Forthcoming Events

29.07.2018 - 03.08.2018, University of New England, Biddeford, US
16.08.2018 - 17.08.2018, South Campus, University of Copenhagen
28.08.2018 - 31.08.2018, Lausanne

News

Fabrizio Carbone promoted Associate Professor of Physics- in the EPFL School of Basic Sciences from 1st of August
New scientific highlights- by MUST PIs Ursula Keller, Gebhard Schertler / Jörg Standfuss, Majed Chergui, Peter Hamm
White Paper Photonics Switzerland- presented June 20, 2018 at the Swissmem "Industrietag"
Ursula Keller portrayed in the NZZ -Laserlicht ist das schönste Licht der Welt
Ursula Keller: Nomination for European Inventor Award 2018Ursula Keller is nominated for the Lifetime Achievement Award - VOTE
New scientific highlights- by MUST PIs Ursula Keller, Thomas Feuer, Majed Chergui, Hans Jakob Wörner, Jean-Pierre Wolf and Ursula Röthlisberger
ERC Advanced Grants for Ursula Keller and Ruth Signorell- a major success for women scientists in the NCCR MUST
ETH WPF General Assembly - 28 March 2018Video-conferencing in Zürich and Lausanne
Thomas Feuer: Vortrag über THz Strahlung (in German)- in der Physikalischen Gesellschaft Zürich (PGZ)
Ursula Keller awarded the IEEE Photonics Award 2018 -for her work as a pioneer in the field of ultrafast lasers
Jean-Pierre Wolf to Receive ZEISS Research Award - for his groundbreaking application of laser pulses in researching the earth’s atmosphere

Scientific highlights

Can ultrashort electron flashes help harvest nuclear energy?


Attosecond coherent control of free-electron wave functions using semi-infinite light fields

Fabrizio and collegues at EPFL have now demonstrated experimentally the ability to coherently manipulate the wave function of a free electron down to the attosecond timescale (10-18 of a second). The team also developed a theory for creating zeptosecond (10-21 of a second) electron pulses, which could also be used to increase the energy yield of nuclear reactions.

Reference: Vanacore, G. M., I. Madan, G. Berruto, K. Wang, E. Pomarico, R. J. Lamb, D. McGrouther, I. Kaminer, B. Barwick, F. J. García de Abajo and F. Carbone (2018). Attosecond coherent control of free-electron wave functions using semi-infinite light fields. Nat. Comm. 9: 2694. (10.1038/s41467-018-05021-x) Vanacore-2018.


July 12, 2018. More >>
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Using attosecond laser pulses, researchers at ETH have measured how the photoelectric effect takes place in molecules

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The photoelectric effect in stereo

When a photon hits a material, it can eject an electron from it provided it has enough energy. Albert Einstein found the theoretical explanation of this phenomenon, which is known as the photoelectric effect, in Bern during his “year of wonders” 1905. That explanation was a crucial contribution to the development of quantum mechanics, which was under way at the time, and it earned him the Nobel Prize in Physics in 1921. A team of physicists led by Ursula Keller at the Institute for Quantum Electronics of the ETH Zurich, with theoretical support by colleagues at the Max-Born-Institute in Berlin, the Max-Planck-Institute for the Physics of Complex Systems in Dresden and the Australian National University in Canberra, has now added a new dimension to the experimental investigation of this important effect. Using attosecond laser pulses they were able to measure a tiny time difference in the ejection of the electron from a molecule depending on the position of the electron inside the molecule.

Reference: Vos, J., L. Cattaneo, S. Patchkovskii, T. Zimmermann, C. Cirelli, M. Lucchini, A. Kheifets, A. S. Landsman and U. Keller (2018). "Orientation-dependent stereo Wigner time delay and electron localization in a small molecule." Science 360 (6395): 1326 (10.1126/science.aao4731) Vos-2018


June 22, 2018. More >>
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Film shows one of the fastest processes in biology

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Biological light sensor filmed in action

Using X-ray laser technology, a team led by researchers of the Paul Scherrer Institute PSI has recorded one of the fastest processes in biology. In doing so, they produced a molecular movie that reveals how the light sensor retinal is activated in a protein molecule. Such reactions occur in numerous organisms that use the information or energy content of light – they enable certain bacteria to produce energy through photosynthesis, initiate the process of vision in humans and animals, and regulate adaptations to the circadian rhythm. The movie shows for the first time how a protein efficiently controls the reaction of the embedded light sensor. The images, now published in the journal Science, were captured at the free-electron X-ray laser LCLS at Stanford University in California. Further investigations are planned at SwissFEL, the new free-electron X-ray laser at PSI. Besides the scientists from Switzerland, researchers from Japan, the USA, Germany, Israel, and Sweden took part in this study. From PSI News.

Reference: Nogly, P., T. Weinert, D. James, S. Carbajo, D. Ozerov, A. Furrer, D. Gashi, V. Borin, P. Skopintsev, K. Jaeger, K. Nass, P. Båth, R. Bosman, J. Koglin, M. Seaberg, T. Lane, D. Kekilli, S. Brünle, T. Tanaka, W. Wu, C. Milne, T. White, A. Barty, U. Weierstall, V. Panneels, E. Nango, S. Iwata, M. Hunter, I. Schapiro, G. Schertler, R. Neutze and J. Standfuss (2018). "Retinal isomerization in bacteriorhodopsin captured by a femtosecond x-ray laser." Science 10.1126/science.aat0094Nogly-2018.

June 14, 2018. More >>
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Scientists from EPFL and Canada have developed a novel and unambiguous way to track energy flow in polyatomic molecules at ultrashort timescales

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Ultrafast X-Ray Spectroscopy of Conical Intersections

A team of scientists from the lab of Majed Chergui at EPFL within the Lausanne Centre for Ultrafast Science, the lab of Albert Stolow (University of Ottawa), and the lab of Michael Schuurman (NRC-Ottawa) have now devised an unambiguous approach to detect conical intersections in polyatomic molecules. The approach uses time-resolved X-ray spectroscopy (pioneered by the group of Majed Chergui) that is capable of detecting electronic structure changes with element selectivity, as the energy flows through the conical intersection.

Reference: Neville, S. P., M. Chergui, A. Stolow and M. S. Schuurman (2018). Ultrafast X-Ray Spectroscopy of Conical Intersections. Phys. Rev. Lett. 120: 243001. (10.1103/PhysRevLett.120.243001) Neville-2018.


June 12, 2018. More >>
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Researchers have placed an electron in a dual state -- neither freed nor bound

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Amplification of intense light fields by nearly free electrons

Half a century ago, Walter Henneberger wondered if it was possible to free an electron from its atom, but still make it stay around the nucleus. Scientists considered it was impossible. For the first time, physicists have managed to control the shape of the laser pulse to keep an electron both free and bound to its nucleus, and were at the same time able to regulate the electronic structure of this atom.

Reference: M. Matthews, F. Morales, A. Patas, A. Lindinger, J. Gateau, N. Berti, S. Hermelin, J. Kasparian, M. Richter, T. Bredtmann, O. Smirnova, J.-P. Wolf, and M. Ivanov, Amplification of intense light fields by nearly free electrons. Nat. Phys., (2018) (10.1038/s41567-018-0105-0). Matthews-2018


April 16, 2018. More >>
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What happens when nuclei can move as fast as electrons?

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Attosecond coupled electron and nuclear dynamics in dissociative ionization of H2

The interaction of an extreme ultraviolet attosecond pulse with a molecular system produces a sudden removal of an electron, which can lead to significant rearrangement of the residual molecular cation. The time scales of the induced electronic and the related nuclear dynamics are usually very different, thus allowing for a separate treatment of their motion. This is certainly the case for molecules containing heavy atoms. However, when light atoms are involved, in particular hydrogen, the space-time correlation between electronic and nuclear motions cannot be ignored. Cattaneo et al. for the first time clearly show that ionization delays in H2 can significantly depend on both photoelectron and the nuclear kinetic energy, which implies that whenever light atoms are involved in the molecular ionization process, the outgoing electron wave packet cannot be disentangled from the nuclear wave packet. The impact of this work goes well beyond the simple H2 molecule because H atoms are present in most organic and biologically relevant molecules, thus is of fundamental importance in many fields of research.

Reference: Cattaneo, L, J Vos, R Y Bello, A Palacios, S Heuser, L Pedrelli, M Lucchini, C Cirelli, F Martín, and U Keller, Attosecond coupled electron and nuclear dynamics in dissociative ionization of H2. Nature Physics, (2018) (10.1038/s41567-018-0103-2). Cattaneo-2018


April 16, 2018. More >>
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Efficient Ab Initio Molecular Dynamics Simulations

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A versatile Multiple Time Step Scheme

A new time-reversible, multiple time step (MTS) method for full QM and hybrid QM/MM Born-Oppenheimer molecular dynamics simulations relies on a fully flexible combination of electronic structure methods, from density functional theory to wavefunction-based quantum chemistry methods, to evaluate the nuclear forces in the reference and in the correction steps.
The possibility of combining different electronic structure methods is based on the observation that exchange and correlation terms only contribute to low frequency modes of nuclear forces. We show how a pair of low/high level electronic structure methods that individually would lead to very different system properties can be efficiently combined in the reference and correction steps of this MTS scheme.

Reference: E. Liberatore, R. Meli, and U. Rothlisberger, A Versatile Multiple Time Step Scheme for Efficient Ab Initio Molecular Dynamics Simulations. J. Chem. Theory Comput., (2018) (10.1021/acs.jctc.7b01189). Liberatore-20181
 

April 16, 2018. More >>
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Attosecond time–energy structure of X-ray free-electron laser pulses

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Angular streaking sets the stage for advanced X-ray pump/probe experiments.

Novel ultrabright X-ray free-electron laser (XFEL) facilities have been developed, which have opened the door to high-intensity X-ray experiments in the physical, chemical, life and material sciences, with implications to state-of-the-art technology and modern medicine. The authors report the measurement of attosecond time–energy information of individual XFEL pulses through angular streaking of X-ray-generated photoelectrons with a circularly polarized infrared (IR) laser pulse, where the time axis is mapped onto the angular axis. From this, we reconstruct the streaking field amplitude and phase as well as the intensity structure and chirp of the XFEL pulses on a single-shot basis with attosecond resolution. We find that the reconstructed substructure of the XFEL pulses is consistent with lower-resolution, indirect X band transverse cavity (XTCAV) measurements as well as with theoretical predictions of the SASE process

Reference:  Hartmann, N., G. Hartmann, R. Heider, M. S. Wagner, M. Ilchen, J. Buck, A. O. Lindahl, C. Benko, J. Grünert, J. Krzywinski, J. Liu, A. A. Lutman, A. Marinelli, T. Maxwell, A. A. Miahnahri, S. P. Moeller, M. Planas, J. Robinson, A. K. Kazansky, N. M. Kabachnik, J. Viefhaus, T. Feurer, R. Kienberger, R. N. Coffee and W. Helml (2018). Attosecond time–energy structure of X-ray free-electron laser pulses. Nat. Photonics. (10.1038/s41566-018-0107-6) Hartmann-2018.

March 5, 2018. More >>


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Attosecond optical-field-enhanced carrier injection into the GaAs conduction band

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Understanding the light-induced electron dynamics in a semiconductor on the attosecond timescale

In a semiconductor, electrons can be excited from the valence into to conduction band via the absorption of laser light. The motion of electrons in the semiconductor under the influence of high-frequency electric fields ultimately determines the material limit for high-speed device performance. The recent progress in few-cycle femtosecond and attosecond pulse generation with full electric-field control extends this frequency regime towards petahertz. Schlaepfer et al. used transient absorption spectroscopy to resolve the attosecond (10-18 s) laser-field-driven response of electrons in gallium arsenide. In addition to the excitation of electrons from one to another band (so-called inter-band transition), carriers can also be accelerated during the light-matter interaction within the individual bands due to the presence of the strong electric laser field (intra-band motion). While only the inter-band transition transfers electrons into the conduction band, Schlaepfer et al. found that the intra-band mechanism significantly enhances the number of these electrons. This finding is unexpected because intra-band motion alone is unable to produce charge carriers in the conduction band. These results represent an important step forward in understanding the light-induced electron dynamics in a semiconductor on the attosecond timescale.

Reference:  Schlaepfer, F., M. Lucchini, S. A. Sato, M. Volkov, L. Kasmi, N. Hartmann, A. Rubio, L. Gallmann and U. Keller (2018). Attosecond optical-field-enhanced carrier injection into the GaAs conduction band. Nat. Phys.: advanced online. (10.1038/s41567-018-0069-0) Schlaepfer-2018


March 12, 2018. More >>
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Anisotropic photoemission time delays close to a Fano resonance

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Direct access to the role of electron correlation in the auto ionization process.

We believe that our angle and spectrally resolved results, presented both for non-resonant and resonant case, represent the most complete study of the atomic time delay evolution across an autoionizing state, and thus allow direct access to the role of electron correlation in the auto ionization process. Our results clearly demonstrate that not only the phase of the photoelectron wave packet is significantly distorted in the presence of resonances, but that this distortion depends on the electron emission angle.
While working on the revision of our paper and attending the 6th International Conference on Attosecond Physics in China in July 2017, we came to know that a similar experiment as the one that we presented had been performed at the Lund University in the group led by Prof. Anne L’Huillier. After sharing the data and discussing the results with the Lund group, we decided that the inclusion of the Lund data into our manuscript would extremely strengthen the paper. Indeed, the results from the two datasets (ETH and Lund) are in excellent agreement in the non-resonant case, and allow the determination of angle and spectrally resolved data in the resonant case.

Reference:  Cirelli, C., C. Marante, S. Heuser, C. L. M. Petersson, Á. J. Galán, L. Argenti, S. Zhong, D. Busto, M. Isinger, S. Nandi, S. Maclot, L. Rading, P. Johnsson, M. Gisselbrecht, M. Lucchini, L. Gallmann, J. M. Dahlström, E. Lindroth, A. L’Huillier, F. Martín and U. Keller (2018). Anisotropic photoemission time delays close to a Fano resonance. Nat. Commun. 9: 955. (10.1038/s41467-018-03009-1) Cirelli-2018.


March 6, 2018. More >>
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Measurement of the Berry curvature of the real solids

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Connecting attosecond science and solid state physics: unveiling electronic properties of solids using high harmonic spectroscopy.

Emission of second harmonic generation in broken inversion symmetry media has been extensively studied since the advent of laser. This second order nonlinear process has been traditionally characterized using phenomenological nonlinear second order susceptibility. Microscopically, second order harmonic was emitted as a consequence of non-vanishing Berry curvature in the bulk of solids, along the investigated direction. Thus, Berry phase and Berry curvature have become ubiquitous concepts in physics, relevant to a variety of additional phenomena such as anomalous, quantum Hall effect, magnetic Bloch bands, piezoelectric effect, etc. Recently, attosecond science is being rapidly extended to the condensed phase and a vast variety of scientific possibilities are enabled. ETH professor Hans Jakob Woerner and one of his postdocs have demonstrated a first step in connecting two seemingly unrelated topics in modern science: attosecond physics and Berry phase effects in condensed phase. By performing polarimetry of high-order harmonic generation from solids, they proposed and showed a method to directly retrieve the Berry curvature using high harmonic spectroscopy. The results are subsequently verified using ab-initio calculations of Berry curvature in α-quartz. The work would have strong implications for condensed matter science as it allows a new class of direct measurement of Berry phase effects in the real solids which was not possible before, and it could serve as a benchmark for theoretical studies.

Reference:  Luu, T. T. and H. J. Wörner (2018). Measurement of the Berry curvature of solids using high-harmonic spectroscopy. Nat. Commun. 9: 916. (10.1038/s41467-018-03397-4) Luu-2018.


March 2, 2018. More >>
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Monitoring positive charges in solar materials

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A novel way of detecting positive charges (holes) and their trapping in solar materials.

The lab of Majed Chergui at EPFL, within the Lausanne Centre for Ultrafast Science, along with scientists from the Paul-Scherrer-Institut and the Argonne National Laboratory (Chicago) have now successfully detected holes and identified their trapping sites after above band-gap photoexcitation using time-resolved element-selective techniques. The researchers used a novel dispersive X-ray emission spectrometer, combined with X-ray absorption spectroscopy. The technique allowed them to directly detect the trapping of holes with a resolution of 80 picoseconds.

Reference:  Penfold, T. J., J. Szlachetko, F. G. Santomauro, A. Britz, W. Gawelda, G. Doumy, A. M. March, S. H. Southworth, J. Rittmann, R. Abela, M. Chergui and C. J. Milne (2018). Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy. Nature Communications 9: 478. (10.1038/s41467-018-02870-4) Penfold-2018

February 2, 2018. More >>
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Photoinduced transitions in magnetoresistive manganites: A comprehensive view

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Using femtosecond x-ray diffraction to study the structural response of  a phase transition induced by laser pulses

We use femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered Pr1−xCaxMnO3 thin films across a phase transition induced by 800 nm laser pulses. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant timescales. The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence fc. For excitations below fc the charge order and the superlattice is only partially suppressed and the ground state recovers within a few tens of nanosecond via diffusive cooling. When exciting above the critical fluence the superlattice vanishes within approximately half a picosecond followed by a change of the unit cell parameters on a 10 picoseconds timescale. At this point all memory from the symmetry breaking is lost and the recovery time increases by many order of magnitudes due to the first order character of the structural phase transition.
 

Reference:  V. Esposito,  L. Rettig, E. Abreu, E. Bothschafter,G. Ingold, M. Kawasaki, M. Kubli, G. Lantz, M. Nakamura, J. Rittman, M. Savoini, Y. Tokura, U. Staub, S. L. Johnson and P. Beaud, Photoinduced transitions in magnetoresistive manganites: A comprehensive view, Phys. Rev. B 97, 014312 (2018). (10.1103/PhysRevB.97.014312) Esposito-2018

January 31, 2018. More >>
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Excess energy of electrons in solar materials

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Majed Chergui and co-workers have shown that after electron delivery to the famous titanium dioxide material, the excess energy is lost to heat at extremely short time scales.

The recent identification of strongly bound excitons in room-temperature anatase TiO2 single crystals and nanoparticles underscores the importance of bulk many-body effects in samples used for applications. Here, for the first time, scientists unravel the interplay between many-body interactions and correlations in highly excited anatase TiO2 nanoparticles using ultrafast two-dimensional deep-ultraviolet spectroscopy. With this approach, under nonresonant excitation, the optical nonlinearities contributing to the bleach of the lowest direct exciton peak are disentangled. This allows the clocking of the ultrafast time scale of the hot electron thermalization in the conduction band with unprecedented temporal resolution, which was determined to be <50 fs, due to the strong electron–phonon coupling in the material. These findings call for the design of alternative resonant excitation schemes in photonics and nanotechnology.

Reference:  Baldini, E., T. Palmieri, E. Pomarico, G. Auböck and M. Chergui (2018). Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles. ACS Photonics. (10.1021/acsphotonics.7b00945) Baldini-2018.

January 11, 2018. More >>
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Short-pulse lasers for weather control

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Review published in Reports on Progress in Physics, by Jean-Pierre Wolf.

Filamentation of ultra-short TW-class lasers recently opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of 0.1–1 mm in diameter, spanning over hundreds of meters in length, and producing a low density plasma (1015–1017 cm−3) along their path. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or non-linear polarization saturation.

Reference:  Wolf, J. P. (2018). Short-pulse lasers for weather control. Rep. Prog. Phys. 81: 026001 (10.1088/1361-6633/aa8488) Wolf-2018

January 10, 2018. More >>
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Clocking the dynamics of effective mass

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A paper by Lamia Kasmi in Optica, with a news item in Science.

Electrons propagating through a solid interact with the crystal lattice, modifying the electronic motion and giving rise to an effective electron mass. The idea of an effective electron mass is based on the assumption of an unbounded crystal lattice, so the question arises of whether this understanding applies at the scale of small electronic devices. Using ultrafast pump-probe spectroscopy, Kasmi et al. studied the temporal dynamics of photoemitted electrons from a copper target. They found that the electrons require up to 350 attoseconds to reach their effective mass, equating to a propagation distance of just two atomic layers. The results could have bearing on the performance of shrinking electronic circuits, as well as in correctly interpreting ultrafast photoemission process from solids (from the News Item in Science.

Reference:  Kasmi, L., M. Lucchini, L. Castiglioni, P. Kliuiev, J. Osterwalder, M. Hengsberger, L. Gallmann, P. Krüger and U. Keller (2017). Effective mass effect in attosecond electron transport. Optica 4: 1492-1497. (10.1364/OPTICA.4.001492) Kasmi-2017 (1.34 MB).

November 31, 2017. More >>
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Coupling between a charge density wave and magnetism in a Heusler material


The magnetic shape memory effect is a phenomenon where a change in magnetization drives a large, macroscopic structural deformation

The lowest temperature phase of the ferromagnetic alloy Ni2MnGa is a prototypical example of this phenomenon, showing magnetically induced lattice strains over 10%.  In this material the effect is thought to be associated with an incommensurate structural modulation along the [110] direction which appears also in an intermediate temperature phase, but it is unclear at present exactly how this modulation is coupled to the magnetism.  In this work measurements of both the magnetism and charge density wave (CDW) amplitude as a function of time after electronic excitation with an ultrafast last pulse show a possible direct, dynamic  connection between magnetism and the CDW.  The magnetization drops in a few hundred femtoseconds, accompanied by a phase transition to an unmodulated cubit structural phase.  The results indicate a transient shift of the q-vector of the CDW that may be driven by the transient drop in magnetization.  This in turn indicates that a long-range structural reordering can take place on time scales exceeding the nominal limits imposed by the time required for the speed of sound to propagate over the probed volume, implying that the changes in long-range order are not initiated at the surface but instead at internal domain boundaries.
 
Reference:  Lantz, G, M J Neugebauer, M Kubli, M Savoini, E Abreu, K Tasca, C Dornes, V Esposito, J Rittmann, Y W Windsor, P Beaud, G Ingold, and S L Johnson, Coupling between a Charge Density Wave and Magnetism in an Heusler Material. Phys. Rev. Lett., (2017) 119: 227207 (10.1103/PhysRevLett.119.227207) Lantz-2017

November 29, 2017. More >>
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The world´s shortest laser pulse

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ETH researchers in the group of Hans Jakob Wörner succeeded in shortening the pulse duration of an X‑ray laser to only 43 attoseconds.

With a time resolution in the range of a few quintillionths of a second, they are now able for the first time to observe the movement of electrons during chemical reactions in slow motion. In order to fully understand the dynamics during a chemical reaction, scientists must be able to study all movements of atoms and molecules on their basic time scale. Molecules rotate in the range of picoseconds (10-12 s), their atoms vibrate in the range of femtoseconds (10‑15 s), and the electrons move in the range of attoseconds (10-18 s). ETH professor Hans Jakob Wörner and his group have now succeeded in generating the world's shortest laser pulse with a duration of only 43 attoseconds. More generally speaking, this laser pulse is the shortest controlled event that has ever been created by humans. The researchers can now observe in high detail how electrons move within a molecule or how chemical bonds are formed.

Reference:  Gaumnitz, T., A. Jain, Y. Pertot, M. Huppert, I. Jordan, F. Ardana-Lamas and H. J. Wörner (2017). Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver. Optics Express 25: 27506-27518. (10.1364/OE.25.027506) Gaumnitz-2017 (4.96 MB).

October 30, 2017. More >>
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New experiments and simulations reveal molecular interactions in extreme phases of water ice

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Researchers from University College London, the University of Groningen and the group of Peter Hamm show how water molecules behave when packed in dense and structurally complex environments

Water is everywhere. But it's not the same everywhere. When frozen under extreme pressures and temperatures, ice takes on a range of complex crystalline structures. Many of the properties and behaviors of these exotic ices remain mysterious, but a team of researchers recently provided new understanding. They analyzed how water molecules interact with one another in three types of ice and found the interactions depended strongly on the orientation of the molecules and the overall structure of the ice. The researchers describe their results in The Journal of Chemical Physics, from AIP Publishing.

Reference: Tran, H., A. V. Cunha, J. J. Shephard, A. Shalit, P. Hamm, T. L. C. Jansen and C. G. Salzmann (2017). 2D IR spectroscopy of high-pressure phases of ice. J. Chem. Phys. 147: 144501 (10.1063/1.4993952) Tran-2017

October 12, 2017. More >>
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Ultrafast light excitation to control the carrier density in multiband materials via hot phonon effects

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Fabrizio Carbone and co-workers open new perspectives for the selective carrier-density manipulation via near-infrared light

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonic modes, with exotic consequences for the emergent electronic properties. Here we show that the population of E2g phonons in the multiband superconductor MgB2 can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the σ-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies and modeled theoretically, revealing the details of the σπ interband scattering mechanism in MgB2.

Reference: Baldini, E., A. Mann, L. Benfatto, E. Cappelluti, A. Acocella, V. M. Silkin, S. V. Eremeev, A. B. Kuzmenko, S. Borroni, T. Tan, X. X. Xi, F. Zerbetto, R. Merlin and F. Carbone (2017). Real-Time Observation of Phonon-Mediated σ−π Interband Scattering in MgB2. Phys. Rev. Lett. 119: 097002 (10.1103/PhysRevLett.119.097002 Baldini-2017 (484 KB)

August 31, 2017. More >>
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Deep-UV probing method detects electron transfer in photovoltaics

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Majed Chergui and co-workers have developed a new method to efficiently measure electron transfer in dye-sensitized transition-metal oxide photovoltaics.

Ultrafast interfacial electron transfer in sensitized solar cells has mostly been probed by visible-to-terahertz radiation, which is sensitive to the free carriers in the conduction band of the semiconductor substrate. Here, we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitized anatase TiO2 and wurtzite ZnO nanoparticles. Our results are compared to those obtained on bare nanoparticles upon above-gap excitation. We show that the signal upon electron injection from the N719 dye into TiO2 is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling. The present approach offers a possible route to detecting interfacial electron transfer in a broad class of systems, including other transition metal oxides or sensitizers.

Reference:  Baldini, E., T. Palmieri, T. Rossi, M. Oppermann, E. Pomarico, G. Auböck and M. Chergui (2017). Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature. J. Am. Chem. Soc. (10.1021/jacs.7b06322) Baldini-2017.

August 14, 2017. More >>
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A new route to manipulate the electronic properties of a material via vibrational excitation.

Nonlinear electron-phonon coupling in doped manganites
 
In this experiment conducted at LCLS we employed resonant trXRD to study the melting of charge order and the associated insulator-to-metal transition in the doped manganite Pr0.5Ca0.5MnO3 after resonant excitation of a high-frequency infrared-active lattice mode. The charge order reduces promptly and - contrary to near-infrared excitation - highly nonlinear as function of excitation fluence. Although here a structural mode is initially excited, the collapse of the electronic order precedes the structural transformation similar as it does in the electronic excitation case. The highly nonlinear fluence dependence, however, points to a distinctive different pathway. Density-functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drives these dynamics.

Reference:  V. Esposito, R. Mankowsky, M. Fechner, H. Lemke, M. Chollet, J. M. Glownia, M. Nakamura, M. Kawasaki, Y. Tokura, U. Staub, P. Beaud, and M. Först, Nonlinear electron-phonon coupling in doped manganites, Phys. Rev. Lett. 118, 247601 (2017). (10.1103/PhysRevLett.118.247601) Esposito-2017

June 15, 2017. More >>
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Imaging ultrafast dynamics of molecules on femtosecond to attosecond timescales

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The group of Hans Jakob wörner observes valence-shell electron and coupled electronic-nuclear dynamics using strong-field photoelectron holography and rescattering.

Strong-field photoelectron holography and laser-induced electron diffraction (LIED) are two powerful emerging methods for probing the ultrafast dynamics of molecules. However, both of them have remained restricted to static systems and to nuclear dynamics induced by strong-field ionization. Here we extend these promising methods to image purely electronic valence-shell dynamics in molecules using photoelectron holography. In the same experiment, we use LIED and photoelectron holography simultaneously, to observe coupled electronic-rotational dynamics taking place on similar timescales. These results offer perspectives for imaging ultrafast dynamics of molecules on femtosecond to attosecond timescales.

Reference:  Walt, S. G., N. Bhargava Ram, M. Atala, N. I. Shvetsov-Shilovski, A. von Conta, D. Baykusheva, M. Lein and H. J. Wörner (2017). Dynamics of valence-shell electrons and nuclei probed by strong-field holography and rescattering.  8: 15651. (10.1038/ncomms15651) Walt-2017 (2.65 MB)

June 15, 2017. More >>
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Genuine binding energy of the hydrated electron


Researchers at ETH Zurich and the University of Kyoto demonstrate the importance of quantitative scattering simulations for a detailed analysis of key properties of the hydrated electron.

A combined photoelectron study on water droplets and a liquid water microjet reveals for the first time the influence of electron scattering on the binding energy and the photoelectron anisotropy of the hydrated electron, and allows the retrieval of corresponding genuine values. Such data are important for a better understanding of the role of pre-hydrated and hydrated electrons in the chain of radiation damage processes in aqueous environment.
 
Reference:  Luckhaus, D., Y.-i. Yamamoto, T. Suzuki and R. Signorell (2017). Genuine binding energy of the hydrated electron. Sci. Adv. 3. (10.1126/sciadv.1603224) Luckhaus-2017 (377 KB).

April 28, 2017. More >>
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Shedding light on the absorption of light by titanium dioxide

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EPFL scientists have uncovered the hidden properties of titanium dioxide, one of the most promising materials for light-conversion technology.

Titanium dioxide (TiO2) is one of the most promising materials for photovoltaics and photocatalysis nowadays. This material appears in different crystalline forms, but the most attractive one for applications is called "anatase". Despite decades of studies on the conversion of the absorbed light into electrical charges in anatase TiO2, the very nature of its fundamental electronic and optical properties was still unknown. EPFL scientists, with national and international partners, have now shed light onto the problem by a combination of cutting-edge steady-state and ultrafast spectroscopic techniques, as well as theoretical calculations. The work is published in Nature Communications.

Reference:  Baldini, E, L Chiodo, A Dominguez, M Palummo, S Moser, M Yazdi-Rizi, G Auböck, B P P Mallett, H Berger, A Magrez, C Bernhard, M Grioni, A Rubio, and M Chergui, Strongly bound excitons in anatase TiO2 single crystals and nanoparticles. Nature Communications, (2017) 8: 13 (10.1038/s41467-017-00016-6) Baldini-2017.

April 13, 2017. More >>
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An ultrafast X-ray source in laboratory format

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Researchers at ETH Zurich and the University of Geneva have succeeded for the first time in using a laboratory X-ray source to demonstrate how two highly fluorinated molecules change within a few quadrillionths of a second, or femtoseconds.

In nature, some processes occur so quickly that even the blink of an eye is very slow in comparison. Many basic physical, chemical and biological reactions take place on the ultrafast time scale of a few femtoseconds (10−15 s) or even attoseconds (10−18 s). In molecules, elementary particles, such as electrons or photons, move in a mere 100 attoseconds (10−16 s). When electrons in a molecule jump from one atom to another, chemical bonds dissolve and new ones arise within a fraction of a femtosecond. The ability to track processes of this kind on the atomic scale in real time is one of the key reasons for development of major new research facilities such as the SwissFEL free electron laser. Now, researchers from the ETH Zurich and the University of Geneva have found a way to study ultrafast processes of this kind in the laboratory, using a soft X-ray source.

Reference:  Pertot, Y., C. Schmidt, M. Matthews, A. Chauvet, M. Huppert, V. Svoboda, A. von Conta, A. Tehlar, D. Baykusheva, J.-P. Wolf and H. J. Wörner (2017). Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source. Science. (10.1126/science.aah6114) Pertot-2017 (1.13 MB)

January 17, 2017. More >>
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Using X-rays to produce a movie of the photosynthesis reaction

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Researchers supported by the NCCR MUST and the ETH FAST program have produced a molecular movie of the events taking place during photosynthesis. Bacteriorhodopsin (bR) is a light-driven proton pump and a model membrane transport protein. Time-resolved serial femtosecond crystallography (TR-SFX) at an x-ray free electron laser was used to visualize conformational changes in bR from nanoseconds to milliseconds following photoactivation. An initially twisted retinal chromophore displaces a conserved tryptophan residue of transmembrane helix F on the cytoplasmic side of the protein while dislodging a key watermolecule on the extracellular side. The resulting cascade of structural changes throughout the protein shows how motions are choreographed as bR transports protons uphill against a transmembrane concentration gradient. The structures were obtained from bR microcrystals suspended in a lipidic cubic phase matrix as described in an earlier highlight.

Reference:  Nango, E., A. Royant, M. Kubo, T. Nakane, C. Wickstrand, T. Kimura, T. Tanaka, K. Tono, C. Song, R. Tanaka, T. Arima, A. Yamashita, J. Kobayashi, T. Hosaka, E. Mizohata, P. Nogly, M. Sugahara, D. Nam, T. Nomura, T. Shimamura, D. Im, T. Fujiwara, Y. Yamanaka, B. Jeon, T. Nishizawa, K. Oda, M. Fukuda, R. Andersson, P. Båth, R. Dods, J. Davidsson, S. Matsuoka, S. Kawatake, M. Murata, O. Nureki, S. Owada, T. Kameshima, T. Hatsui, Y. Joti, G. Schertler, M. Yabashi, A.-N. Bondar, J. Standfuss, R. Neutze and S. Iwata (2016). A three-dimensional movie of structural changes in bacteriorhodopsin. Science 354: 1552 (10.1126/science.aah3497) Nango-2016 (1.52 MB)

January 3, 2017. More >>
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Chemically Modified Insulin Is Available More Rapidly

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Insulin retains its efficacy but is available more rapidly to the organism when a hydrogen atom is replaced by an iodine atom. Markus Meuwly, Krystel El Hage, Vijay Pandyarajan and co-workers at the University of Basel undertook quantitative atomic-level simulations of 3-I-TyrB26-insulin to predict its structural features and (ii) tested these predictions by X-ray crystallography. Inspired by quantum chemistry and molecular dynamics, such “halogen engineering” promises to extend principles of medicinal chemistry to proteins.

Reference: El Hage, K., V. Pandyarajan, N. B. Phillips, B. J. Smith, J. G. Menting, J. Whittaker, M. C. Lawrence, M. Meuwly and M. A. Weiss (2016). Extending Halogen-Based Medicinal Chemistry to Proteins: Iodo-Insulin as a Case Study. J. Biol. Chem. (10.1074/jbc.M116.761015) El-Hage-2016 (2.42 MB).

November 14, 2016. More >>
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Evidence that cations structure the hydrogen-bond network in water

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The structural and dynamical properties of water are known to be affected by ion solvation. However, a consistent molecular picture that describes how and to what extent ions perturb the water structure is still missing. Here Andrey Shalit, Saima Ahmed, Janne Savolainen and Peter Hamm apply 2D Raman–terahertz spectroscopy to investigate the impact of monatomic cations on the relaxation dynamics of the hydrogen-bond network in aqueous salt solutions. The inherent ability of multidimensional spectroscopy to deconvolute heterogeneous relaxation dynamics is used to reveal the correlation between the inhomogeneity of the collective intermolecular hydrogen-bond modes and the viscosity of a salt solution. Specifically, they demonstrate that the relaxation time along the echo direction t1= t2 correlates with the capability of a given cation to ‘structure’ water. Moreover, the authors provide evidence that the echo originates from the water–water modes, and not the water–cation modes, which implies that cations can structure the hydrogen-bond network to a certain extent.

Reference: Shalit, A., S. Ahmed, J. Savolainen and P. Hamm (2017). Terahertz echoes reveal the inhomogeneity of aqueous salt solutions. Nature Chem. 9, 273–278. (10.1038/nchem.2642) Shalit-2016 (2.84 MB).

October 31, 2016. More >>
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An Ultrafast Method to Track the Movement of Light and Electrons in Nanostructured Surface

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Tom Lummen, Fabrizio Carbone and co-workers demonstrate the in situ visualization of photoinduced plasmonic interference patterns confined to otherwise inaccessible buried interfaces.  This provides a critical tool for the investigation and development of complex plasmonic heterostructures and advanced multilayer devices. The experiments demonstrate the feasibility of ultrafast imaging of plasmon dynamics using photon-induced near-field electron microscopy (PINEM), which makes it possible to measure the carrier wavelength and propagation speed of surface plasmon polaritons (SPPs) travelling along buried interfaces directly in the time domain. Furthermore, the authors show that transient plasmonic interference patterns can be shaped, manipulated and controlled through both the polarization of the excitation light and the nanopatterning architecture. This facilitates a widely tunable range of nanoscale near-field structures. Finally, the results presented here represent a considerable advance towards the realization of the recently proposed methodology involving inelastic electron diffraction from transient plasmonic gratings.

Reference: Lummen, T. T. A., R. J. Lamb, G. Berruto, T. LaGrange, L. Dal Negro, F. J. García de Abajo, D. McGrouther, B. Barwick and F. Carbone (2016). Imaging and controlling plasmonic interference fields at buried interfaces. Nature Commun. 7: 13156. (10.1038/ncomms13156, http://www.nature.com/articles/ncomms13156 - suppl-info) Lummen-2016 (1.7 MB).

October 25, 2016. More >>
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A new technique opens up advanced solar cells

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Organic photovoltaics (OPVs) are a major promise for future solar technologies thanks to a combination of attractive properties: light weight, low manufacturing cost, availability in different colors and on flexible substrates. Beside these advantages, so far OPVs only achieve about 10% light-to-electricity power conversion efficiency, while silicon solar cells achieve values over 20%. To overcome this disadvantage, a deeper mechanistic understanding of the fundamental processes in OPVs is necessary. Light, when absorbed by a blend of donor and acceptor organic semiconductors, generates bound negative and positive charges (exitons) that need to be separated and driven to the electrodes of the solar cell. Scientists from Prof. Natalie Banerji’s group (University of Fribourg) in collaboration with Imperial College London and EPF Lausanne could follow the fate of the initially generated excitons for a variety of donor-acceptor blends by using a new ultrafast spectroscopic technique: electro-modulated differential absorption (EDA). EDA allows to monitor the separation distance of charges in real time by taking advantage of the Stark effect (shift of the absorption spectrum in an electric field).The results of this study, recently published in Nature Communications, provide a deeper understanding of the initial processes in OPVs, which are strongly affected by the arrangement of the donor and the acceptor, and will help to improve the design of future materials and devices.

Reference: Causa, M., J. De Jonghe-Risse, M. Scarongella, J. C. Brauer, E. Buchaca-Domingo, J.-E. Moser, N. Stingelin and N. Banerji (2016). The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics. Nature Commun. 7: 12556. (10.1038/ncomms12556) Causa-2016 (26.44 MB)

September 2, 2016. More >>
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Is there a material limit for high-speed electronics?

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The ultrafast electron motion driven by a high-frequency electric field ultimately determines the material limit for high-speed device performance. Ultrafast laser sources with few-cycle femtosecond pulses and with full electric field control allow us to fully bridge the gap between electronics and optics with frequencies approaching the petahertz regime. In a recent experiment at ETH Zurich, Matteo Lucchini, Lukas Gallmann, Ursula Keller and co-workers have investigated the response of electrons in thin films (50 nm) of diamond to electric fields oscillating at optical frequencies of about half a petahertz. They exposed the diamond to a few-femtosecond infrared light pulse and probed the absorption changes induced by the resulting electron motion with attosecond extreme ultraviolet pulses. Through collaboration with the theory group of Katsuhiro Yabana (Tsukuba University, Japan) and with the help of state-of-the-art numerical calculations, the time-varying structures in the signal could be assigned to the so-called dynamical Franz-Keldysh effect, which was observed here for the first time on a few-femtosecond time scale. Furthermore they could show that intra-band transitions dominate the response over inter-band transitions in the solid. This finding helps to resolve a controversy about the importance of inter- vs. intra-band mechanisms in strong-field driven interactions that was triggered by contradicting interpretations of results reported in recent publications. On the application side our observations indicate that electrons in the solid can indeed be steered at close to petahertz frequencies of our light pulses. While other physical processes may limit practical device performance of future electronic components, this experiment shows that no such speed barrier is to be expected anytime soon with respect to our ability to manipulate the charges with electric fields.

Reference: Lucchini, M., Sato, S. A., Ludwig, A., Herrmann, J., Volkov, M., Kasmi, L., Shinohara, Y., Yabana, K., Gallmann, L., and Keller, U. (2016). Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond. Science 353, 6302, 916-919. DOI: 10.1126/science.aag1268, Lucchini-2016 (1.87 MB).

August 26, 2016. More >>
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Attosecond Delays in Molecular Photoionization

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Attosecond experiments have revealed that the photoionization of electrons occurs with measurable delays in atomic gases and solids. However, compared to atoms (or solids made of only one atomic species), measuring photoionization delays in molecules is more challenging. This is because molecules have both a higher density of states and an anisotropic electrostatic potential arising from the molecular geometry. These two factors complicate the interpretation of ionization measurements. Now, Martin Huppert, Hans Jakob Wörner and co-workers , at the Swiss Federal Institute of Technology (ETH) in Zurich, have overcome these challenges, obtaining measurements of attosecond ionization delays in two molecules for a range of photon energies. They find that ionization delays in a water molecule are as short as those of the simplest atom (hydrogen), while the delays for another molecule (N2O) can be much larger for photon energies corresponding to characteristic molecular resonances.

Reference: Huppert, M., Jordan, I., Baykusheva, D., von Conta, A., and Wörner, H. J. (2016). Attosecond Delays in Molecular Photoionization. PhysRevLett.117.093001 Huppert-2016 (585 KB)

August 22, 2016. More >>
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Catching proteins in the act with a lipidic cubic phase injector

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Some of the fastest processes in our body run their course in proteins activated by light. The protein rhodopsin sees to it that our eyes can rapidly take in their ever-changing surroundings. Free-electron X-ray lasers now make it possible for the first time to catch such processes in flagranti. Free-electron X-ray lasers generate extremely short and intense pulses of X-ray light. An international team under the leadership of the PSI (Schertler & Standfuss) has now successfully shown how the ultrafast processes by which proteins do their work can be studied with free-electron X-ray lasers. As a model organism, they used a simple microbe that can convert light into chemical energy. The researchers injected bacteriorhodopsin crystals into the X-ray beam with a special injector. In this injector the crystals, just a few micrometres in size, are embedded in an extremely viscous fluid: the lipidic cubic phase.

Reference: Nogly, P., et al. (2016). Lipidic cubic phase injector is a viable crystal delivery system for time-resolved serial crystallography. Nature Commun. 7: 12314. 10.1038/ncomms12314 Nogly-2016 (1.87 MB).


August 22, 2016. More >>
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Computer Simulation Renders Transient Chemical Structures Visible

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Maksym Soloviov, Akshaya K. Das, and Markus Meuwly use computational chemistry to characterize the motion of individual atoms of the protein myoglobin. Myoglobin plays an important role in the transport of oxygen within cells and is found mainly in muscle tissue. Nitrogen monoxide, which is formed in the cells, is a short-lived and reactive messenger that is important in regulating vasodilation under hypoxia.

Using computational chemistry, it is possible to characterize the motion of individual atoms of a molecule. Today, the latest simulation techniques allow scientists to quantitatively describe the dynamics of molecules and systems containing hundreds of thousands of atoms. These techniques are important, above all, for characterizing molecular states that are difficult to observe directly in experiments due to their short lifetime. Here, computer simulations are a source of valuable complementary insight.

Reference: Soloviov, M., A. K. Das and M. Meuwly (2016). Structural Interpretation of Metastable States in Myoglobin–NO. Angew. Chem. Int. Ed.: n/a-n/a. (10.1002/anie.201604552) Soloviov-2016 (2.28 MB)


July 14, 2016. More >>
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Coupled local and itinerant demagnetization dynamics

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The exchange interaction between different magnetic moments is the dominating force determining spin ordering in magnetic materials. In particular the interplay of itinerant d-electron and localized f-electron magnetic moments in 4f metals and their alloys opens up new routes to control their magnetic behavior on ultrafast timescales.

Reference: Rettig, L, C Dornes, N Thielemann-Kühn, N Pontius, H Zabel, D L Schlagel, T A Lograsso, M Chollet, A Robert, M Sikorski, S Song, J M Glownia, C Schüßler-Langeheine, S L Johnson, and U Staub, Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho. Phys. Rev. Lett., (2016) 116: 257202 (10.1103/PhysRevLett.116.257202). Rettig-2016


June 24, 2016. More >>
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Laser vaporization of cirrus-like ice particles with secondary ice multiplication

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Laser blasts might help scientists tweak Earth’s thermostat by shattering the ice crystals found in cirrus clouds. Zapping tiny ice particles in the lab forms new, smaller bits of ice, Mary Matthews, Jean-Pierre Wolf and co-workers from the University of Geneva and from Karlsruhe Institute of Technology in Germany report May 20 in Science Advances. Since clouds with more numerous, smaller ice particles reflect more light, the technique could combat global warming by causing the clouds to reflect more sunlight back into space. They injected water drops into a chilled chamber that mimics the frigid conditions high in the atmosphere, where wispy cirrus clouds live. The water froze into spherical ice particles, which the scientists walloped with short, intense bursts of laser light.

Reference: Matthews, M., F. Pomel, C. Wender, A. Kiselev, D. Duft, J. Kasparian, J.-P. Wolf and T. Leisner (2016). Laser vaporization of cirrus-like ice particles with secondary ice multiplication. Sci. Adv. 2. (10.1126/sciadv.1501912) Matthews-2016 (896 KB).


May 20, 2016. More >>
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First measurement of multi-harmonics generation from a single nanoparticle

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Jean-Pierre Wolf, Luigi Bonacina and co-workers report on the first measurement of multi-harmonics generation from a single nanoparticle and its related nonlinear optical tensors. In addition to the fundamental interest of this study, we propose to use multi-harmonics emission from nonlinear nanoparticles for improving the selectivity of probes when imaging biological tissues. Endogenous fluorescence from tissues eventually limits the identification of the probes, and the co-localization of different harmonics from the same nanoparticle allows to unambiguously determine their position.  We demonstrated this approach for applications like cell tracking (stem cells, cancer cells), in collaboration with Max Planck for Experimental Medicine and Institut Curie.

Reference: Schmidt, C., J. Riporto, A. Uldry, A. Rogov, Y. Mugnier, R. L. Dantec, J.-P. Wolf and L. Bonacina (2016). Multi-Order Investigation of the Nonlinear Susceptibility Tensors of Individual Nanoparticles. Sci. Rep. 6: 25415. (10.1038/srep25415) Schmidt-2016 (707 KB)

May 3, 2016. More >>
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Dynamical Symmetries of Atoms and Molecules revealed by Bicircular High-Harmonic Spectroscopy

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Symmetry is a fundamental concept in science and plays a central role in our understanding of matter, and is also at the origin of selection rules that govern spectroscopy. These rules have been essential in determining molecular structures. Access to symmetry on subfemtosecond time scales would open new avenues in time-resolved spectroscopy. Denitsa Baykusheva, Hans Jakob Wörner and co-workers introduce bicircular high-harmonic spectroscopy as a new method to probe dynamical symmetries of atoms and molecules and their evolution in time. Bicircular HHS has a broad range of innovative applications, such as the generation of isolated elliptically polarized attosecond pulses. Extended to solids it will open up promising directions, such as the time-resolved study of symmetry and symmetry breaking in crystals.

Reference: Baykusheva, D., M. S. Ahsan, N. Lin and H. J. Wörner (2016). Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules. Phys. Rev. Lett. 116: 123001. (10.1103/PhysRevLett.116.123001) Baykusheva-2016 (703 KB).

March 25, 2016. More >>
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Electron transfer dynamics observed over 8 orders of magnitude in time

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In a collaboration between the MUST-groups of Markus Meuwly and Jean-Pierre Wolf, they and their co-workers investigated the dynamics of sequential proton coupled electron transfer (PCET) over 8 orders of magnitude in time.

Charge transfer mechanisms lay at the heart of chemistry and biochemistry. Proton coupled electron transfers (PCET) are central in biological processes such as photosynthesis and in the respiratory chain, where they mediate long range charge transfers. These mechanisms are normally difficult to harness experimentally due to the intrinsic complexity of the associated biological systems. Metal-peptide cations experience both electron and proton transfers upon photo-excitation, proving an amenable model system to study PCET.

Reference: MacAleese, L., S. Hermelin, K. El Hage, P. Chouzenoux, A. Kulesza, R. Antoine, L. Bonacina, M. Meuwly, J.-P. Wolf and P. Dugourd (2016). Sequential Proton Coupled Electron Transfer (PCET): Dynamics Observed over 8 Orders of Magnitude in Time. J. Am. Chem. Soc. (10.1021/jacs.5b12587) MacAleese-2016 (807 KB).

March 23, 2016. More >>
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Ptychographic reconstruction of attosecond pulses (free software)

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Thomas Feurer, Ursula Keller and co-workers demonstrate a new attosecond pulse reconstruction modality which uses an algorithm that is derived from ptychography. In contrast to other methods, energy and delay sampling are not correlated, and as a result, the number of electron spectra to record is considerably smaller. Together with the robust algorithm, this leads to a more precise and fast convergence of the reconstruction.

In March 2016, the paper was choosen for inclusion in OSA Spotlight on Optics. From the OSA letter: "Spotlight on Optics (Spotlight) showcases research produced in our journals-research and information that would be impossible without your talent and contribution. Your paper is in excellent company. Only two papers are highlighted from our respective journals each month from among the scores of fine articles published."

In the context of this publication, the authors offer a minimal example MATLAB code that demonstrates the capabilities of this method based on four example data sets. Use of this software is free under the condition, that you include a reference to the original publication (below) whenever you make use of it.

Download software (10.56 MB).

Lucchini, M., Brügmann, M.H., Ludwig, A., Gallmann, L., Keller, U., and Feurer, T. (2015) Ptychographic reconstruction of attosecond pulses. Optics Express  23,  29502-29513 (10.1364/OE.23.029502).

November 4, 2015. More >>
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Ionization Charge Dynamics Tracked

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In a step toward steering electrons inside molecules to control chemical reactivity, researchers report following electron-hole migration in iodoacetylene (H–C≡C–I) with 100-attosecond resolution after ionizing the molecule with a laser. How the electron hole migrates depends on the orientation of the molecule relative to the direction the laser light is polarized.
Led by ETH Zurich’s Hans Jakob Wörner, the researchers used a technique called high harmonic generation in which a laser pulse causes an electron to tunnel out and away from an atom—in this case, primarily the iodine of iodoacetylene. When the electron and hole recombine, the process releases a burst of attosecond-duration X-rays. If the molecule is perpendicular to the laser polarization field when it is ionized, the hole initially localizes on the iodine. The hole then delocalizes over the molecule before localizing on the carbons. If the molecule is parallel to the laser polarization field, the hole localizes mostly on the carbons.

Kraus, P.M., Mignolet, B., Baykusheva, D., Rupenyan, A., Horný, L., Penka, E.F., Grassi, G., Tolstikhin, O.I., Schneider, J., Jensen, F., Madsen, L.B., Bandrauk, A.D., Remacle, F., and Wörner, H.J. (2015) Measurement and laser control of attosecond charge migration in ionized iodoacetylene. Science 350, 790-795 (10.1126/science.aab2160) Kraus-20151 (1.64 MB)

October 22, 2015. More >>
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Dissecting the electronic dynamics of a photovoltaic material

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Transition metal oxides are among the most promising materials for the conversion of solar energy into electricity (photovoltaics) or into chemical energy such as the splitting of water (photocatalysis). Their structure makes them ideal for generation, transport and trapping of charge carriers, such as electrons and electron holes. Titanium dioxide is a promising transition metal oxide, but determining its electron dynamics at room temperature has proven very difficult. EPFL, ETHZ and PSI scientists have solved the problem by using X-ray absorption spectroscopy (XAS). Published in Scientific Reports, the study reveals new information about electron movement in the surface region of titanium dioxide, opening new potential for photovoltaic and photocatalytic systems.

Santomauro, F.G., Lübcke, A., Rittmann, J., Baldini, E., Ferrer, A., Silatani, M., Zimmermann, P., Grübel, S., Johnson, J.A., Mariager, S.O., Beaud, P., Grolimund, D., Borca, C., Ingold, G., Johnson, S.L., and Chergui, M. (2015) Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2. Scientific Reports  5,  14834 (10.1038/srep14834)

October 8, 2015. More >>
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Tracking a biological process with atomic specificity

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Using “time-resolved X-ray absorption spectroscopy” (XAS), which can capture detailed information about the electronic structure of specific atoms in a molecular system, as well as the geometry around them, Majed Chergui and co-workers were able to confirm for the first time that an intermediate, “domed” structure, does indeed occur after nitric oxide rebinds to heme, unlike with any of the other diatomic ligands (cyanide, carbon monoxide etc). They also showed that the 200-picosecond timescale is actually due to nitric oxide molecules coming from remote docking sites in the protein. This work opens the way to a detailed investigation of metalloproteins using subpicosecond X-ray spectroscopy at free electron lasers.

Silatani M, Lima FA, Penfold TJ, Rittmann J, Reinhard M, Rittmann-Frank H, Borca C, Grolimund D, Milne CJ, Chergui M. NO binding kinetics in Myoglobin investigated by picosecond Fe K-edge absorption spectroscopy. PNAS 05 October 2015. DOI: 10.1073/pnas.1424446112.

October 6, 2015. More >>
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Understanding single-photon ionization dynamics - is the Wigner time delay valid?


Cirelli, Keller and co-workers demonstrate that the Wigner time delay (related to the electron wave packet group delay) can correctly explain the “classical trajectory” of the center of an electron wave packet only up to a certain level. We are able to show experimentally that the Wigner time delay can reproduce correctly the general trend of the measured delays but it does not capture all the observed features.

Sabbar, M., Heuser, S., Boge, R., Lucchini, M., Carette, T., Lindroth, E., Gallmann, L., Cirelli, C., and Keller, U. (2015) Resonance Effects in Photoemission Time Delays. Phys Rev Lett  115,  133001 (10.1103/PhysRevLett.115.133001)

September 23, 2015. More >>
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Are the laws of optics still valid at the ultimate scaling limits of electronic and optoelectronic devices?


The laws of optics describing phenomena such as reflection or refraction are very well tested and established. However, they essentially describe the macroscopic and quasi-static response of matter to the electromagnetic light fields. While this view provides the correct description for most applications, the question arises whether the same optics laws can also be transferred to atomic length and time scales, which represent the ultimate scaling limits of electronic and optoelectronic devices. The authors answer this question by probing a metal surface with atomic length and attosecond time resolution.

Lucchini, M., Castiglioni, L., Kasmi, L., Kliuiev, P., Ludwig, A., Greif, M., Osterwalder, J., Hengsberger, M., Gallmann, L., and Keller, U. (2015) Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models. Phys Rev Lett  115,  137401 (10.1103/PhysRevLett.115.137401).

September 22, 2015. More >>
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Spintronics just got faster

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In a tremendous boost for spintronic technologies, EPFL scientists have shown that electrons can jump through spins much faster than previously thought. Electrons spin around atoms, but also spin around themselves, and can cross over from one spin state to another. A property which can be exploited for next-generation hard drives. However, "spin cross-over" has been considered too slow to be efficient. Using ultrafast measurements, EPFL scientists have now shown for the first time that electrons can cross spins at least 100,000 times faster than previously thought. Aside for its enormous implications for fundamental physics, the finding can also propel the field of spintronics forward. The study is published in Nature Chemistry.

Auböck, G., and Chergui, M. (2015) Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+. Nature Chem. (DOI: 10.1038/NCHEM.2305)

July 20, 2015. More >>
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Plasmonic Tipless Pyramid arrays for Cell Poration

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Improving the efficiency, cell survival, and throughput of gene transfection methods in living cells is of great benefit to regenerative medicine. In collaboration with the group of E. Mazur at Harvard, J.P. Wolf and his team developed a nanostructured substrate made of tipless pyramids for plasmonic-induced DNA transfection. By optimizing the geometrical parameters for an excitation wavelength of 800 nm, they demonstrate a 100-fold intensity enhancement of the electric near field at the cell−substrate contact area, while the low absorption typical for gold is maintained. They further demonstrated that such substrate can induce transient poration of cells by a purely optically induced process, eliminating the need for viral vectors.

Courvoisier, S., Saklayen, N., Huber, M., Chen, J., Diebold, E.D., Bonacina, L., Wolf, J.P., and Mazur, E. (2015) Plasmonic Tipless Pyramid arrays for Cell Poration. Nano Lett 15,  4461-4466 (10.1021/acs.nanolett.5b01697)

June 16, 2015. More >>
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How long does it take to remove electrons from noble metal surfaces?

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In an NCCR MUST collaboration between the Hengsberger/Osterwalder group from the University of Zurich and the attosecond science team from the Keller group at ETH Zurich, we have successfully extended an interferometric attosecond technique (RABBITT, reconstruction of attosecond beating by interference of two-photon transitions) to solid-state samples. RABBITT in combination with a unique experimental setup enabled us to extract the surface specific photoemission delays for Ag(111) and Au(111) surfaces as a function of excitation energy.

The energy dependence of the photoemission delays deviates considerably from the expectations based on a simple model using scattering theory and ballistic transport. The observed deviation highlights the importance of final state effects in the photoemission dynamics from solids – a contribution that was neither accessible nor considered in earlier studies.

Locher, R., Castiglioni, L., Lucchini, M., Greif, M., Gallmann, L., Osterwalder, J., Hengsberger, M., and Keller, U. (2015) Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry. Optica  2,  405-410 (10.1364/OPTICA.2.000405)

April 23, 2015. More >>
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Electron transfer challenges fluorescence resonance analysis

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Using advanced technology unique to EPFL, scientists have uncovered evidence that challenges one of the most widespread techniques in biology.

Tryptophan is an amino acid, one of the building blocks of proteins. It is used extensively to study how proteins change their 3D structure, and also how they interact with other proteins and molecules. This is studied with a fluorescence technique called FRET, which measures the transfer of energy from tryptophan to another molecule. But in some cases, FRET data could be distorted because tryptophan transfers an electron instead of energy. Using a unique spectroscopic technique, scientists at EPFL have now confirmed for the first time that this is indeed the case. The study, which has far-reaching implications for the effectiveness of FRET, is published in PNAS.

Monni, R., Al Haddad, A., van Mourik, F., Auböck, G., and Chergui, M. (2015) Tryptophan-to-heme electron transfer in ferrous myoglobins. Proc Natl Acad Sci USA (10.1073/pnas.1423186112)

April 20, 2015. More >>
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The first ever photograph of light as both a particle and wave

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Light behaves both as a particle and as a wave. Since the days of Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.
Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

Piazza, L., Lummen, T.T.A., Quiñonez, E., Murooka, Y., Reed, B.W., Barwick, B., and Carbone, F. (2015) Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field. Nat Commun 6, 6407 (10.1038/ncomms7407).

March 2, 2015. More >>
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Ultrafast Structural Dynamics of the Fe-Pnictide Parent Compound BaFe2As2


Femtosecond time-resolved x-ray diffraction at FEMTO is used to investigate the structural dynamics in the Fe-pnictide parent compound BaFe2As2. We observe fluence dependent intensity oscillations of two specific Bragg reflections with a period of ~200 fs. Their distinctly different sensitivity to the pnictogen height h demonstrates the coherent excitation of the A1𝑔 phonon mode and allows us to quantify the coherent modifications of the Fe-As tetrahedra. By a comparison with time-resolved photoemission data we derive the electron-phonon deformation potential for this particular mode, which is comparable to theoretical predictions. Our results demonstrate the importance of this structural degree of freedom for the electron-phonon coupling in the Fe pnictides and indicate a transient increase of the Fe magnetic moments on an ultrafast timescale.

Reference: Rettig, L, S O Mariager, A Ferrer, S Grübel, J A Johnson, J Rittmann, T Wolf, S L Johnson, G Ingold, P Beaud, and U Staub, Ultrafast Structural Dynamics of the Fe- Pnictide Parent Compound BaFe2As2. Phys. Rev. Lett., (2015) 114: 067402 (10.1103/PhysRevLett.114.067402) Rettig-2015
February 13, 2015. More >>
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Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser

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The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000–5000, wavelength stability of 10−4, and an increase in peak brightness by a factor of 2–5 across the photon energy range of 500–1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

Ratner, D., Abela, R., Amann, J., Behrens, C., Bohler, D., Bouchard, G., Bostedt, C., Boyes, M., Chow, K., Cocco, D., Decker, F.J., Ding, Y., Eckman, C., Emma, P., Fairley, D., Feng, Y., Field, C., Flechsig, U., Gassner, G., Hastings, J., Heimann, P., Huang, Z., Kelez, N., Krzywinski, J., Loos, H., Lutman, A., Marinelli, A., Marcus, G., Maxwell, T., Montanez, P., Moeller, S., Morton, D., Nuhn, H.D., Rodes, N., Schlotter, W., Serkez, S., Stevens, T., Turner, J., Walz, D., Welch, J., and Wu, J. (2015) Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser. Phys Rev Lett 114, 054801 (10.1103/PhysRevLett.114.054801
 
February 6, 2015. More >>
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Bagiante, S., Enderli, F., Fabiańska, J., Sigg, H., and Feurer, T. (2015) Giant Electric Field Enhancement in Split Ring Resonators Featuring Nanometer-Sized Gaps. Sci Rep 5, 8051 (10.1038/srep08051)
 
Today’s pulsed THz sources enable us to excite, probe, and coherently control the vibrational or rotational dynamics of organic and inorganic materials on ultrafast time scales. Driven by standard laser sources THz electric field strengths of up to several MV/m have been reported and in order to reach even higher electric field strengths the use of dedicated electric field enhancement structures has been proposed.
Thomas Feurer and co-workers demonstrate resonant electric field enhancement structures, which concentrate the incident electric field in sub-diffraction size volumes and show an electric field enhancement as high as ~14,000 at 50 GHz. These values have been confirmed through a combination of near-field imaging experiments and electromagnetic simulations.

January 27, 2015. More >>
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A. Ludwig, J. Maurer, B.W. Mayer, C.R. Phillips, L. Gallmann, and U. Keller (2014) Breakdown of the Dipole Approximation in Strong-Field Ionization. Phys. Rev. Lett 113, 243001 (10.1103/PhysRevLett.113.243001)
 
Ionization of atoms and molecules with strong laser pulses is a fundamental process in atomic, molecular and optical physics and must be understood to control and improve the generation of high harmonics and attosecond pulses. Two important parameters that control the nature of the ionization process are the wavelength and intensity of the ionizing laser pulses. The resulting parameter plane was explored only selectively with different experimental approaches. However, theory predicts that the plane can be divided into different interaction regimes.
With the advent of optical systems delivering high-energy few-cycle pulses on the long-wavelength side of the visible spectrum around 3.4 µm, light-matter-interaction can now be studied in an area where the magnetic field component of the light pulses can be expected to play a measurable role. Thus far, this component could be neglected in the region of the parameter space where the majority of strong field ionization experiments take place. The authors showed that beyond this region the electron dynamics is altered by the magnetic field component of the light as well as the ion’s Coulomb force onto the escaping electron. Thus the so-called “Dipole Approximation” fails.

December 15, 2014. More >>
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Landsman, A.S., Weger, M., Maurer, J., Boge, R., Ludwig, A., Heuser, S., Cirelli, C., Gallmann, L., and Keller, U. (2014) Ultrafast resolution of tunneling delay time. Optica 1, 343-349 (10.1364/OPTICA.1.000343).

How quickly does a quantum particle tunnel through a barrier?  This fundamental question has been hotly debated (as time is not a quantum operator) since the early days of quantum mechanics.  Conclusive experiments were not possible.  In modern ultrafast science, the reconstruction of electron dynamics, e.g., in several recent Science and Nature papers, implicitly relies on instantaneous tunneling time.  Our experimental resolution shows tunneling time is neither instantaneous nor deterministic; most existing theory fails.  Moreover, the time-scales involved significantly impact dynamics of valence shell electrons – and hence the chemical properties of molecules, with implications likely even for molecular biology research.

November 17, 2014. More >>
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Beaud, P., Caviezel, A., Mariager, S.O., Rettig, L., Ingold, G., Dornes, C., Huang, S.W., Johnson, J.A., Radovic, M., Huber, T., Kubacka, T., Ferrer, A., Lemke, H.T., Chollet, M., Zhu, D., Glownia, J.M., Sikorski, M., Robert, A., Wadati, H., Nakamura, M., Kawasaki, M., Tokura, Y., Johnson, S.L., and Staub, U. (2014) A time-dependent order parameter for ultrafast photoinduced phase transitions. Nat Mater 13, 923-927 (10.1038/nmat4046).
 
The exploration of the interaction of structural and electronic degrees of freedom in strongly correlated electron systems on the femtosecond time scale is an emerging area of research. One goal of these studies by Paul Beaud and his collegues is to advance our understanding of the underlying correlations, another to find ways to control the exciting properties of these materials on an ultrafast time scale. So far a general model is lacking that provides a quantitative description of the correlations between the structural and electronic degrees of freedom.

August 3, 2014. More >>
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Hartmann, N., Helml, W., Galler, A., Bionta, M.R., Güntert, J., Molodtsov, S. L., Ferguson, K.R., Schorb, S., Swiggers, M.L., Carron, S., Bostedt, C., Castagna, J.C., Bozek, J., Glownia, J.M., Kane, D.J., Fry, A.R., White, W.E., Hauri, C.P., Feurer, T., and Coffee, R.N. (2014) Sub-femtosecond precision measurement of relative X-ray arrival time for free-electron lasers. Nature Photon 8, 706-709 (10.1038/nphoton.2014.164)

Thomas Feurer and co-workers report a unique two-dimensional spectrogram measurement of the relative X-ray/optical delay. This easily scalable relative delay measurement already surpasses previous techniques by an order of magnitude with its sub-1 fs temporal resolution and opens up the prospect of time-resolved X-ray measurements to the attosecond community.
 
July 27, 2014. More >>
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J. Savolainen, F. Uhlig, S. Ahmed, P. Hamm and P. Jungwirth (2014) Direct Observation of the Collapse of the Delocalized Excess Electron in Water, Nature Chem 6, 697–701 (10.1038/nchem.1995)
 
It is generally believed that, after being generated, an excess electron in water shrinks from a strongly delocalized to a localized state in about a picosecond. Now, these early stages in the behaviour of this electron have been observed using a combination of transient THz spectroscopy and ab initio molecular dynamics simulations.

July 6, 2014. More >>
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Teresa Kubacka et al. (2014) Observed live with x-ray laser: electricity controls magnetism. Science 343, 1333-1336.
 
Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich, including MUST PIs Paul Beaud, Steve Johnson, Christoph Hauri and Urs Staub have now changed the magnetic arrangement in a material much faster than is possible with today’s hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled.

March 6, 2014. More >>
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Staedler et al. (2014) Deep UV generation and direct DNA photo-interaction by harmonic nanoparticles in labelled samples. Nanoscale 6, 2929 (10.1039/C3NR05897B) (Jean-Pierre Wolf and co-workers)

Malignant human cell lines labelled by harmonic nanoparticles are targeted with a biophotonics approach based on the nonlinear optical process of second harmonic generation. The method enables independent imaging and therapeutic action, selecting each modality by simply tuning the excitation laser wavelength from infrared to visible. In particular, the generation of deep ultraviolet radiation at 270 nm allows direct interaction with nuclear DNA in the absence of photosensitizing molecules.

January 30, 2014. More >>
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Piazza, L. et al. (2014) Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite. Struct. Dynam. 1, 014501 (10.163/1.4835116) (Fabrizio Carbone and co-workers)

State of the art femtosecond electron microscopy experiments on a Praseodimium-doped bi-layered manganite helps to unravel the details of the response of diferent orbitals to photo-induced structural distortions. Carbone and co-workers show the dynamical response of the electronic structure of a Pr-doped manganite in a very broad spectroscopic range (more then 60 eV) together with the dynamical response of the crystal obtained in diffraction.

January 21, 2014. More >>
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Marchioro, A. et al. (2014) Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells. Nat. Photon. 8, 250–255 (10.1038/nphoton.2013.374)

Jacques Moser and co-workers show using transient laser spectroscopy and microwave photoconductivity measurements that primary charge separation in hybrid organic–inorganic solid-state solar cells occurs at both junctions, with TiO2 and the hole-transporting material, simultaneously, with ultrafast electron and hole injection taking place from the photoexcited perovskite over similar timescales. Charge recombination is shown to be significantly slower on TiO2 than on Al2O3 films.

January 19, 2014. More >>
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Savolainen, J. et al. (2013) Two-dimensional Raman-terahertz spectroscopy of water. Proc Natl Acad Sci USA 110, 20402-20407 (10.1073/pnas.1317459110)

Peter Hamm and co-workers present two-dimensional Raman-terahertz (THz) spectroscopy as a multidimensional spectroscopy directly in the far-IR regime. The method is used to explore the dynamics of the collective intermolecular modes of liquid water at ambient temperatures that emerge from the hydrogen-bond networks water forming.

December 17, 2013. More >>
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Peter Hamm and co-workers: vibrational dynamics of isotope-diluted ice Ih

August 29, 2013

Using three-dimensional infrared (3D-IR) spectroscopy, Peter Hamm and co-workers have investigated the vibrational dynamics of isotope-diluted ice Ih. .

Perakis, F., Borek, J., and Hamm, P. (2013) Three-dimensional infrared spectroscopy of isotope-diluted ice Ih. J Chem Phys 139, 014501 (DOI: 10.1063/1.4812216).

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Christoph Hauri and co-workers: Magnetization Controlled at Picosecond Intervals

August 11, 2013

A terahertz laser developed at the Paul Scherrer Institute makes it possible to control a material’s magnetisation at a timescale of picoseconds (0.000 000 000 001 seconds).

C. Vicario, C. Ruchert, F. Ardana-Lamas, P.M. Derlet, B. Tudu, J. Luning and C.P. Hauri (2013) Offresonant magnetization dynamics phase-locked to an intense phase-stable THz transient. Nature Photonics, Advance Online publication, 11 August 2013
DOI: 10.1038/nphoton.2013.209

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Majed Chergui and co-workers: Solving electron transfer in water

July 2, 2013

EPFL scientists have shown how a solvent can interfere with electron transfer by using unprecedented time resolution in ultrafast fluorescence spectroscopy: paper in Nature Communications.

Fabrizio Messina, Olivier Bräm, Andrea Cannizzo, Majed Chergui. Real-time observation of the charge transfer to solvent dynamics. Nature Communications, 2013; 4 DOI: 10.1038/ncomms3119

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Bill Pedrini, Rafael Abela, Bruce Patterson and co-workers: new paper in Nature Communications

April 3, 2013

B. Pedrini, A. Menzel, M. Guizar-Sicairos, V.A. Guzenko, S. Gorelick, C. David, B.D. Patterson & R. Abela published in Nature Communications: Two-dimensional structure from random multiparticle X-ray scattering images using cross-correlations. DOI: 10.1038/ncomms2622.

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Hans Jakob Wörner / Jean-Pierre Wolf and co-workers: Direct Amplitude Shaping of High Harmonics in the Extreme Ultraviolet

February 18, 2013

Foundations for the first coherent control experiments of core and valence electrons on attosecond timescales
In the framework of our MUST-collaboration, we recently demonstrated direct shaping of attosecond pulse trains after their generation using a reflective micromirror array based on micro-electro-mechanical-system (MEMS) technology.

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Majed Chergui and co-workers: An ultraviolet analogue of 2D NMR

February 15, 2013

Unravelling electron and energy transfer processes of amino-acid residues in bio-systems
Recently, the group of Prof. Chergui has implemented the first experimental set-up for 2D UV spectroscopy and in a recent article in Science, they demonstrated its capabilities in the case of heme proteins.

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Gebhard Schertler and co-workers: A glimpse inside the control centres of cell communication

(from the PSI website, February 14, 2013)

Researchers detect characteristic constructional features in a family of sensors that process signals in the human body and control physiological processes.
The cells within the human body continually communicate with one another in order to fulfil their various tasks. For that purpose, they are equipped with sensors with which they receive signals from their environment. Sensors on cell surfaces are known as receptors. Numerous processes taking place within our body – such as sight, smell or taste – are performed by an important family of receptors known as G protein-coupled receptors (GPCR). 


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Eric Vauthey and co-workers: Bimolecular Photoinduced Electron Transfer

July 2, 2012

Electron transfer processes are ubiquitous chemical reactions, involved for example in the conversion of light into chemical energy in the photosynthetic apparatus of plants or into electricity in photovoltaic devices. Additionally, electron transfer is the simplest chemical reaction and, as such, it has attracted much attention from theoreticians. Using ultrafast spectroscopy, we could observe the initial stages of the reaction in viscous environments and evidence the complex interplay of diffusion and reaction that requires state-of-the-art theoretical models to be correctly analysed.

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Majed Chergui and co-workers: A Setup for Ultrafast Broadband Two-Dimensional UV Spectroscopy

June 8, 2012

Inspired by NMR techniques, the implementation multidimensional spectroscopies in the infrared regime (vibrational multidimensional spectroscopy) over the last 20 years made it possible to obtain molecular dynamical and structural information way beyond conventional (one-dimensional) time-resolved techniques. More recently the spectral range of these techniques was extended to the visible (electronic multidimensional spectroscopies).

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Peter Hamm and co-workers: Towards 2D Raman-THz spectroscopy

March 7, 2012

Water is a complex liquid due to the fast dynamics of the hydrogen-bond network that is responsible for its peculiar properties. We know from ultrafast vibrational spectroscopy that the memory time of water at room temperatures, i.e. the typical time a given water molecule stays in its hydrogen bond environment, is a few picoseconds at most. These studies concentrate on the high-frequency OH-stretch vibration of water and make use of the fact that its vibrational frequency is a relatively sensitive probe of the strength of hydrogen bonding of a given OH group to its environment.

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Markus Meuwly and Peter Hamm: Temperature dependence of the heat diffusivity of proteins

November 2, 2011

In a combined experimental–theoretical study, we have investigated the transport of vibrational energy from the surrounding solvent into the interior of a heme protein, the sperm whale myoglobin double mutant L29W-S108L (left Figure [1]), and its dependence on temperature between 20 and 70 K [2].

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