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Highlights from 2012
For earlier years see Archive

 


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.

Eichmann

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

Contact

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.

WoeIsBn

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

Contact

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.

Eichmann

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

Contact

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.

Steinmeyer

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

Contact

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

 
     
 


Dr. Maria Richter of the Max Born Institute receives the Dissertation Award 2016 of the Leibniz Association

23rd November 2016

MRichter

At its annual meeting the Leibniz Association in Berlin awarded physicist Maria Richter for her outstanding doctoral thesis.

Dr. Maria Richter - Photo: David Ausserhofer  

Dr. Maria Richter developed in her PhD thesis "Imaging and controlling electronic and nuclear dynamics in strong laser fields" new theoretical and practical methods to visualize the ultrafast response of atoms and molecules to strong laser fields. Usually it is assumed that a strong laser field leads to ionization of the atom, where one or several electrons are ejected from the atomic shell, leaving a positively charged ion behind. However, Maria Richter showed with numerical calculations that instead of the emission of electrons, a new system can form - the so-called "atom plus superatomic field" -, which supports stable electronic states. The structure of the new atom is strongly modified compared to the field-free atomic structure. The new electronic structure of an atom exposed to a strong laser field has been calculated by Richter in the case of the potassium atom, and with that it has then been shown how the new structure can be imaged in experiments using photoelectron spectroscopy. Thereby she developed methods to identify the exotic laser-dressed atomic structure in a direct and unambiguous fashion with strong fields. Furthermore, she presents a new way to control the charge and energy flow in molecules, by modifying the electronic structure of the molecule with intense laser fields. Her findings are important steps towards a better understanding and control of photo-induced chemical, physicochemical and biophysical reactions in molecules, which, for instance, play a role in the stability of DNA against irradiation or in the first step of vision.

Maria Richter presented her results on several international conferences. More than three months before defending her thesis, she joined a world-renowned research group in the Department of Chemistry at Universidad Autónoma de Madrid.

Publication::
M. Richter, F. Bouakline, J. González-Vázquez, L. Martínez-Fernández, I. Corral, S. Patchkovskii, F. Morales, M. Ivanov, F. Martín, F., O. Smirnova, "Sub-laser-cycle control of coupled electron-nuclear dynamics at a conical intersection", New Journal of Physics, 17, 113023 (2015). DOI: 10.1088/1367-2630/17/11/113023
M. Richter, S. Patchkovskii, F. Morales, O. Smirnova, M. Ivanov, Misha,"The role of the Kramers-Henneberger atom in the higher-order Kerr effect". New Journal of Physics, 15, 083012 (2013). DOI: 10.1088/1367-2630/15/8/083012.
F. Morales, M. Richter, S. Patchkovskii, O. Smirnova, "Imaging the Kramers-Henneberger atom", Proceedings of the National Academy of Sciences, 108, 16906-16911 (2011). DOI: 10.1073/pnas.1105916108.

About the Prize:
With this Dissertation Award the Leibniz Association annually recognizes the best Ph.D. theses of the past year in the categories "humanities and social sciences" and "natural sciences and engineering".

See for further information:

https://www.leibniz-gemeinschaft.de/ueber-uns/auszeichnungen/promotionspreis/

Contact

Dr. Maria Richter Tel. (030) 6392 1358

 
     
 


Ocean rogue waves: a mystery unveiled?

12 October 2016

Rogue waves are extremely high ocean waves that exceed the significant wave height by more than a factor of 2. Extreme waves are also very rare; less than one in 100,000 waves exceeds the rogue wave criterion. While their existence was long disputed throughout the 1990s, thousands of rogue waves have been recorded on oil rigs in the past 20 years. Nevertheless, the origin of rogue waves is still disputed, with a multitude of competing theories that fall into two basic categories: linear theories consider incidental random interference the origin of rogue waves. This means that it is just bad luck when your ship is hit by a rogue wave. Then nothing can be done to foresee such an event. Recently nonlinear theories gained increasing popularity as they promise that certain characteristic wave patterns may possibly precede a rogue event. While this appears very appealing, neither theory can sufficiently explain the measured probabilities of rogue waves in the ocean.

In a collaborative effort, the group of Günter Steinmeyer at the Max-Born-Institut in Berlin together with colleagues from the Leibniz-University in Hannover and the Technical University in Dortmund now report a new approach to shed more light on the rogue wave mystery. To this end, they suggest a new metric for the complexity of the wave motion, namely, the so-called phase space dimension. This metric measures the effective number of waves that interfere at one given location on the ocean surface. More importantly, they also propose a way to measure the dimension, and this measurement could readily be implemented on ships, possibly providing an early warning of rogue waves.

In fact, it seems that the capability of the ocean to form rogue waves is variable. The study suggests that the ocean surface movement is fairly simply structured throughout most of the time. Even in heavy storms, mostly conditions prevail that do not enable rogue wave formation. However, the complexity of the wave patterns may suddenly increase when crossing seas are generated, resulting in rogue-wave prone situations. Using the suggested dimensional analysis, it is exactly these rogue-wave prone situations that can be detected. Nevertheless, the individual rogue wave event remains unforeseeable. Moreover the study suggests that the ocean dynamics are ruled by linear yet still very complex dynamics.

The study therefore opens a new perspective for a better understanding of ocean rogue waves. Much research went into ocean nonlinearities, but it appears that the latter play a minor role for rogue wave formation. In contrast, winds have found very little attention in the rogue wave discussion so far. As winds are ultimately the drivers behind ocean wave formation in general, it therefore seems perfectly possible to identify rogue-wave prone situations from meteorological analysis, identifying situations that may give rise to crossing seas early on. The appearance of an individual rogue wave may remain a mystery, but at least, we may soon be able to predict the "rogueness" of ocean weather hours or days in advance.

Steinmeyer

Abb. 1 (click to enlarge)

Fig. 1: Numerical simulations of prototypical rogue waves in the ocean. Top left: Normal sea state. Top right: Rogue hole. Bottom left: Rogue wave. Bottom right: Rogue wave group, also known as "three sisters". Ocean nonlinearities are only required to explain why rogue holes are even more rare than positive rogue waves. Whether rogue waves appear isolated or in a group depends on the spectral width of the sea state.
Steinmeyer Fig 2: Probability of exceeding the significant wave height by a given factor. Shown are model calculations for a varying number of effectively interfering waves. If this number is smaller than 10, then no rogue waves can appear. The danger of rogue wave emergence increases when more and more waves are interacting with each other. If waves from different locations propagate towards the same location in the ocean, then the risk for rogue waves becomes extreme. Also shown for comparison are results of longterm averaged observations as well as the empirical Forristall distribution.
Fig. 2 (click to enlarge)  

Original publication: Nature / Scientific Reports 6, Article number: 35207 (2016) doi:10.1038/srep35207
Ocean rogue waves and their phase space dynamics in the limit of a linear interference model
Simon Birkholz, Carsten Brée, Ivan Veselic, Ayhan Demircan & Günter Steinmeyer

click for (Movie)

Contact

Dr. Günter Steinmeyer Tel. 030 6392 1440

 
     
 


Benjamin Fingerhut is the winner of the 2016 Robin Hochstrasser Young Investigator Award

23rd September 2016

smirnova

Dr. Benjamin Fingerhut, junior group leader at the Max Born Institute (MBI) receives the 2016 Robin Hochstrasser Young Investigator Award. The award is granted by an international scientific committee, consisting of members of the editorial board of the journal Chemical Physics, in order to support excellent early career researchers.

Dr. Benjamin P. Fingerhut - Photo: MBI  

To honor Robin Hochstrasser and support young scientists Elsevier has initiated for Chemical Physics the Robin Hochstrasser Young Investigator Award. Professor Hochstrasser was one of the pioneers in ultrafast spectroscopy of molecular systems and has made seminal contributions to our understanding of condensed phase structure and dynamics. His group was the first to introduce 2D IR spectroscopy in 1998 as optical analogue of nuclear magnetic resonance (NMR) spectroscopy. Today, this technique is among the most important in ultrafast science. At MBI, it has been extended into the terahertz range and is being applied to biophysical problems.

The Robin Hochstrasser Young Investigator Award of Chemical Physics is granted to excellent scientists younger than 40 years of age on the basis of their scientific contributions. An international committee of scientists, consisting of five members of the editorial board of Chemical Physics, selects the winner from the nominations.

Benjamin Fingerhut joined the MBI in 2014 and is currently supported by an Emmy Noether Early Career Grant of the German Research Foundation (DFG) which allowed him to establish the new Junior Research Group: Biomolecular Dynamics at the MBI. His research involves the development of state of the art spectroscopic simulation techniques and their application to the real-time determination of ultrafast structural dynamics of molecular and biomolecular systems. The group combines analytical and computational approaches for novel simulation protocols suited to investigate excited-state non-adiabatic dynamics as well as vibrational dynamics of spacio-selective probes like phosphate groups to explore fluctuation induced decoherence dynamics in aqueous and biological environments.

For further information see:

https://www.elsevier.com/awards/global/robin-hochstrasser-young-investigator-award

Contact

Dr. Benjamin P. Fingerhut Tel. (030) 6392 1404

 
     
 


Quantum Friction: Beyond the local equilibrium approximation

1st September 2016

Systems out of thermodynamic equilibrium are very common in nature. In recent years they have attracted constantly growing attention because of their relevance for fundamental physics as well as for modern nanotechnology. In a collaborative effort, the Theoretical Optics and Photonics group at the Max-Born-Institut and Humboldt-Universität zu Berlin together with colleagues from the Universität Potsdam, Yale University and the Los Alamos National Laboratory now report on detailed new physical insights of non-equilibrium atom-surface quantum friction.

A particular class of non-equilibrium phenomena is represented by dynamical van der Waals/Casimir forces acting between atoms, molecules and surfaces. These forces, whose origin is deeply rooted in quantum theory, are at the origin of contactless (quantum) friction between two objects that, when separated by a few tens of nanometers, move relative to each other. Unfortunately, the detailed quantitative description of non-equilibrium systems is rather challenging and the most common approaches rely on the assumption that corrections to the associated equilibrium characteristics are relatively small. However, the validity of these procedures and of the corresponding approximations has been scarcely verified, inevitably limiting the reliability of the results.

In stark contrast with widely accepted assumptions that dominate the existing literature, the researchers have shown that the local thermal equilibrium (LTE) approximation, which treats interacting subsystems in a general non-equilibrium system as being locally in equilibrium with their immediate environment, fails dramatically when applied to the study of quantum friction.

Using general quantum statistical arguments and exactly solvable models, the researchers determined that the LTE approximation underestimates the magnitude of the drag force by approximately 80%. Considering that the LTE approximation has been the workhorse for the theoretical description of many non-equilibrium phenomena, ranging from thermal energy transport to non-equilibrium dispersion forces, these results demonstrate that LTE-based calculations lack rigorous justification and have to be re-examined.

Besides addressing fundamental questions in the highly interdisciplinary field of van der Waals/Casimir forces, these new results will have considerable impact on many other applications of current interest in non-equilibrium physics, such as miniaturized traps for ultra-cold gases (atom chips), nano-electromechanical systems (NEMS) and near-field radiative heat transfer. Altogether, this work provides a quantitative analysis whose conclusions represent a substantial advance in the understanding of non-equilibrium quantum physics.

Mikosch

Fig. 1 (click to enlarge)

Fig. 1: Schematic representation of the difference between the local thermal equilibrium approximation (a) and the full non-equilibrium description (b) for quantum friction. In the first case it is assumed that the atom and the surface are separately in thermal equilibrium with their immediate local environments. However, quantum correlations between the atom and surface (pictorially represented by the black arrows in (b)) lead to a failure of this approximation, which underestimates the magnitude of quantum friction by approximately 80 %.

Original Publication: Phys. Rev. Lett., 117, 100402 (2016) doi: 10.1103/PhysRevLett.117.100402
"Failure of local thermal equilibrium in quantum friction"
F. Intravaia, R.O. Behunin, C. Henkel, K. Busch, and D.A.R. Dalvit

Contact

Dr. Francesco Intravaia Tel. 030 6392 1261

 
     
 


Ultrastrong, ultrafast and local: water induces electric fields at the surface of DNA

28 July 2016

The structure and dynamics of the DNA double helix are influenced in a decisive way by the surrounding water shell. New experiments in the ultrafast time domain show that the first two water layers at the DNA surface generate electric fields of up to 100 megavolts/cm which fluctuate on the femtosecond time scale and are limited to a spatial range on the order of 1 nm.

DNA molecules are the carrier of genetic information and form a double helix structure in their native aqueous environment. This structure consists of two opposite twisted strands of nucleotides (Fig. 1A). An alternating arrangement of negatively charged phosphate groups and polar sugar units forms the backbone of the double helix structure which interacts directly with the surrounding water molecules. The overall negative charge of the double helix is compensated for by positively charged counterions such as sodium ions which in an aqueous environment are located in direct proximity to the DNA surface. The interaction of the electric dipoles of water molecules with the charges of the counterions and phosphate groups as well as the polar units generates electric fields at the DNA surface. Such fields are being discussed in a highly controversial way, even after decades of intense research, reflecting the structural complexity of this many-body system and its thermal fluctuations on short time scales.

Scientists from the Max-Born-Institute in Berlin have now succeeded for the first time in determining the strength, range, and ultrafast dynamics of electric fields at a native DNA surface. In a recent paper published in the Journal of Physical Chemistry Letters, they report how vibrations of the backbone of native salmon DNA serve as probes for mapping electric interactions in space and time. Electric fields at the surface directly influence the shape and dynamics of vibrational resonances which are recorded in real-time on the femtosecond time scale (1 fs = 10-15) s by a method referred to as two-dimensional infrared spectroscopy (Fig. 1B). The water content of the DNA samples has been varied in a systematic way to discern different contributions to the fluctuating electric fields at the DNA surface.

The experiments and a detailed theoretical analysis performed in parallel show that water molecules in the first two layers at the DNA surface generate an extremely strong electric field whereas the ionic groups and outer water layers play a minor role. The spatial range of the field is approximately 1 nm, its strength reaches values up to 100 megavolts/cm (hundred million volts per centimeter) as shown in Fig. 1C. Thermal motions of the water molecules result in field fluctuations of 25 MV/cm on a 300 fs time scale. The time scale of fluctuations demonstrates a hindrance of water motions due to the coupling to the corrugated DNA surface and a slowing down compared to neat water. This new quantitative insight is important for understanding the key role of water and its dynamics at biological interfaces such as charged cell membranes and the surface of proteins.

DNASiebert

Fig. 1 (click to enlarge)

Fig. 1: (A) Surface of a DNA double helix. The twisted strands of the helix are represented by oxygen atoms (red) of the phosphate groups. Counterions are shown in blue, the small angled structures are water molecules. (B) Two-dimensional infrared spectrum of the DNA backbone. Nonlinear vibrational signals are plotted as a function of excitation and detection frequency. The lineshapes of the resonances on the diagonal (identical excitation and detection ferquency) are directly influenced by fluctuating electric fields. The off-diagonal signals originate from couplings between different backbone vibrations. (C) Time-averaged electric field (blue) as a function of the distance from the DNA surface. Water molecules in the first layer (around 0.4 nm) generate approximately 70% of the total field, the second water layer contributes some 20%.

Original Publication: The Journal of Physical Chemistry Letters, 7, 3131-3136 (2016)
Range Magnitude and Ultrafast Dynamics of Electric Fields at the Hydrated DNA Surfaces
T. Siebert, B. Guchhait, Y. Liu, B. P. Fingerhut, T. Elsaesser

Contact:

Dr. Torsten Siebert Tel. 030 6392 1414
Dr. Benjamin Fingerhut Tel. 030 6392 1404
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400

 
     
 


Attosecond Science opens new Avenues in Femtochemistry

6th July 2016

Attosecond Science is a new exciting frontier in contemporary physics, aimed at time-resolving the motion of electrons in atoms, molecules and solids on their natural timescale. Electronic dynamics derives from the creation and evolution of coherence between different electronic states and proceeds on sub-femtosecond timescales. In contrast, chemical dynamics involves position changes of atomic centers and functional groups and typically proceeds on a slower, femtosecond timescale inherent to nuclear motion.

Nonetheless, there are exciting ways in which chemistry can hugely benefit from the technological developments pushed forward in the vibrant field of Attosecond Science. This was exploited in the work recently published by Lorenz Drescher and coworkers. Attosecond pulses are generated in the process of High Harmonic Generation (HHG), in which infrared photons are upconverted to the extreme ultraviolet (XUV) frequency domain in a highly non-linear interaction of intense coherent light and matter. The short duration of attosecond pulses implies a frequency spectrum with photon energies spanning from a few electron volts (eV) to hundreds of eV. Such broad and continuous frequency spectra are ideally suited for core shell absorption measurements in molecules.

Core shell to valence shell transitions are a unique probe of molecular structure and dynamics. Core-to-valence transitions are element specific, due to the highly localized nature of core orbitals on specific atoms. On the other hand the intramolecular local environment of specific atomic sites is encoded, since an electron is lifted from a core orbital to a hole in the valence shell, affected by chemical bonding (see Fig. 1). Importantly, these transitions typically correspond to very short lifetimes of only a few femtoseconds. The use of ultrashort XUV pulses hence gives a new twist to the ultrafast studies of chemistry: It allows to probe chemical dynamics, initiated by a UV pump laser pulse, from the perspective of different reporter atoms within a molecule in an XUV transient absorption experiment. This is now beginning to be explored by a number of groups around the world.

In the experiment carried out by Drescher and coworkers at the MBI, photodissociation of iodomethane (CH3I) and iodobenzene (C6H5I) was studied with time-resolved XUV transient absorption spectroscopy at the iodine pre-N4,5 edge, using femtosecond UV pump pulses and XUV probe pulses from HHG (see Fig.2). For both molecules the molecular core-to-valence absorption lines were found to fade immediately, within the pump-probe time-resolution. Absorption lines converging to the atomic iodine product however emerge promptly in CH3I but are time-delayed in C6H5I. In CH3I, we interpret this observation as the creation of an instantaneous new target state for XUV absorption by the UV pump pulse, which is then subject to relaxation of the excited valence shell as the molecule dissociates. This relaxation shows in a continuous shift in energy of the emerging atomic absorption lines in CH3I, which we measured in the experiment. In contrast, the delayed appearance of the absorption lines in C6H5I is indicative of a UV created vacancy, which within the molecule is initially spatially distant from the iodine reporter atom and has to first travel intramolecular before being observed. This behaviour is attributed to the dominant π → σ* UV excitation in iodobenzene, which involves the π orbital of the phenyl moiety.

While in the current work only a simplistic independent particle model was used to rationalize the observed experimental findings, MBI with its newly created theory department provides unique opportunities for joint experimental and theory studies on XUV transient absorption of photochemical processes. This will involve a new theoretical approach developed recently by researchers from MBI together with colleagues in Canada, the UK and Switzerland, which was recently submitted as a publication.

Mikosch

Fig. 1 (click to enlarge)

Fig. 1: Fig.1: XUV absorption from the core shell to vacancies in the valence shell are element specific and sensitive to the local chemical environment around the reporter atom.
Mikosch

Fig. 2 (click to enlarge)

Fig.2: (a) Transient XUV absorption action spectrum of photodissociation of CH3I, i.e. the difference in the XUV absorption spectrum before and after photodissociation. Lines associated with molecular CH3I are depleted while atomic iodine lines have appeared. (b), (c) Absorption lines converging to atomic iodine appear promptly in CH3I, but time-delayed in C6H5I, as seen by comparing to the UV-XUV cross-correlation, convoluted with a step function (red).

Original Publication: Journal of Chemical Physics Communication, 145, 011101 (2016)
XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation
L. Drescher, M.C.E. Galbraith, G. Reitsma, J. Dura, N. Zhavoronov, S. Patchkovskii, M.J.J. Vrakking, and J. Mikosch

Contact

Dr. Jochen Mikosch Tel. 030 6392 1240

 
     
 


Fluctuating liquid structure induces ultrabroad infrared absorption: The hydrated proton on ultrafast time scales

4th July 2016

The elusive infrared absorption continuum of protons in aqueous environment has been topic of intense controversial debate since half a century. A team of scientists from the Max Born Institute and the Ben Gurion University of the Negev, Israel, show for the case of the Zundel cation (H2O...H+...OH2) H5O2+ that the surrounding liquid induces fluctuating electrical forces onto the proton, modulating its vibrational motions between the two water molecules. This mechanism, together with low-frequency thermal motions, results in the extreme broadening of the infrared spectrum.

The proton (H+), the positively charged nucleus of a hydrogen atom, plays a fundamental role for many processes in nature. In liquid water, the transport of electrical charge is dominated by moving excess protons while proton motions across cell membranes are at the heart of cell respiration. In spite of this relevance, the molecular nature and dynamics of excess protons interacting with water molecules in their environment are not fully understood. Vibrational, in particular infrared spectroscopy has helped to identify limiting molecular structures of hydrated protons such as the Eigen and Zundel cations where the latter displays an extremely broad unstructured infrared absorption, a so-called "Zundel continuum" (Figure 1). In liquid water, such structures are unstable and expected to undergo rapid changes on a time scale of femto- to picoseconds (1 picosecond = 1 ps = 10-12 s). The mechanisms underlying the absorption continua have remained highly controversial.

Researchers from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin and the Ben Gurion University of the Negev in Beer-Sheva, Israel have now applied nonlinear infrared spectroscopy with femtosecond time resolution to elucidate the nature of the broadband continuum. For the particular model case H5O2+, the Zundel cation consisting of two water molecules held together by a proton (H2O...H+...OH2), they dynamically dissect the Zundel continuum from the regular OH stretching and bending vibrations of the two water molecules (Figure 2). As they report in Angewandte Chemie Int. Ed. (DOI: 10.1002/anie.201602523 ), a judicious choice of femtosecond vibrational excitation allows for isolating the transient continuum absorption. The different excitations show lifetimes below 60 fs, much shorter than the OH stretching and bending vibrations of neat water.

A theoretical analysis of the results demonstrates that the extreme broadening of the infrared absorption is caused by motions of the inner proton exerted by the strong, rapidly fluctuating electrical fields that originate from the surrounding polar solvent molecules. The energy of proton motions along the so-called proton transfer coordinate, the direction connecting the two water molecules in (H2O...H+...OH2), is strongly modulated by these external fields, resulting in a concomitant modulation of vibrational transition energies. On a time scale faster than 100 fs, the system explores a broad range of transition energies. Together with vibrational overtones, combination tones and modes changing the distance between the two water molecules the field modulated transitions lead to the observed extreme broadening of the infrared absorption. Due to the extremely fast structural fluctuations, particular H+ arrangements are washed out very rapidly, i.e., the system has an extremely short-lived structural memory.

This new view at the Zundel cation clearly goes beyond the many studies of gas phase cluster work on hydrated protons, where due to the low temperature conditions, the Zundel continuum is not observed. The results are of relevance for many dynamic aspects of hydrated protons, be it for proton transport in water by the infamous von Grotthuss mechanism, in hydrogen fuel cells, or biological systems functioning with proton translocation mechanisms.

dahms

Abb. 1 (click to enlarge)

Fig. 1: Hydration of protons goes beyond the hydronium (H3O+) species typically mentioned in chemistry textbooks. Eigen and Zundel cations have been named after two leading German scientists Manfred Eigen and Georg Zundel who proposed these structures in the 1960s. The mid-infrared spectrum of the Zundel cation shows the marked contributions of OH stretching and bending vibrations, and the significant broadband Zundel continuum. This Zundel continuum is caused by the ultrafast fluctuating potential of the proton transfer coordinate that modulates fundamental, overtone and combination tone transitions.
dahms Fig. 2:Transient IR spectra showing the distinct response of the OH stretching mode and the Zundel continuum after femtosecond excitation
Abb. 2 (click to enlarge)  

Original publication: Angewandte Chemie International Edition
The Hydrated Excess Proton in the Zundel Cation H5O2+: The Role of Ultrafast Solvent Fluctuations
Fabian Dahms, Rene Costard, Ehud Pines, Benjamin P. Fingerhut, Erik T. J. Nibbering, Thomas Elsaesser

Contact

Dr. Erik T. J. Nibbering Tel. 030 6392 1477
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400

 
     
 


Dr. Olga Smirnova installed as Professor at the Technical University Berlin

30th June 2016

smirnova

The Technical University Berlin has installed Dr. Olga Smirnova as a Professor for "Theoretical Physics with a Focus on Atomic and Molecular Laser Physics" on 30 June 2016. The S-Professorship is anchored in the Institute for Optics and Atomic Physics on the TU.

President Prof. Dr. Christian Thomsen und Prof. Dr. Olga Smirnova - Photo: MBI  

The theoretical physicist Smirnova obtained her Ph.D. degree in 2000 at the Moscow State University. In 2003 she moved to the Technical University of Vienna as a Lise-Meitner fellow, in 2005 she joined the theory group at the Steacie Institute for Molecular Sciences, NRC Canada, becoming a permanent staff member in 2006. She came to the Max Born Institute in 2009 to start her own theory group. In 2010 Smirnova was the recipient of the Karl-Scheel-Preis awarded by the Physikalische Gesellschaft zu Berlin. In her research, she focusses on using intense light fields to image and control attosecond dynamics in atoms and molecules. At the TU Berlin she will strengthen the curriculum in the fields of modern optics, intense light-matter interaction and ultrafast science.

We congratulate Prof. Dr. Smirnova to her new position as a professor at the TU Berlin!

Contact

Prof. Olga Smirnova

 
     
 


Spin polarization by strong field ionization

13th June 2016

Strong field ionization has been studied for over half a century. Yet, the role of electron spin during this process has been largely overlooked. Surprisingly, our joint experimental and theoretical study shows that a chance of detaching spin-up or spin-down electron from an atom can be very different.

As a fundamental property of the electron, the spin plays a decisive role in the electronic structure of matter, from molecules and atoms to solids, where it determines, for example, the magnetic properties of matter. Ultrashort pulses of electrons are unique tools for studying materials, both their structure and dynamics, opening a rich field of ultrafast diffraction imaging. Since electron spin is an essential variable in diffraction, ultrashort pulses of spin-polarized electrons would add a completely new dimension to this field. But where could one get such pulses?

One way is to use ionization in strong laser fields. This process naturally produces electrons in ultrashort bursts. The bursts last only a small fraction of the laser cycle when they are released from the confines of the binding potential. But would these electron bursts be spin-polarized? Surprisingly, until very recently, this question has never been asked.

This situation has now changed with the joint experimental and theoretical work of Alexander Hartung et al, inspired by the earlier theoretical prediction of I. Barth and O. Smirnova (Phys. Rev. A 88, 013401, 2013). Using the gas of Xe atoms, the authors present the first experimental detection of electron spin polarization created by strong-field ionization. The measured spin-polarization, shown in Fig.1, was found to be as high as 30%, changing its sign with the electron energy. This work opens the new dimension of spin to strong-field physics. It paves the way to the production of sub-femtosecond spin-polarized electron pulses with applications ranging from probing the magnetic properties of matter at ultrafast timescales to testing chiral molecular systems with sub-femtosecond temporal and sub-ångström spatial resolutions. The paper also shows that spin polarization is important during laser-driven electron recollision with the parent ion, when such recollision is induced by the elliptical laser field. Since in the laser-driven electron collision with the parent ion the electron is fully controlled by the laser field, the dynamics can now be studied not only with attosecond temporal and angstrom spatial resolution, but also with spin sensitivity. This would allow chiral molecules to be probed with sub-femtosecond temporal resolution and sub-ångström spatial resolution. Finally, spin polarization of the ejected electron is firmly linked to the creation of the parent ion in an initially spin-polarized state. Spin-orbit coupling then leads to internal circular electron and spin currents.

Morales

Fig. 1 (click to enlarge)

Fig. 1: Spin polarization measured as a function of electron energy. The blue curve is a theoretical prediction, while the red dots with error-bars show experimental results. The measurement was done for Xe atom.

Original Publication: Nature Photonics (2016), 10, doi:10.1038/nphoton.2016.109
Electron spin polarization in strong-field ionization of xenon atoms
Alexander Hartung, Felipe Morales, Maksim Kunitski, Kevin Henrichs, Alina Laucke, Martin Richter, Till Jahnke, Anton Kalinin, Markus Schöffler, Lothar Ph. H. Schmidt, Misha Ivanov, Olga Smirnova, Reinhard Dörner

Contact

Prof. Olga Smirnova Tel. 030 6392 1340
Prof. Misha Ivanov Tel. 030 6392 1210
Dr. Felipe Morales Tel. 030 6392 1358

 
     
 


Quantum Swing - a pendulum that moves forward and backwards at the same time

28 April 2016

Two-quantum oscillations of atoms in a semiconductor crystal are excited by ultrashort terahertz pulses. The terahertz waves radiated from the moving atoms are analyzed by a novel time-resolving method and demonstrate the non-classical character of large-amplitude atomic motions.

The classical pendulum of a clock swings forth and back with a well-defined elongation and velocity at any instant in time. During this motion, the total energy is constant and depends on the initial elongation which can be chosen arbitrarily. Oscillators in the quantum world of atoms and molecules behave quite differently: their energy has discrete values corresponding to different quantum states. The location of the atom in a single quantum state of the oscillator is described by a time-independent wavefunction, meaning that there are no oscillations.

Oscillations in the quantum world require a superposition of different quantum states, a so-called coherence or wavepacket. The superposition of two quantum states, a one-phonon coherence, results in an atomic motion close to the classical pendulum. Much more interesting are two-phonon coherences, a genuinely non-classical excitation for which the atom is at two different positions simultaneously. Its velocity is nonclassical, meaning that the atom moves at the same time both to the right and to the left as shown in the movie. Such motions exist for very short times only as the well-defined superposition of quantum states decays by so-called decoherence within a few picoseconds (1 picosecond = 10-12s). Two-phonon coherences are highly relevant in the new research area of quantum phononics where tailored atomic motions such as squeezed and/or entangled phonons are investigated.

In a recent issue of Physical Review Letters, researchers from the Max Born Institute in Berlin apply a novel method of two-dimensional terahertz (2D-THz) spectroscopy for generating and analyzing non-classical two-phonon coherences with huge spatial amplitudes. In their experiments, a sequence of three phase-locked THz pulses interacts with a 70-μm thick crystal of the semiconductor InSb and the electric field radiated by the moving atoms serves as a probe for mapping the phonons in real-time. Two-dimensional scans in which the time delay between the three THz pulses is varied, display strong two-phonon signals and reveal their temporal signature [Fig. 1]. A detailed theoretical analysis shows that multiple nonlinear interactions of all three THz pulses with the InSb crystal generate strong two-phonon excitations.

This novel experimental scheme allows for the first time to kick off and detect large amplitude two-quantum coherences of lattice vibrations in a crystal. All experimental observations are in excellent agreement with theoretical calculations. This new type of 2D THz spectroscopy paves the way towards generating, analyzing, and manipulating other low-energy excitations in solids such as magnons and transitions between ground and excited states of excitons and impurities with multiple-pulse sequences.

WoeIsBn

Fig. 1 (click to enlarge)

Fig. 1: Experimental data: (a) Two-dimensional (2D) scan of the sum of the electric fields E(τ,t) of the three driving THz pulses A, B, and C as a function of the coherence time τ and the real time t. The contour plot is colored red for positive electric fields and blue for negative fields. (b) 2D scan of electric field ENL(τ,t) nonlinearly emitted by the two-phonon coherence in InSb. The orange dashed line indicates the center of pulse A. (c) Electric field transient ENL(0,t) for the coherence time τ=0.
Movie Movie: Visualization of nonclassical quantum coherences in matter. The two parabolas (black curves) show the potential energy surfaces of harmonic oscillators representing the oscillations of atoms in a crystalline solid around their equilibrium positions, i.e., the so called phonons. Blue curves: probability of presence of atoms at different spatial positions in thermal equilibrium. The quantum mechanical uncertainty principle demands a finite width of such distribution functions. Red curves: time-dependent probability distributions of coherent oscillating states in matter. One-phonon coherence (left panel): the quantum mechanical motion of atoms resembles the classical motion of a pendulum (cyan ball). The latter moves during the oscillation either from left to right or vice versa. Two-phonon coherence (right panel): quantum mechanics allows also for kicking off a nonclassical state with the quantum-mechanical property that the atom can be at two positions simultaneously. The velocity of the atoms behaves also nonclassical, i.e., the atom moves at the same time both to the right and to the left. In the case of a perfect harmonic oscillator the currents of the two parts of the atom exactly cancel each other. Thus, a small anharmonicity is necessary to observe the emission of a coherent electric field transient as shown in Fig. 1(c).

Original publication: Physical Review Letters 116, 177401
Two-Phonon Quantum Coherences in Indium Antimonide Studied by Nonlinear Two-Dimensional Terahertz Spectroscopy
Carmine Somma, Giulia Folpini, Klaus Reimann, Michael Woerner, and Thomas Elsaesser

An additional article with the focus on the experimental technique has been published as well in:

The Journal of Chemical Physics 144, 184202
Phase-resolved two-dimensional terahertz spectroscopy including off-resonant interactions beyond the χ(3) limit
Carmine Somma, Giulia Folpini, Klaus Reimann, Michael Woerner, and Thomas Elsaesser

Contact

Prof. Klaus Reimann Tel. 030 6392 1476
Dr. Michael Wörner Tel. 030 6392 1470
Prof. Dr. Thomas Elsässer Tel. 030 6392 1400

 
     
 


Ultrafast photoelectron imaging grasps competition in molecular autoionization

22nd April 2016

Using time-, energy- and angular-resolved photoelectron imaging a team of researchers from the Max Born Institute in Berlin, in collaboration with colleagues from Milan and Padova, has been able to make snapshots of coupled Rydberg orbitals evolving in time during an ultrafast autoionization process.

Electronic autoionization is a process in which multiple electrons in an excited atom or molecule rearrange in order to "kick out" one of them. Notwithstanding its long research history, the theoretical description of this phenomenon still meets with significant challenges, especially in cases where several electronic autoionizing resonances overlap. These challenges are fundamental, since most of the theories approach the inherently time-dependent autoionization process from an energy-domain perspective, thanks to the prevailing experimental information that is collected in the energy domain. However, recent advances in ultrafast laser spectroscopy and, especially, the generation of ultrashort XUV pulses, allowed the researchers to look at autoionization in nitrogen molecules on its natural time scale.

In a recent publication (M. Eckstein et al, Phys. Rev. Lett. 116, 163003 (2016)), the experimental team has used a newly constructed XUV time delay compensating monochromator beamline to excite one of the complex autoionizing resonances in a nitrogen molecule. In the femtosecond pump-probe experiment, a second time-delayed infrared (IR) laser pulse was able remove the electron from the excited orbitals before the autoionization had a chance to take place, i.e. at a timescale of less than 15 fs. The resulting photoelectrons were detected using a Velocity Map Imaging spectrometer, which delivers both energy- and angular-resolved distributions of photoelectrons. The analysis of the angular distributions, which gives direct information about the shape of the involved electronic orbitals, showed that the photoelectron emission angles change within the lifetime of the resonance (see. Fig. 1). Immediately after the excitation, the emission is more or less isotropic, i.e. the electrons are emitted with equal probability in all directions. However, with increasing pump-probe time delay, the electrons more and more tend to fly out in the direction of the laser light polarization. This observation can only be understood, if one assumes that two different electronic states with substantially different lifetimes are simultaneously probed by the IR pulse. The existence of these two states was indeed predicted by theory more than 30 years ago. The present experiment gives the first confirmation of this old prediction.

The two overlapping electronic states with long and short lifetimes observed by the team suggest a role for the phenomenon of interference stabilization, previously suggested in the field of laser-dressed atoms and in atomic Rydberg physics. In the framework of this theory two overlapping resonances influence each other in such way that one of the two becomes stabilized at the expense of the other. Quantum interferences lead to a counterintuitive effect: the stronger the resonances interact, the more one of them is stabilized. The present work draws parallels between these interference phenomena in laser-dressed atoms and in molecular autoionization. Further experimental and theoretical research will shed light on how general this phenomenon is and will help to achieve a new level of understanding of autoionization dynamics.

Original Publication: Physical Review Letters 116, 163003
Direct Imaging of Transient Fano Resonances in N2 Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy

Full Citation:
Martin Eckstein, Chung-Hsin Yang, Fabio Frassetto, Luca Poletto, Giuseppe Sansone, Marc J. J. Vrakking, Oleg Kornilov
"Direct Imaging of Transient Fano Resonances in N2 Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy"

DOI: 10.1103/PhysRevLett.116.163003

Contact

Dr. Oleg Kornilov, Tel. 030 6392 1246

betaRyIRKornilov

Fig. 1: Angular distributions of photoelectrons emitted upon ionization of an excited nitrogen molecule by a weak IR pulse. The insets show individual angular distributions for time-delays marked by black arrows. The green and blue curves quantify the angular distributions in terms of angular asymmetry parameters - the relative weights of the second and forth Legendre polynomials in the angle distributions.

 

Fig. 1 (click to enlarge)  
 
     
 


Thomas Elsaesser is the 2016 recipient of the Ellis R. Lippincott Award

8 March 2016

Thomas Elsaesser, Director at the Max-Born-Institute and Professor for Experimental Physics at Humboldt University, Berlin, receives the Ellis R. Lippincott Award for his "seminal contributions to the understanding of the ultrafast coherent and incoherent vibrational dynamics of hydrogen bonds in liquids and biomolecules".

The prize recognizes his pioneering work elucidating molecular processes and interactions in water, hydrogen bonded dimers, nucleobase pairs, and biomolecules in an aqueous environment such as hydrated DNA and phospholipids. This research is based on methods of nonlinear infrared spectroscopy in the pico- and femtosecond time domain.
The prestigious Ellis R Lippincott Award was established in 1975 by the Optical Society of America, the Coblentz Society and the Society for Applied Spectroscopy to honor the unique contributions of Ellis R. Lippincott to the field of vibrational spectroscopy. It is presented to an individual who has made significant contributions to vibrational spectroscopy as judged by his or her influence on other scientists. Because innovation was a hallmark of Lippincott's work, this quality must also be demonstrated by candidates for the award. The award is presented in Fall 2016 at the national meeting of one of the sponsoring societies.

Link press release of the Optical Society of America Prestigious Awards and Medals 2016

Contact

Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400

 
     
 


Amplification of Sound Waves at Extreme Frequencies

18 February 2016

An electric current through a semiconductor nanostructure amplifies sound waves at ultrahigh frequency. This method allows for novel, highly compact sources of ultrasound, which can serve as diagnostic tool for imaging materials and biological structures with very high spatial resolution.

Ultrasound is an acoustic wave at a frequency well above the human audible limit. Ultrasound in the megahertz range (1 MHz = 106 Hz = 1 million oscillations per second) finds broad application in sonography for, e.g., medical imaging of organs in a body and nondestructive testing of materials. The spatial resolution of the image is set by the ultrasound wavelength. To image objects on the nanoscale (1 nanometer = 10-9 m = 1 billionth of a meter), sound waves with a frequency of several hundreds of gigahertz (1 gigahertz (GHz) = 1000 MHz) are required. To develop such waves into a diagnostic tool, novel sources and sound amplification schemes need to provide sufficient sound intensities.

In a recent publication (K. Shinokita et al., Phys. Rev. Lett. 116, 075504 (2016)), researchers from the Max-Born-Institut in Berlin together with colleagues from the Paul-Drude-Institut, Berlin, and the École Normale Supérieure, Paris, have demonstrated a new method for sound amplification in a specially designed semiconductor structure consisting of a sequence of nanolayers. Sound waves with a frequency of 400 GHz are generated and detected with short optical pulses from a laser. The sound is amplified by interaction with an electric current traveling through the semiconductor in the same direction as the sound waves. The sound amplification is based on a process called "SASER", the Sound Amplification by Stimulated Emission of Radiation, in full analogy to the amplification of light in a laser. The sound wave stimulates electrons moving with a velocity higher than the sound velocity, to go from a state of high energy to a state of lower energy and, thus, make the sound wave stronger. To achieve a net amplification, it is necessary that there are more electrons in the high-energy than in the lower-energy state. In this way, a 400 GHz sound wave is amplified by a factor of two.

The present work is a proof of principle. For a usable source of high-frequency sound waves, it is necessary to further increase the amplification, which should be possible by improving the design of the structure and, most importantly, better cooling of the semiconductor device. Once such a source is available, it can be used for extending the spatial resolution of sonography towards the scale of viruses, a length scale much shorter than the wavelength of visible light.

Reimann

Fig. 1 (click to enlarge)

Fig. 1: Changes of the sample reflectivity as a function of the delay time after the pump pulse. The observed oscillations are proportional to the instantaneous amplitude of the sound wave. The blue curve shows the results without the current through the superlattice, the red curve with a current of 1 A. With current the amplitude is always larger than without current. The amplification (the ratio between the red and blue curves) is most pronounced at delay times of 300 ps (1 picosecond (ps) is 10-12 s, one millionth of a millionth of a second), since the amplification takes time.
Movie Movie: The sample consists of alternating layers of gallium arsenide and aluminum gallium arsenide (here shown in yellow and red). A short laser pulse (arrow from the left) can generate an acoustic wave, seen as periodic changes of the layer thicknesses. Whereas the amplitude of the acoustic wave increases with time with an electric current (moving electrons, shown as blue dots), it stays constant without a current (upper part).

Original Publication: Physical Review Letters 116, 075504
Strong Amplification of Coherent Acoustic Phonons by Intraminiband Currents in a Semiconductor Superlattice

Keisuke Shinokita, Klaus Reimann, Michael Woerner, Thomas Elsaesser, Rudolf Hey, Christos Flytzanis

This article was chosen as an Editor's suggestion, see also: Pumping up the sound

Contact

Prof. Klaus Reimann Tel. 030 6392 1476
Dr. Michael Wörner Tel. 030 6392 1470
Prof. Dr. Thomas Elsässer Tel. 030 6392 1400

 
     
 


Invisible light flash ignites nano-fireworks

19th January 2016

A team of researchers from the Max Born Institute in Berlin and the University of Rostock demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode.

Intense laser pulses can transform transparent material into a plasma that captures energy of the incoming light very efficiently. Scientists from Berlin and Rostock discovered a trick to start and control this process in a way that is so efficient that it could advance methods in nanofabrication and medicine. The light-matter encounter was studied by a team of physicists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin and from the Institute of Physics of the University of Rostock.

The researchers studied the interaction of intense near-infrared (NIR) laser pulses with tiny, nanometer-sized particles that contain only a few thousand Argon atoms - so-called atomic nanoclusters. The visible NIR light pulse alone can only generate a plasma if its electromagnetic waves are so strong that they rip individual atoms apart into electrons and ions. The scientists could outsmart this so-called ignition threshold by illuminating the clusters with an additional weak extreme-ultraviolet (XUV) laser pulse that is invisible to the human eye and lasts only a few femtoseconds (a femtosecond is a millionth of a billionth of a second). With this trick the researchers could "switch on" the energy transfer from the near-infrared light to the particle at unexpectedly low NIR intensities and created nano-fireworks, during which electrons, ions and colourful fluorescence light are sent out from the clusters in different directions (Figure 1). Their results open unprecedented opportunities for both fundamental laser-matter research and applications and was published in the latest issue of Physical Review Letters.

The experiments were carried out at the Max Born Institute at a 12 meter long high-harmonic generation (HHG) beamline. "The observation that argon clusters were strongly ionized even at moderate NIR laser intensities was very surprising", explains Dr. Bernd Schütte from MBI, who conceived and performed the experiments. "Even though the additional XUV laser pulse is weak, its presence is crucial: without the XUV ignition pulse, the nanoparticles remained unaffected and transparent for the NIR light (Figure 2)." Theorists around Prof. Thomas Fennel from the University of Rostock modelled the light-matter processes with numerical simulations and uncovered the origin of the observed synergy of the two laser pulses. They found that only a few seed electrons created by the ionizing radiation of the XUV pulse are sufficient to start a process similar to a snow avalanche in the mountains. The seed electrons are heated in the NIR laser light and kick out even more electrons. "In this avalanching process, the number of free electrons in the nanoparticle increases exponentially", explains Prof. Fennel. "Eventually, the nanoscale plasma in the particles can be heated so strongly that highly charged ions are created."

The novel concept of starting ionization avalanching with XUV light makes it possible to spatially and temporally control the strong-field ionization of nanoparticles and solids. Using HHG pulses paves the way for monitoring and controlling the ionization of nanoparticles on attosecond time scales, which is incredibly fast. One attosecond compares to a second as one second to the age of the universe. Moreover, the ignition method is expected to be applicable also to dielectric solids. This makes the concept very interesting for applications, in which intense laser pulses are used for the fabrication of nanostructures. By applying XUV pulses, a smaller focus size and therefore a higher precision could be achieved. At the same time, the overall efficiency can be improved, as NIR pulses with a much lower intensity compared to current methods could be used. In this way, novel nanolithography and nanosurgery applications may become possible in the future.

Original Publication: Physical Review Lett3001
Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons

Full Citation:
Bernd Schütte, Mathias Arbeiter, Alexandre Mermillod-Blondin, Marc J. J. Vrakking, Arnaud Rouzée, Thomas Fennel
"Ionization Avalanching in Clusters Ignited by Extreme-Ultraviolet Driven Seed Electrons"

DOI: 10.1103/PhysRevLett.116.033001

Contact

Dr. Bernd Schütte

Schuette

Fig. 1: Nano-fireworks in an argon nanoparticle are ignited by a moderately intense and invisible XUV laser pulse. A subsequent visible laser pulse heats the nanoparticle very efficiently, resulting in its explosion. Electrons and ions move in different directions and send out fluorescence light in various colors. Without the XUV pulse the nanoparticle would remain intact.

 

Abb. 1 (click to enlarge)  
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Fig. 2: Ion charge spectra measured from argon nanoparticles. Using an XUV ignition pulse, only a few singly-charged ions are observed (black spectrum). By adding an NIR heating pulse, highly charged ions up to Ar8+ are generated (red spectrum).

Fig. 2 (click to enlarge)  
 
     

 
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