction in ammonium sulfate crystals [(NH₄)₂SO₄] in the most recent issue of The Journal of Chemical Physics. Using an advanced femtosecond laser system, they generate a blue pulse of 50 femtosecond duration (1 fs = 10^-15 seconds) which initiated the chemical reaction. After a very short period they probe the structure of the excited material with high spatial resolution using a synchronized x-ray flash with only 100 fs duration. The x-ray pulse is diffracted off a powder made of small crystallites (the so called Debye-Scherrer method. Measuring simultaneously many different x-ray reflections the physicists could reconstruct the transient distances of atomic lattice planes and in turn the three-dimensional distribution of electronic charge within the crystal. Taking x-ray snap shots at various delay times after triggering the reaction, they created a molecular movie according to the well known stroboscope effect.
Surprisingly, they observed a reversible chemical reaction which is fundamentally different from the already known, slow thermal phase transitions of ammonium sulfate. First, the blue flash caused a release of both a proton (positive charge) from the ammonium ion (NH₄)+ and an electron (negative charge) from the sulfate ion (SO₄)-. The two elementary particles merged to a hydrogen atom which jumps back and forth between two distant spatial positions. The corresponding motion is illustrated in the attached movie which was derived from the x-ray snapshots. The circles indicate the initial positions of the protons. The red blots show the motion of the hydrogen atoms after the chemical reaction.
Femtosecond x-ray powder diffraction demonstrated here for the first time can be applied to many other systems, for instance for investigating molecular magnets or for monitoring electron motions in (bio)molecular light harvesting complexes used in solar cells.

More information:

Search and Discovery article of Johanna Miller in Physics Today
Journal of Chemical Physics: Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction
J. Chem. Phys. 133, 064509 (2010); doi:10.1063/1.3469779
Contact: Dr. Michael Wörner, Tel +4930 63921470
s. auch Pressemitteilung Forschungsverbund

Electrons in motion

11th August 2010

The world of atoms and molecules is very different from our everyday experience. We like to think of electrons as little particles. "And that is to some extent a valid picture" explains Marc Vrakking, director at the Max Born Institute in Berlin, "but quantum mechanics also forces us sometimes to consider the wave-nature of electrons". Using this abstract representation of an electron, physicists can explain intriguing phenomena that in the end connect to our simple picture of the electron as a particle.
Because it is not possible to observe the motions of electrons directly, the European research team characterized this motion by a number of separate measurements that completely characterized the wave-nature of the electron, or, as Vrakking likes to call it "the electronic wavepacket". With this, the particle-like motion of the electron is completely defined.
In the experiment, the researchers have exploited the fact that waves can be made to interfere. In many respects their experiment resembled the famous double-slit experiment that Thomas Young performed at the start of the 19th century, where light fell onto a pair of slits and an interference pattern was observed on a screen placed downstream. The way to understand this observation is to view the incident light as a wave, and to view the interference pattern as a consequence of the fact that light passing through one slit may be in- or out-of-phase with light passing through the other. This observation cannot be explained when viewing the incident light as a collection of particles ("fotons").
In order to characterize an electronic wavepacket, the researchers have also used interference, and have exploited the interference between an (“unknown”) electronic wavepacket that they wanted to characterize and a reference ("known") wavepacket, that they produced with an attosecond laser pulse, by ionizing the atom. The properties of this reference wavepacket are fully known because they follow from the properties of the fully-known attosecond pulse. By overlapping and interfering the two wavepackets, and observing the interference pattern that resulted, they could extract all the properties of the unknown wavepacket.
Marc Vrakking explains the method: "Initially the reference wavepacket produced by the attosecond laser has a much higher energy than the unknown wavepacket In order to create the interference pattern, we therefore had to lift the unknown wavepacket to the same energy. We have done this using an infrared laser pulse. By varying the time-delay between the attosecond pulse and the infrared pulse we could acquire a whole series of interference patterns, which allowed to extract all that there is to be known about the unknown wavepacket." A full characterization of the electronic wavepacket follows from a knowledge of the energies, the populations and the relative phases of all the states that contribute to the wavepacket. Once known, the time-dependent motion of the electronic wave can be pictured, and the picture of a particle-like motion re-emerges, bringing us back to our everyday world.

More information:

Physical Review Letters: Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique
http://dx.doi.org/10.1103/PhysRevLett.105.053001
Contact: Prof. Dr. Marc Vrakking, Tel +4930 63921200
see also press releases Forschungsverbund

A look into the Interior of Molecule

10th Juni 2010

In order to not only observe, but also really understand a chemical reaction, scientists have to know how electrons move within molecules. Until now it was technically impossible to observe how electrons move within a molecule, because they move so incredibly fast. However, a group of European researchers including scientists from the Max Born Institute has now achieved this goal with the help of attosecond laser pulses.

An attosecond is a billionth of a billionth of a second. During an attosecond, light covers a distance of less than one millionth of a millimeter –equivalent to the distance from one end of a small molecule to the other. Physicists are therefore making great efforts to create attosecond laser pulses: using attosecond laser pulses they can “photograph” the movement of electrons within molecules, just like in a photo shoot.

In the European research team, Prof. Marc Vrakking, Director of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin worked together with groups from – among others - Milan, Amsterdam, Lund (Sweden), Garching, Lyon and Madrid. The physicists examined the hydrogen molecule (H₂) – which is nature’s simplest molecule, with just two protons and two electrons. The researchers wanted to find out exactly how ionization takes place in a hydrogen molecule. In this process, one electron is removed from the molecule, and the remaining electron undergoes a rearrangement. Marc Vrakking explains, “In our experiment we were able to show for the first time that with the help of an attosecond laser we really have the ability to observe the movement of electrons in molecules. First we irradiated a hydrogen molecule with an attosecond laser pulse. This led to the removal of an electron from the molecule – the molecule was ionized. In addition, we split the molecule into two parts using an infrared laser beam, just like with a tiny pair of scissors. This allowed us to examine how the charge distributed itself between the two fragments – since one electron is missing, one fragment will be neutral and the other positively charged. We knew where the remaining electron could be found, namely in the neutral part.”

Ever since the 1980s, scientists have been examining molecules and atoms with the help of femtosecond lasers – a femtosecond is one millionth of one billionth of a second, thus a thousand times slower than an attosecond. Using femtosecond lasers, the movement of atoms and molecules can be tracked, but not that of electrons. In 2001, researchers were able for the first time to produce a laser flash with a length of 250 attoseconds. This was followed by considerable efforts on the development of attosecond lasers as well as the control and measurement of the pulses. More recently, scientists are beginning to use the attosecond pulses to address problems in natural science, such as the first molecular application that was published now.

Although the European team’s experiments with attosecond lasers produced some of the results they had hoped for, there was also one surprise in store for the scientists. In order to be able to interpret the measurements even better, they involved a group of theoreticians from the University of Madrid in the project. The Spanish researchers’ work brought completely new insights. Dr. Felipe Morales from Madrid, who is currently working as a postdoctoral fellow at MBI, reports, “We nearly reached the limits of our computer capacity. We spent one and a half million hours of computer time to understand the problem.” The calculations showed that the complexity of the problem was far greater than previously thought. For example, it was shown that the electron that is removed from the molecule by the attosecond laser pulse also plays an important role in the subsequent dynamics of the molecular ion that is left behind. Vrakking describes it as follows, “We have not – as we originally expected - solved the problem. On the contrary, we have merely opened a door. But in fact this makes the entire project much more important and interesting.”

Nature-Paper: Electron localization following attosecond molecular photoionization
http://dx.doi.org/10.1038/nature09084

World record for shortest controllable time

10th May 2010

Lasers can now generate light pulses down to 100 attoseconds (1 attosecond = 10-18s = one billionth of the billionth of a second) thereby enabling real-time measurements on ultrashort time scales that are inaccessible by any other methods. Scientist at the Max Born Institute for Nonlinear Optics and Short Time Spectroscopy (MBI) in Berlin, Germany have now demonstrated timing control with a residual uncertainty of 12 attoseconds. This constitutes a new world record for the shortest controllable time scale. They reported about in Nature Photonics.
Light is an electromagnetic wave of very high frequency. In the visible domain, a single oscillation of the electric field only takes about 1200-2500 attoseconds. Consequently, an ultrashort laser pulse is comprised of a few of these oscillations. However, pulses from conventional short-pulse laser sources exhibit strong fluctuations of the positions of the field maximum relative to the pulse center. For maximum field strength, the center of the pulse has to coincide with a maximum of the electric field, as shown in Fig. 1 as a red curve. Consequently, methods have been developed to stabilize the position of the field maximum, i.e., the phase of the pulse.
Together with Vienna based laser manufacturer Femtolasers, MBI researchers in the group of Günter Steinmeyer have now developed a new method to control the phase of the pulse outside of the laser. In contrast to previous approaches, no manipulation inside the laser is necessary, which completely eliminates fluctuations of laser power and pulse duration and guarantees a strongly improved long-term stability. Correction of the pulse phase relies on a so-called acousto-optic frequency shifter, which is directly driven by the measured signal. Dr. Steinmeyer is convinced: "This direct correction of the phase dramatically simplifies many experiments in attosecond physics and frequency metrology."
Previously, stabilization of the position of the field maxima was only possible with a precision of about 100 attoseconds (10-16 s, corresponding to 1/20 of the wavelength), which is comparable to the shortest duration of attosecond pulses demonstrated so far. The new method allowed to push this limitation down to 12 attoseconds (1.2 x 10-17 s, 1/200 of the wavelength), which surpasses the atomic unit of time (24 attoseconds) by a factor of two. As the atomic unit of time marks the fastest possible time scale of processes in the outer shells of an atom, the new stabilization method will enable significant progress in the research on the fastest processes in nature.
This success strongly relied on a close collaboration with laser manufacturer Femtolasers who provided a specifically optimized laser for the joint experiment and is currently developing products based on this new method.
Contakt: Dr. habil. Günter Steinmeyer, Tel +4930 63921440
see also press release Forschungsverbund

Negative Mass and High Speed: How Electrons Go Their Own Ways

12th April 2010

Isaac Newton found in the 17th century that a force causes a body to accelerate. The inertial mass of the body is the ratio between force and acceleration, thus, given the same force, a light body is accelerated more strongly than a heavy body. A body's mass is positive, meaning that the acceleration is in the same direction as the force. Charged elementary particles as the free electron, which has a mass of only 10^-30 = 0. ...(29 zeros !)...1 kilograms, can be accelerated in electric fields to extremely high speeds. If the electric field is small, the motion of electrons in crystals is governed by the same laws. In this regime, the mass of a crystal electron is only a small part of the mass of a free electron.
Wilhelm Kuehn, Michael Woerner, Klaus Reimann and Thomas Elsaesser from the MBI have now demonstrated that crystal electrons in extremely high electric fields exhibit a completely different behavior. Their mass even becomes negative. They report in the latest issue of Physical Review Letters 104, 146602 (2010) that the electron is accelerated within the extremely short time of 100 femtoseconds = 0.000 000 000 000 1 seconds to a speed of 4 million kilometers per hour. Afterwards the electron comes to a stop and even moves backward. This means that the acceleration is in opposite direction to the force, which can only be explained by a negative inertial mass of the electron.

In the experiments, electrons in the semiconductor crystal gallium arsenide are accelerated by an extremely short electrical pulse with a field strength of 30 million Volts per meter. At the same time the speed of the electrons is measured with high precision as a function of time. The duration of the electric pulse is only 300 femtoseconds. This extremely short duration is essential as otherwise the crystal could be damaged.

DThe new results agree with calculations of the Nobel Prize winner Felix Bloch undertook more than 80 years ago. They open up a new regime of charge transport with new possibilities for future microelectronics devices. The observed frequencies are in the terahertz range (1 THz = 1000 GHz = 10¹² Hz), about 1000 times higher than the clock rate of the newest PCs.
see also press release Forschungsverbund

Olga Smirnova receives the Karl Scheel Prize 2010

5th. February 2010

Dr. Olga Smirnova, head of MBI's Juniorgroup in Theory on "Attosecond multielectron dynamics in molecules", receives the Kar Scheel Preis of the year 2010. The prestigeous prize of the Physikalische Gesellschaft zu Berlin (PGzB) is awarded for outstanding scientific work typically achieved after PhD. It includes a cash award of 5.000 €. More about the prize ...

Molecules in real-time – how hydrogen bonds determine structure and function.

5th January 2010

The European Research Council (ERC) has awarded Prof. Thomas Elsaesser an 'Advanced Grant' of 2.49 Million Euros. The project aims at elucidating extremely fast processes which determine the properties of hydrogen bonds in molecular systems.
Thomas Elsaesser who is with the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin, Germany, is one of the leading researchers in ultrafast science, studying ultrafast processes in condensed matter. His project is devoted to unraveling changes of molecular structures on the length scale of a chemical bond and the ultrashort time scale of molecular motions. This work will cover aspects of physics, chemistry and biology. ERC Advanced Grants allow exceptional established research leaders to pursue frontier research of their choice.

Hydrogen bonds are weak chemical bonds and represent a fundamental interaction in Nature. They determine the structure of biomolecules such as deoxyribonucleic acid (DNA), the basic carrier of genetic information in cells. On the other hand, they undergo fluctuations due to their weak binding forces. In water, this leads to extremely fast changes in the arrangement of molecules including the breaking and reformation of hydrogen bonds. Although hydrogen bonds have been studied for a long time, their structural dynamics which occur in the femtosecond time domain (1 femtosecond = 10^-15 s = one millionth of a billionth of a second), are understood only in part.
Within the project, novel methods of ultrafast optics in a wavelength range from the far-infrared to hard x-rays will be applied for investigating hydrogen bonds. A key goal consists in measuring molecular structures in real-time by initiating and reading out structure changes with ultrashort light pulses. X-ray pulses of a wavelength comparable to the length of a chemical bond allow for generating a sequence of 'snapshots' of molecular structure. Infrared pulses give insight into local motions of specific molecular groups. In the experiments, the interaction of DNA with its aqueous environment will be studied, i.e., the coupling of water molecules to different functional units of the DNA double helix, the fluctuations of the water shell around DNA, and the role of water for the redistribution and the transport of energy from DNA into the environment. Hydrogen bonds play a key role for a broad range of biochemical processes and, thus, the results are expected to be of broad relevance. In a second part of the project, structures generated by charge and/or proton transfer will be studied in hydrogen bonded molecular crystals. Such elementary chemical processes govern the electrical properties of the materials which are of interest for applications in novel ferroelectric devices.

Biographical information on Thomas Elsaesser is available at http://staff.mbi-berlin.de/elsasser/

Light pressure – the route to efficient laser ion acceleration

9th December 2009

One of the recent challenges in light-matter interaction consists in the unidirectional acceleration of charged particles by laser light. This can occur through a variety of laser-induced plasma phenomena or, more directly, through transfer of the unidirectional momentum of a propagating laser field, the so-called light pressure.
Utilization of the light pressure requires rather ambitious parameters of laser intensity and temporal pulse shape. In return, theory predicts rather favorable energy conversion efficiencies and narrow ion energy distributions which both are a prerequisites for many applications.
Scientists from the Max Born Institute (MBI) Berlin and from the Max Planck Institute for Quantum Optics (MPQ) Garching and LMU Munich were able to demonstrate this principle in recent experiments (Phys. Rev. Lett. 103 (24), 245003(2009)). The key in the process is to favor the momentum exchange between the laser photons and the target while suppressing unwanted electron heating. Two technologies are essential for this purpose: Ultra-high temporal contrast laser pulses (delivered by the High-Field-Laser at MBI-Berlin) on the one hand and ultimate thin diamond like carbon foils (produced at MPQ/LMU) on the other. The results demonstrate efficient ion beam generation while simultaneously reducing the kinetic energy spread of the ions.
See also Informationsdienst Wissenschaft in english and at Informationsdienst Wissenschaft in german

Carbonic acid now measured in liquid water

Carbonic acid, the hydrated form of carbon dioxide, is one of the most abundant molecules on Earth. Carbonic acid (H₂CO₃), has until now only been detected as isolated molecule in the gas phase and frozen in ice matrices. Adamczyk et al. now describe in a publication in Science Express (12 November 2009) how carbonic acid can be generated using photoacids and detected with transient infrared spectroscopy.
More information: see press releases (in English, German, French and Dutch), highlight and detailed project pages.

Acceleration of neutral atoms in strong short-pulse laser fields

The force experienced by a charged particle in an oscillating electric field is proportional to the cycle-averaged intensity gradient. Extremely strong kinematic forces are now observed on neutral atoms in short-pulse laser fields; the ponderomotive force on electrons is identified as the driving mechanism, leading to probably the highest observed acceleration on neutral atoms in an external field to date.
Scientists of the Max-Born-Institute reported about in the current issue of Nature. These results are featured in the cover story of Nature doi:10.1038/nature08445.
see also
press release Forschungsverbund
Making the paper, Nature 461, 1171 (2009)
Nature Cover
See also articles in:
Physics Update, Physics World, ProPhysik, scinexx, spektrumdirekt, chemie.de

Miniature particle accelerator: Micro-water droplets as a source for laser driven ion acceleration

The cover of Physical Review Letters (Vol. 103 Issue 13) shows a result of a recent MBI publication. In the underlying work (T.Sokollik et al., PRL 103, 135003 (2009)) ion acceleration from isolated spherical targets was investigated by proton imaging for the first time. Already in a previous work (S. Ter Avetisyan et al. PRL 2006), scientists from the Max-Born-Institute in Berlin found that laser irradiated water (or heavy water) droplets can generate a quasi-monoenergetic proton (or deuteron) beam. On the base of simulations they could argue that this, besides additional premises might be connected to a spatially asymmetric field structure which favours a directional emission. Using proton imaging now, the evidence for an advantageous field structure was found which leads to a directional ion beam emission using a micro-sphere target which is a versatile target system. The great advantage of such a system is the MHz repetition rate of droplet generation with a liquid jet. On the other hand, the use of evaporating liquids seems to have a drawback. In case of such targets which evaporate in vacuum, the presence of an ambient plasma counteracts the energy transfer between laser and ion beam. Current investigations aim to avoid these disadvantages of liquids and to explore further fundamental processes of isolated targets.

High harmonic interferometry of multi-electron dynamics in molecules

High harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and recombines with the parent molecular ion. The emission provides a snapshot of the structure and dynamics of the recombining system, encoded in the amplitudes, phases and polarization of the harmonic light. Here we show with CO2 molecules that high harmonic interferometry can retrieve this structural and dynamic information: by measuring the phases and amplitudes of the harmonic emission, we reveal 'fingerprints' of multiple molecular orbitals participating in the process and decode the underlying attosecond multi-electron dynamics, including the dynamics of electron rearrangement upon ionization. These findings establish high harmonic interferometry as an effective approach to resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the de Broglie wavelength of the recombining electron, and attosecond temporal resolution arising from the timescale of the recombination event.
The scientists Dr. Olga Smirnova of Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie and from National Research Council of Canada published their results at the current issue of Nature (22 July 2009) doi:10.1038/nature08253.

Multispot writing in fused glass

Nature photonics selected recent work from the ongoing collabortion between MBI and Université Jean Monnet at Saint Etienne, France, as one of their Research Highlights. We quote: "Multispot writing in fused glass, Opt. Express 17, 3531–3542 (2009). Owing to its highly deterministic and nonlinear absorption process, infrared femtosecond laser writing offers the means to create buried, localized structural modifications in transparent materials. By moving the sample with respect to the laser's focal point, three-dimensional structures can be inscribed. However, the fabrication of complex structures often involves long processing times. Cyril Mauclair and co-workers from France and Germany have now demonstrated that the problem of speed can be solved by parallel photoinscription that uses multiple laser spots with reconfigurable patterns. The trick is to use a periodical binary phase mask to spatially modulate the wavefront of the laser beam. By varying the period (cycling frequency) of the binary phase, the team show that a simple grating phase mask and therefore dynamic double-spot operation can be achieved. The team use a liquid-crystal spatial light modulator, addressed optically, to create the binary phase mask. A 800-nm Ti:sapphire laser emitting 150-fs pulses at a repetition rate of 10 kHz and with a power of 30 mW is used for the process. By controlling the motion of the sample, the team succeeded in manufacturing three-dimensional light dividers and fabricating wavelength-division demultiplexing devices in fused silica. They are confident that with sufficient energy, more machining foci can be used."

Hot Electrons in Carbon – Graphite behaves like a semiconductor

Markus Breusing, Claus Ropers und Thomas Elsaesser, three scientists from the Max-Born-Institute in Berlin, have now investigated the behavior of electrons in thin graphite films in real time. As they now report in Physical Review Letters (Volume 102, 086809/1-4 (2009)), they recorded the dynamics of electrons with an unprecedented temporal resolution of only 10 femtoseconds (one femtosecond is a millionth of a billionth of a second). Electrons were excited to high energy states with ultrashort laser pulses, and their return to equilibrium was observed. The individual steps of this process are temporally resolved, and the momentary distribution of electrons in the material is identified. Within 30 femtoseconds, electrons form a hot gas with temperatures of 2500 °C, which cools down to about 200 °C in only 500 femtoseconds. The energy dissipated in this process is transferred to the crystal lattice. After this process, the electrons slowly return to their initial states. For the first time, the study shows conclusively that, on ultrashort time scales, graphite behaves like a semiconductor, such as silicon or gallium arsenide, and not like a metal.
More...

Ionisation dynamics in the light of elliptically polarised femtosecond laser pulses

Scientists from the MBI (I.V. Hertel, I. Shchatsinin, T. Laarmann, N. Zhavoronkov, H.-H. Ritze, and C. P. Schulz) have shown, that elliptically polarized, ultrashort light pulses allow a particularly clear view into the dynamics of ionisation processes in intense laser fields. They found e.g. convincing evidence for a so called „doorway state“ in the football molecule C60 (Buckminsterfullerene), which is populated in a first step prior to ejecting an electron from the molecule. Subsequently the molecule is so strongly deformed, that many other electrons can participate in the process and several of them can finally leave the system – on a time scale of a few femtoseconds. The work, recently published in the renowned Journal Physical Review Letters (Phys. Rev. Lett. 102, 023003 (2009)) has also been included in the Virtual Journals on "Ultrafast Science" and "Nanoscale Science & Technology".

Carl Ramsauer Award 2008 for two former PhD students of the Max Born Institute
11. November 2008

Dr. Claus Ropers (31) and Dr. Anke B. Schmidt (31) are two of the winners of this year’s Carl-Ramsauer prize of the Berlin Physical Society. Claus Ropers did his doctor degree at the Humboldt University and Anke Schmidt is awarded for her doctoral thesis at the Free University. They did their research work at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy. The prize will be awarded on the 19th of November.
More (in german) ...

Hydrogen bonds: Scientists find new mechanism

Water’s unrivalled omnipresence and the crucial role it plays in life drives scientists to understand every detail of the processes underlying waters unusual properties on a molecular level. Bernd Winter and his colleagues from the Max Born Institute (MBI) and the Berlin Electron Storage Ring for Synchrotron-Radiation (BESSY) have now been able to study a hitherto unknown property of the negatively charged hydroxide ion (OH-) of water. They report about this in the prestigious science magazine Nature (E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, und Winter, B., Nature, 455, 89-91,2008)

The crystal strikes back

The dashing start of electrons in a crystal does not remain without consequences for their further fate. This is reported by the Berlin researchers Peter Gaal, Wilhelm Kuehn, Klaus Reimann, Michael Woerner and Thomas Elsaesser of the Max Born Institute and Rudolf Hey of the Paul Drude Institute in the latest issue of the magazine Nature (Vol. 450, Page 1210). They examined the ultrafast movement of electrons in a gallium arsenide crystal exposed for a short time to a very high electrical field. This conceptually new experiment shows for the first time a collective, oscillatory motion of the electrons with ultrahigh frequency, which arises additionally to the well-known drift motion of these particles. This newly discovered effect could play an important role in connection with the miniaturization of electronic devices.
More information: Project 3-02,  Press release of AlphaGalileo

Molecular Pirouettes - Researchers in Berlin and Munich watch closely molecules as they reorient themselves during ultrafast photochemical reactions

Ultrafast intramolecular electronic charge separation during photo-chemical reactions cause up to tenthousand surrounding molecules to perform aligning pirouettes. Researchers observed for the first time such light induced reorientations in an organic molecular crystal. Scientists of the Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy and of the Ludwig-Maximilians-University in Munich report their recent experimental results in the current issue of the journal Physical Review Letters (vol. 98, page 248301).
In their study they initiated a separation of positive and negative electronic charge in a small number of particular molecules with extremely short light pulses. In turn the surrounding molecules responded by aligning their respective dipole axes along the photoinduced electric fields. The researchers observed this fundamental process for the first time by means of femtosecond x-ray diffraction with high spatial precision and in real time.
More information: Project 3-04

Nature features two recent MBI publications

Recent publications from the MBI on aqueous proton transfer and on ultrashort spatially confined electron pulses have been featured in the News and Views section of Nature magazine.
In the 15 March 2007 issue, James T. Hynes highlights recent work on the base-induced solvent switch model for aqueous proton transfer. More information: see contribution in News and Views of Nature, the original publication in Angewandte Chemie International Edition and our own project pages.
In the 29 March 2007 issue, Herman Batelaan and Kees Uiterwaal discuss recent work on the generation of femtosecond electron pulses from nanoscale metal tips. More information: see contribution in News and Views of Nature, the original publication in Physical Review Letters and our own project pages.

Cover Picture ChemPhysChem 5/2007: Excited-State Relaxation of Protonated Adenine

The MBI has a strong tradition investigating dynamic processes in isolated chromophores. Dirk Nolting and coworkers now extended such time-resolved investigations to protonated biomolecules. For the DNA base adenine, an accelerated internal conversion from electronic to nuclear energy was observed. Such fast processes may prevent potentially destructive photochemical processes.
The cover picture shows the potential energy scheme of protonated adenine. The adenine molecule in front is in the ground-state equilibrium geometry whereas the molecule in the back shows the nonplanar structure after absorption of an UV photon. In their Article on page 751 Nolting et al. investigate the excited state dynamics of protonated adenine by femtosecond pump-probe transient mass spectrometry in the gas phase.
More information: ChemPhysChem (April 2, 2007)

Electron flashes for the nanoworld – a new source of ultrashort electron pulses

Researchers at the MBI have presented a novel source of extremely short electron pulses. The electron source is based on an ultra-sharp metallic needle illuminated with ultrashort laser pulses. The particular excitation conditions result in an extremely short duration of the electron pulses of less than 0.02 picoseconds (20 femtoseconds) which allows for studying ultrafast processes in nanosystems.
More information: C. Ropers et al., Physical Review Letters, Vol. 98, 043907 (2007), press release.

The external evaluation of the MBI has been finished. The assessment is excellent.

On Nov. 23, 2006 the senate of the Leibniz Association recommended to continue the funding without limitations for the next 7 years. The senate emphasizes that the MBI belongs to the worldwide leading institutes in the field of nonlinear optics and ultrafast dynamics of the interaction of light and matter.
Press release of the Leibniz Association, press release of the Forschungsverbund Berlin e.V.

"J. Phys. B's 2006 Highlights", chosen by the Editorial Board of Journal of Physics B

Two of the 24 highlights of the year 2006 are publications that emerged from the project 2-02.

Topical Review: "Above-threshold ionization by few-cycle pulses", D. B. Milosevic, G. G. Paulus, D. Bauer, and W. Becker, J. Phys. B 39, R203-262 (2006)
The term Above-Threshold Ionization (ATI) refers to the features of the angle-resolved electron spectrum of atoms ionized by an intense laser pulse. For the past 25 years, this effect has been instrumental for understanding the interaction of an intense laser pulse with an atom. Recently, due the availability of laser pulses of only a few cycles' duration with controlled temporal evolution (stable carrier-envelope phase), ATI has revealed many more fascinating facets.
Letter to the Editors: "Attosecond electron thermalization by laser-driven electron recollision in atoms", X. Liu, C. Figueira de Morisson Faria, W. Becker, and P. B. Corkum, J. Phys. B 39, L305-311 (2006)
The paper investigates electron recollision as the mechanism of nonsequential multiple ionization of atoms. A very simple, almost analytically solvable, statistical model is introduced, which turns out to describe very well the data produced by the Heidelberg group for triple and quadruple ionization of neon [K. Zrost et al., J. Phys. B 39, 40 (2006)]. The model assumes a delay between the time of the recollion and the later time when the electrons are blown off, which is on the attosecond time scale.

Fund ranking of the Deutsche Forschungsgemeinschaft (DFG)

The MBI achieved an excellent position in the most recent ranking of funding through the Deutsche Forschungsgemeinschaft (DFG). Acquiring funding from the DFG is considered one of the key indicators for excellent research in Germany. The report mainly focusses on the universities but also lists about 170 non-university research institutions in Germany. On this list, the MBI holds position 16. Also, the MBI is one of the very few non-university institutions specifically mentioned for good networking with universities. This aspect becomes even more evident from the DFG-maps on networking in physics and in chemistry.

Klaus Tschira Award 2006 for former MBI Ph.D. student

Dr. Nils Huse has received the Klaus Tschira Award for 2006 for his tutorial presentation of research results on "The short memory of water". In his contribution he describes how with modern methods of experimental physics one can investigate the anomalies of water. The award ceremony took place on 12th October 2006 at the University of Heidelberg. More information: see contribution in Bild der Wissenschaft Plus (in German) and project pages.

Lise-Meitner Award 2006 for former MBI Ph.D. student

Dr. Nils Huse has received the Lise-Meitner Award 2006 of the Freunde und Förderer der Physik der HU Berlin and the Institut für Physik at the Humboldt Universität zu Berlin for his dissertation "Multidimensional Vibrational Spectroscopy of Hydrogen-Bonded Systems in the Liquid Phase Coupling Mechanisms and Structural Dynamics". The award ceremony took place on 20th July 2006 at the Humboldt Universität zu Berlin. More information: see press release (in German) and project pages.

Walter Schottky Award 2006 of the German Physical Society (DPG) for Heisenberg scholar

Dr. Manfred Fiebig and his colleagues at MBI demonstrated for the first time that electric and magnetic properties of multiferroics are correlated and that magnetic structures of multiferroics may be purposefully controlled by applying electric fields.
More information: see press release (in German)

One of "J. Phys. B's 2005 Highlights", chosen by the Editorial Board of Journal of Physics B
"Strong laser field ionization of Kr: first-order relativistic effects defeat rescattering"

E Gubbini, U Eichmann, M Kalashnikov and W Sandner, Max-Born-Institute Berlin

The magnetic component of a light wave is, in most cases, negligible compared to the electric one. This changes in ultra-strong laser fields, where electrons may be ionized and acclererated to relativistic speeds. Trajectory deviations of the order of a few atomic diameters, caused by magnetic light forces, have been measured with great precision in the present experiment.
J. Phys. B: At. Mol. Opt. Phys. 38 No 6 (28 March 2005) L87-L93
More information: see Project 2-02

Carl-Ramsauer-Award 2005 for former MBI student

Dr. Helmut Lippert has received the Carl-Ramsauer-Award 2005 of the Physical Society in Berlin for his dissertation "Ultra short spectroscopy of isolated and micro-solvated biochromophors". The award ceremony took place on 16th November 2005 at the university of Potsdam.
More information: see press release (in German)

Scientific highlight from a collaboration between the MBI and the Ben Gurion University of the Negev published in SCIENCE Volume 310 of 7 October 2005, p. 83-86.

Mohammed et al. report on a sequential, von Grotthuss-type, proton hopping mechanism through water bridges in aqueous acid-carboxylic base reactions.
More information: see Breaking news in Projekt 2-04

Scientific highlights from a collaboration between the MBI and the University of Toronto published in NATURE Volume 434, Number 7030, Issue of 10 March 2005.

Cowan et al. (p. 199) demonstrate that the fastest hydrogen bond fluctuations in neat liquid water (H2O) occur on a sub-50 femtosecond time scale, resulting in an extremely fast loss of structural memory. This has been revealed by multi-dimensional vibrational spectroscopy of the O-H stretching vibration of water.
More information: see Highlights in 2005

Nano-Motion Pictures: One goal of ultrafast x-ray structural studies is to image atomic motions in materials in a nondestructive manner. Bargheer et al. (p. 1771) imaged coherent atomic motions in a GaAs/AlGaAs superlattice that were induced by exciting electron-hole pairs in the GaAs subband. This excitation process weakens the bonding in the GaAs layers, which causes them to expand and the AlGaAs layers to contract. From their analysis of the small changes they observed in weak reflections, the authors argue that the layers cycle between expansion and contraction every 3.5 picoseconds and launch coherent acoustic standing waves.