Attosecond measurement on electrons in water clusters

Researchers in the group of Hans Jakob Wörner at ETH Zurich have developed a method that enables time-​resolved measurements of electron motion in water clusters lasting only a few attoseconds. The technique can be used for more detailed studies of water as well as faster electronics.

Reference: Gong, X., Heck, S., Jelovina, D., Perry, C., Zinchenko, K., Lucchese, R., and Wörner, H.J. (2022) Attosecond spectroscopy of size-resolved water clusters. Nature (https://doi.org/10.1038/s41586-022-05039-8)

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Tracking chirality in real time

Scientists at EPFL have developed a new laser-based technique that can measure ultrafast changes in the structural symmetry of molecules, called chirality, tracking their conformational shifts in real time. In a collaboration with researchers from the Universities of Geneva and Pisa, the breakthrough resolves a long-standing issue on how an important class of metal complexes switch their magnetic properties when triggered by a flash of light, and can have implications for magnetic data storage applications.

Reference: Oppermann, M., Zinna, F., Lacour, J., and Chergui, M. (2022) Chiral control of spin-crossover dynamics in Fe(II) complexes. Nature Chem. (10.1038/s41557-022-00933-0)

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Explaining the Efficiency of Photosynthesis

Do biological organisms exploit the power of quantum mechanics? Contrary to what is often assumed, new work from the group of Jeremy Richardson at the ETH Zürich uses theory and simulation to show that photosynthesis is equally efficient with classical as with quantum vibrations.

Reference: Runeson, J.E., Lawrence, J.E., Mannouch, J.R., and Richardson, J.O. (2022) Explaining the Efficiency of Photosynthesis: Quantum Uncertainty or Classical Vibrations? J. Phys. Chem. Lett. 13, 3392-3399 (10.1021/acs.jpclett.2c00538)

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How light energy is converted into kinetic energy for chloride translocation

For the first time, a molecular movie has captured in detail the process of an anion transported across the cell membrane by a light-​fuelled protein pump. Publishing in Science, the researchers have unravelled the mystery of how light energy initiates the pumping process − and how nature made sure there is no anion leakage back outside.

Reference: Mous, S., Gotthard, G., Ehrenberg, D., Sen, S., Weinert, T., Johnson Philip, J.M., James, D., Nass, K., Furrer, A., Kekilli, D., Ma, P., Brünle, S., Casadei Cecilia, M., Martiel, I., Dworkowski, F., Gashi, D., Skopintsev, P., Wranik, M., Knopp, G., Panepucci, E., Panneels, V., Cirelli, C., Ozerov, D., Schertler, G., Wang, M., Milne, C., Standfuss, J., Schapiro, I., Heberle, J., and Nogly, P. (2022). Dynamics and mechanism of a light-driven chloride pump. Science, eabj6663 (10.1126/science.abj6663)

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Measuring tunneling dynamics through barriers

The tunnelling of a microparticle through a barrier is one of the most fundamental and ubiquitous quantum processes. In strong-field ionization, the oscillating electric field of laser pulses creates a time-dependent potential barrier in atoms or molecules allowing the bound electron to tunnel. The authors, including Hans Jakob Wörner and co-workers  present a robust measurement of tunnelling dynamics including the electron sub-barrier phase and amplitude. They combine the attoclock technique with two-colour phase-of-phase (POP) spectroscopy to accurately calibrate the angular streaking relation and to probe the non-stationary tunnelling dynamics by manipulating a rapidly changing potential barrier.

Reference: Han, M., P. Ge, J. Wang, Z. Guo, Y. Fang, X. Ma, X. Yu, Y. Deng, H. J. Wörner, Q. Gong and Y. Liu (2021). "Complete characterization of sub-Coulomb-barrier tunnelling with phase-of-phase attoclock." Nature Photon. (10.1038/s41566-021-00842-7)

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Ionization in intense laser fields beyond the dipole approximation: review

The frontier laser research with the carrier envelope offset (CEO) phase stabilization enabled for example the invention of the attoclock technique, single attosecond pulse generation and petahertz electronics. In all these cases with neglect the magnetic field in the strong laser field interaction. We confirmed the predicted low frequency limit of the dipole approximation in 2014. The electric dipole approximation is widely used in atomic, molecular and optical physics and is typically related to a regime for which the wavelength is much larger than the atomic structure. However, it was predicted that in strong laser fields another regime exists where the dipole approximation breaks down. In this case during the ionization process the photoelectrons can reach large enough velocities such that the magnetic field component of the laser field becomes significant and the dipole approximation breaks down. This paper is an invited review paper about ionization beyond the dipole approximation which has become a hot topic in the attosecond field.

Reference: Maurer, J., and Keller, U. (2021). Ionization in intense laser fields beyond the electric dipole approximation: concepts, methods, achievements and future directions. J. Phys. B: Atom. Mol. Opt. Phys. 54, 094001 (10.1088/1361-6455/abf731)

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The shape of light changes our vision

Vision is a complex process that has been successfully deciphered by many disciplines –physics, biochemistry, physiology, neurology, etc.–: The retina captures light, the optic nerve transmits electrical impulses to the brain, which ultimately generates the perception of an image. Although this process takes some time, recent studies have shown that the first stage of vision, the perception of light itself, is extremely fast. But the analysis of this decisive step was carried out on molecules in solution in the laboratory. Scientists from the University of Geneva (UNIGE), in collaboration with the EPFL and the University Hospitals of Geneva (HUG), Switzerland, reproduced the experiment on mice, in order to observe the processing of light by a living organism in all its complexity.

Reference: Gaulier, G., Dietschi, Q., Bhattacharyya, S., Schmidt, C., Montagnese, M., Chauvet, A., Hermelin, S., Chiodini, F., Bonacina, L., Herrera, P.L., Röthlisberger, U., Rodriguez, I., and Wolf, J.-P. (2021). Ultrafast pulse shaping modulates perceived visual brightness in living animals. Sci Adv 7, eabe1911. (10.1126/sciadv.abe1911)

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Uniquely sharp X-ray view at SwissFEL

Researchers at the Paul Scherrer Institute PSI, EPFL, MIT, Fermi FEL and other partners, including many current and former NCCR MUST researchers and two fellows of the FP-RESOMUS program have succeeded for the first time in looking inside materials using the method of transient grating spectroscopy with ultrafast X-rays at SwissFEL. The experiment at PSI is a milestone in observing processes in the world of atoms. The researchers are publishing their research results in the journal Nature Photonics.

Reference: Rouxel, J.R., Fainozzi, D., Mankowsky, R., Rösner, B., Seniutinas, G., Mincigrucci, R., Catalini, S., Foglia, L., Cucini, R., Döring, F., Kubec, A., Koch, F., Bencivenga, F., Haddad, A.A., Gessini, A., Maznev, A.A., Cirelli, C., Gerber, S., Pedrini, B., Mancini, G.F., Razzoli, E., Burian, M., Ueda, H., Pamfilidis, G., Ferrari, E., Deng, Y., Mozzanica, A., Johnson, P.J.M., Ozerov, D., Izzo, M.G., Bottari, C., Arrell, C., Divall, E.J., Zerdane, S., Sander, M., Knopp, G., Beaud, P., Lemke, H.T., Milne, C.J., David, C., Torre, R., Chergui, M., Nelson, K.A., Masciovecchio, C., Staub, U., Patthey, L., and Svetina, C. (2021). Hard X-ray transient grating spectroscopy on bismuth germanate. Nature Photon. (10.1038/s41566-021-00797-9)

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Molecular simulations contribute to the research on perovskite solar cells with world record efficiency

In this study, collaborative research efforts between several groups at the EPFL (including Ursula Röthlisberger) led by Michael Grätzel and UNIST, Korea led by Jin Young Kim, have developed an innovative chemical synthesis technology to maximize the solar to the power conversion efficiency of FAPbI₃ based PSCs. The resulting solar cells achieved a power conversion certified record efficiency of 25.21%, thereby surpassing other solar cell technologies of CdTe, CIGS, and the current market-leader, polycrystalline silicon.

Reference: Jeong, J., Kim, M., Seo, J., Lu, H., Ahlawat, P., Mishra, A., Yang, Y., Hope, M.A., Eickemeyer, F.T., Kim, M., Yoon, Y.J., Choi, I.W., Darwich, B.P., Choi, S.J., Jo, Y., Lee, J.H., Walker, B., Zakeeruddin, S.M., Emsley, L., Röthlisberger, U., Hagfeldt, A., Kim, D.S., Grätzel, M., and Kim, J.Y. (2021). Pseudo-halide anion engineering for α-FAPbI₃ perovskite solar cells. Nature. (10.1038/s41586-021-03406-5).

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Transient 2D IR spectroscopy from micro- to milliseconds

Peter Hamm introduces a new application of high-repetition rate, femtosecond Yb-laser/amplifier systems: transient 2D IR spectroscopy covering the time range from micro- to milliseconds. This approach intertwines the measurement of 2D IR spectra with the time separation from an actinic pump pulse and utilizes the high repetition rate of these lasers systems in two ways: by offering a high time resolution (10 µs) and by enabling the measurement of many 2D IR spectra. The well-studied photocycle of bacteriorhodopsin is used as a demonstration object in this proof-of-principle experiment.

Reference: Hamm, P. (2021). Transient 2D IR spectroscopy from micro- to milliseconds. J Chem Phys 154, 104201. (10.1063/5.0045294).

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Weak electronic excitations induce fast atomic displacements

Urs Staub, Steve Johnson and co-workers find that the structural distortion and the underlying electronic structure of the charge density wave in TiSe₂ show different energetics at ultrafast timescales by using resonant and non-resonant x-ray diffraction on an x-ray free electron laser. This indicates that the lattice distortion stabilizes the charge density wave.

Reference: Burian, M., Porer, M., Mardegan, J.R.L., Esposito, V., Parchenko, S., Burganov, B., Gurung, N., Ramakrishnan, M., Scagnoli, V., Ueda, H., Francoual, S., Fabrizi, F., Tanaka, Y., Togashi, T., Kubota, Y., Yabashi, M., Rossnagel, K., Johnson, S.L., and Staub, U. (2021). Structural involvement in the melting of the charge density wave in 1T−TiSe2. Phys Rev Research 3, 013128. (10.1103/PhysRevResearch.3.013128)
 

March 1, 2021. More >>

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How fast is electronic relaxation?

Technical advances in extending spectroscopy to the attosecond time scale has caused considerable interest in experimental studies of various ultrafast processes that were previously inaccessible. Using attosecond transient absorption around the carbon K-edge accompanied by ab initio quantum dynamics simulations, Zinchenko et al. investigated ultrafast, non-adiabatic dynamics in the ethylene cation. They directly observed the electronic D₁ → D₀ relaxation mediated by the conical intersection taking place within 7 femtoseconds, far shorter than any vibrational period or any previously reported electronic relaxation. The demonstrated technique is directly applicable to liquids and solutions, enabling further studies of charge- and energy-transfer dynamics of organic molecules in chemical and biochemical systems.

Reference: Zinchenko, K.S., Ardana-Lamas, F., Seidu, I., Neville, S.P., van der Veen, J., Lanfaloni, V.U., Schuurman, M.S., and Wörner, H.J. (2021). Sub-7-femtosecond conical-intersection dynamics probed at the carbon K-edge. Science 371, 489 (10.1126/science.abf1656)

January 29, 2021. More >>

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Efficient spin excitation via ultrafast damping-like torques in antiferromagnets

Antiferromagnets (AFMs) are a promising material class for spintronic applications. Their robustness against external magnetic fields, along with the possibility of picosecond antiferromagnetic switching, could help to substitute semiconductor-based information technologies with antiferromagnetic spintronic devices. The authors show that both field-like and damping-like torques – a combination that has proved to be powerful for the electrical control of magnetic order – are available optically, and can be utilised to act on the magnetic order. Thus, optically generated torques might provide the long sought-after tool enabling the efficient realisation of ultrafast coherent precessional switching of AFMs.

Reference: Tzschaschel, C., Satoh, T., and Fiebig, M. (2020). Efficient spin excitation via ultrafast damping-like torques in antiferromagnets. Nature Commun 11, 6142. (https://doi.org/10.1038/s41467-020-19749-y)

December 1, 2020. More >>

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Compact design delivers hard X-rays

Beneath a forest in Villigen, Switzerland, a new compact free-electron laser facility is generating brilliant X-ray flashes. About 10 years ago, researchers at the Paul Scherrer Institut (PSI) in Villigen, Switzerland, conceived an ambitious plan to build a high-performance yet cost-effective XFEL source, SwissFEL. After completing a conceptual design report in 2012, they started construction in 2013, observed the first lasing in 2016, and commenced regular user operations in 2019. Now, writing in Nature Photonics, the team report the capabilities of their source, which yields outstanding performance from a compact design. The team generated a pulse energy of 0.5 mJ at a 1 Å wavelength with a 100 Hz repetition rate from their compact XFEL source, with a total length of only 0.74 km and a moderate electron beam energy of 5.8 GeV.

Reference: Prat, E., Abela, R., Aiba, M., Alarcon, A., Alex, J., Arbelo, Y., Arrell, C., Arsov, V., Bacellar, C., Beard, C., Beaud, P., Bettoni, S., Biffiger, R., Bopp, M., Braun, H.H., Calvi, M., Cassar, A., Celcer, T., Chergui, M., Chevtsov, P., Cirelli, C., Citterio, A., Craievich, P., Divall, M.C., Dax, A., Dehler, M., Deng, Y.P., Dietrich, A., Dijkstal, P., Dinapoli, R., Dordevic, S., Ebner, S., Engeler, D., Erny, C., Esposito, V., Ferrari, E., Flechsig, U., Follath, R., Frei, F., Ganter, R., Garvey, T., Geng, Z.Q., Gobbo, A., Gough, C., Hauff, A., Hauri, C.P., Hiller, N., Hunziker, S., Huppert, M., Ingold, G., Ischebeck, R., Janousch, M., Johnson, P.J.M., Johnson, S.L., Juranic, P., Jurcevic, M., Kaiser, M., Kalt, R., Keil, B., Kiselev, D., Kittel, C., Knopp, G., Koprek, W., Laznovsky, M., Lemke, H.T., Sancho, D.L., Lohl, F., Malyzhenkov, A., Mancini, G.F., Mankowsky, R., Marcellini, F., Marinkovic, G., Martiel, I., Marki, F., Milne, C.J., Mozzanica, A., Nass, K., Orlandi, G.L., Loch, C.O., Paraliev, M., Patterson, B., Patthey, L., Pedrini, B., Pedrozzi, M., Pradervand, C., Radi, P., Raguin, J.Y., Redford, S., Rehanek, J., Reiche, S., Rivkin, L., Romann, A., Sala, L., Sander, M., Schietinger, T., Schilcher, T., Schlott, V., Schmidt, T., Seidel, M., Stadler, M., Stingelin, L., Svetina, C., Treyer, D.M., Trisorio, A., Vicario, C., Voulot, D., Wrulich, A., Zerdane, S., and Zimoch, E. (2020). A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam. Nature Photon 14, 748-+. (10.1038/s41566-020-00712-8)
 

November 20, 2020. More >>

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Molecular dynamics simulations play a key role in manufacturing high efficiency metal-halide perovskites

In a collaborative research efforts at EPFL led by Michael Grätzel and Anders Hafgeldt, with the help of the group of Ursula Röthlisberger, an innovative chemical deposition method has been developed that overcomes these issues while maintaining more than 23% power-conversion efficiency and long-term operational and thermal stability. The fabricated solar cells also featured low (330 mV) open-circuit voltage loss and a low (0.75 V) turn-on voltage for electroluminescence.

Reference: Lu, H., Liu, Y., Ahlawat, P., Mishra, A., Tress, W.R., Eickemeyer, F.T., Yang, Y., Fu, F., Wang, Z., Avalos, C.E., Carlsen, B.I., Agarwalla, A., Zhang, X., Li, X., Zhan, Y., Zakeeruddin, S.M., Emsley, L., Röthlisberger, U., Zheng, L., Hagfeldt, A., and Grätzel, M. (2020). Vapor-assisted deposition of highly efficient, stable black-phase FAPbI₃; perovskite solar cells. Science 370, eabb8985 (science.sciencemag.org/content/370/6512/eabb8985).

October 2, 2020. More >>

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Molecular dynamics simulations play a key role in manufacturing high efficiency metal-halide perovskites

In a collaborative research efforts at EPFL led by Michael Grätzel and Anders Hafgeldt, with the help of the group of Ursula Röthlisberger, an innovative chemical deposition method has been developed that overcomes these issues while maintaining more than 23% power-conversion efficiency and long-term operational and thermal stability. The fabricated solar cells also featured low (330 mV) open-circuit voltage loss and a low (0.75 V) turn-on voltage for electroluminescence.

Reference: Lu, H., Liu, Y., Ahlawat, P., Mishra, A., Tress, W.R., Eickemeyer, F.T., Yang, Y., Fu, F., Wang, Z., Avalos, C.E., Carlsen, B.I., Agarwalla, A., Zhang, X., Li, X., Zhan, Y., Zakeeruddin, S.M., Emsley, L., Röthlisberger, U., Zheng, L., Hagfeldt, A., and Grätzel, M. (2020). Vapor-assisted deposition of highly efficient, stable black-phase FAPbI₃; perovskite solar cells. Science 370, eabb8985 (science.sciencemag.org/content/370/6512/eabb8985).

October 2, 2020. More >>

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Monitoring energy flow in light-matter states

Using state-of-the-art laser spectroscopy, researchers at EPFL - led by Majed Chergui -  and at the University of Gothenburg have found how energy flows in real-time among hybrid light-matter states, providing unique insight into the dynamics of these states. The work will help develop novel applications that can use light to tailor-make properties of a material.

Reference: Mewes, L., Wang, M., Ingle, R.A., Börjesson, K., and Chergui, M. (2020). Energy relaxation pathways between light-matter states revealed by coherent two-dimensional spectroscopy. Commun Phys 3, 157 (10.1038/s42005-020-00424-z)

September 11, 2020. More >>

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Most robust exciton discovered in titanium dioxide

An international team of physicists led by Majed Chergui at EPFL has demonstrated that excitons in the semiconductor anatase titanium dioxide can withstand the highest concentration of electrons ever reported.

Reference: Baldini, E., Palmieri, T., Dominguez, A., Rubio, A., and Chergui, M. (2020). Giant Exciton Mott Density in Anatase TiO2. PhysRev Lett 125, 116403 (10.1103/PhysRevLett.125.116403)

September 10, 2020. More >>

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A step toward a better understanding of molecular dynamics

At EPFL's Laboratory of Theoretical Physical Chemistry (LCPT), the research team of Jiri Vanicek studying the dynamics of polyatomic molecules — molecules made up of several atoms — found that electrons in these molecules move quite differently from what would be expected in isolated atoms.

Reference: Golubev, N.V., Begušić, T., and Vaníček, J. (2020). On-the-Fly Ab Initio Semiclassical Evaluation of Electronic Coherences in Polyatomic Molecules Reveals a Simple Mechanism of Decoherence. Phys. Rev. Lett. 125, 083001 (10.1103/PhysRevLett.125.083001)

August 20, 2020. More >>

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A universal structural deformation in all heme proteins

In a new experiment, a team of scientists led by Majed Chergui at EPFL’s School of Basic Sciences, with Chris Milne and colleagues at the Paul-Scherrer Institut and the European X-ray Free Electron Laser (Hamburg) have found that Cytochrome c also undergoes doming, a typical deformation of respiratory heme proteins. This shows that doming is a universal feature of all heme proteins and is not limited to respiratory ones.

Reference: Bacellar, C., Kinschel, D., Mancini, G.F., Ingle, R.A., Rouxel, J., Cannelli, O., Cirelli, C., Knopp, G., Szlachetko, J., Lima, F.A., Menzi, S., Pamfilidis, G., Kubicek, K., Khakhulin, D., Gawelda, W., Rodriguez-Fernandez, A., Biednov, M., Bressler, C., Arrell, C.A., Johnson, P.J.M., Milne, C.J., and Chergui, M. (2020). Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies. Proc Natl Acad Sci USA 117, 21914 (10.1073/pnas.2009490117)

September 8, 2020. More >>

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Electron movements in liquid measured in super-slow motion

Electrons are able to move within molecules, for example when they are excited from outside or in the course of a chemical reaction. For the first time, scientists led by Hans Jakob Wörner at the ETH Zürich have now succeeded in studying the first few dozen attoseconds of this electron movement in a liquid.

Reference: Jordan, I., Huppert, M., Rattenbacher, D., Peper, M., Jelovina, D., Perry, C., von Conta, A., Schild, A., and Wörner, H.J. (2020). Attosecond spectroscopy of liquid water. Science 369, 974-979 (10.1126/science.abb0979)

August 20, 2020. More >>

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Unraveling the initial molecular events of respiration

A team of scientists led by Majed Chergui at EPFL's School of Basic Sciences have solved the question whether the transition from low-spin planar to a high-spin domed heme in myoglobin is by thermal relaxation or by a cascade among electron spin states. The researchers detached the small molecule from the heme using short, energizing laser pulses. They then used another short, hard X-ray pulse from an X-ray free-electron laser to induce X-ray emission (XES), a very sensitive fingerprint of the spin state of molecules, which monitored the heme's changes as a function of time. They could thus determine that the passage from planar to domed and back is caused by a cascade among spin states.

Reference: Kinschel, D., Bacellar, C., Cannelli, O., Sorokin, B., Katayama, T., Mancini, G.F., Rouxel, J.R., Obara, Y., Nishitani, J., Ito, H., Ito, T., Kurahashi, N., Higashimura, C., Kudo, S., Keane, T., Lima, F.A., Gawelda, W., Zalden, P., Schulz, S., Budarz, J.M., Khakhulin, D., Galler, A., Bressler, C., Milne, C.J., Penfold, T., Yabashi, M., Suzuki, T., Misawa, K., and Chergui, M. (2020). Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins. Nature Commun 11, 4145. (10.1038/s41467-020-17923-w)

August 18, 2020. More >>

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The quantum future of microscopy: Wave function engineering of electrons, ions, and nuclei

A perspective: the utilization of quantum methods in electron microscopy based on interference effects and phase-manipulation of an electron wave function will lead to many potential new and independent research fields. Dose reduction approaches would be beneficial in biological applications, whereas wave function engineering approaches have clear benefits for condensed matter problems. At the same time, these tools and approaches can also be applied to other matter-waves associated with composite particles, possibly bringing revolutionizing technologies for high-energy physics and energy domains.

Reference: Madan, I., Vanacore, G.M., Gargiulo, S., LaGrange, T., and Carbone, F. (2020). The quantum future of microscopy: Wave function engineering of electrons, ions, and nuclei. Appl Phys Lett 116, 230502. (10.1063/1.5143008)

June 11, 2020. More >>

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Electron Scattering in Liquid Water and Amorphous Ice: A Striking Resemblance

The lack of accurate low-energy electron scattering cross sections for liquid water is a substantial source of uncertainty in the modeling of radiation chemistry and biology. In this paper, Ruth Signorell (ETH Zürich) compares experimental photoemission data of liquid water with corresponding predictions using amorphous ice cross sections, with the aim of resolving the debate regarding the difference of electron scattering in liquid water and amorphous ice. We find very similar scattering properties in the liquid and the ice for electron kinetic energies up to a few hundred electron volts.
 

Reference: Signorell, R. (2020). Electron Scattering in Liquid Water and Amorphous Ice: A Striking Resemblance. Phys Rev Lett 124, 205501 (10.1103/PhysRevLett.124.205501)

May 24, 2020. More >>

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Elucidating the mechanism of a light-driven sodium pump

Researchers at the Paul Scherrer Institute PSI have succeeded for the first time in recording, in action, a light-driven sodium pump from bacterial cells.  The findings promise progress in the development of new methods in neurobiology. The researchers used the new X-ray free-electron laser SwissFEL for their investigations. They have published their findings today in the journal Nature.

Reference: Skopintsev, P., Ehrenberg, D., Weinert, T., James, D., Kar, R.K., Johnson, P.J.M., Ozerov, D., Furrer, A., Martiel, I., Dworkowski, F., Nass, K., Knopp, G., Cirelli, C., Arrell, C., Gashi, D., Mous, S., Wranik, M., Gruhl, T., Kekilli, D., Brünle, S., Deupi, X., Schertler, G.F.X., Benoit, R.M., Panneels, V., Nogly, P., Schapiro, I., Milne, C., Heberle, J., and Standfuss, J. (2020). Femtosecond-to-millisecond structural changes in a light-driven sodium pump. Nature (10.1038/s41586-020-2307-8)

May 20, 2020. More >>

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Publication of the very first Pilot experiment results from Alvra - SwissFEL

OLED technology beyond small or expensive devices requires light-emitters, luminophores, based on earth-abundant elements. Understanding and experimental verification of charge transfer in luminophores are needed for this development. An organometallic multicore Cu complex comprising Cu–C and Cu–P bonds represents an underexplored type of luminophore. To investigate the charge transfer and structural rearrangements in this material, the authors - including several current and former MUST researchers, bold in the reference below - apply complementary pump-probe X-ray techniques: absorption, emission, and scattering including pump-probe measurements at the X-ray free-electron laser SwissFEL.

Reference: Smolentsev, G., Milne, C.J., et al. (2020). Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays. Nature Commun. 11, 2131. (10.1038/s41467-020-15998-z)

May 1, 2020. More >>

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One of the first ultrafast soft x-ray resonant inelastic x-ray scattering (RIXS) study on solid material

This paper investigates how the orbitals reconstruct on ultrafast timescales, through a photo-induced insulator to metal transition, in the Mott-Hubbard material V₂O₃. The authors show how time-resolved RIXS can be used at an X-ray free-electron laser to study ultrafast orbital dynamics in a correlated material.

Reference: Parchenko, S., Paris, E., McNally, D., Abreu, E., Dantz, M., Bothschafter, E.M., Reid, A.H., Schlotter, W.F., Lin, M.-F., Wandel, S.F., Coslovich, G., Zohar, S., Dakovski, G.L., Turner, J.J., Moeller, S., Tseng, Y., Radovic, M., Saathe, C., Agaaker, M., Nordgren, J.E., Johnson, S.L., Schmitt, T., and Staub, U. (2020). Orbital dynamics during an ultrafast insulator to metal transition. Phys Rev Research 2, 023110 (https://link.aps.org/doi/10.1103/PhysRevResearch.2.023110)

April 30, 2020. More >>

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Following Femtosecond Dynamics in Liquids with a High Harmonic Generation Source in the Water Window Regime

Femtosecond time–resolved soft–X–ray liquid-phase transient-absorption measurements in soft X-ray spectral range rely on a combination of a sub–micrometer–thin flat liquid jet with a high–harmonic table–top source covering the entire water-window range (284 - 538 eV). In a  400-nm pump and soft-X-ray probe scheme,we observe differences in rise-time of the induced transient changes in X-ray absorption in methanol and ethanol indicating ultrafast proton dynamics after ionization in those systems.

Reference: Smith, Adam D., Balciunas, Tadas,  Chang, Yi-Ping,  Schmidt, Cedric., Zinchenko, Kristina, Nunes, Fernanda B.,  Rossi, Emanuele,  Svoboda, Vít, Yin, Zhong, Wolf, Jean-Pierre, Wörner, Hans Jakob, Journal of Physical Chemistry Letters, 2020, vol. 11, n° 6, p. 1981-1988, (doi: 10.1021/acs.jpclett.9b03559)
 

February 19, 2020. More >>

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New quasiparticle unveiled in room temperature semiconductors

Predicted back in 1967 by Gerald Mahan

The group of Majed Chergui at EPFL, in collaboration with Alexander Steinhoff (University of Bremen), Ana Akrap (University of Fribourg), and the group of László Forró (EPFL) unveiled fingerprints of the long-sought particle known as the Mahan exciton in the room temperature optical response of the popular methylammonium lead halide perovskites. On the fundamental side, these findings deepen our knowledge of many-body phenomena in condensed matter systems, paving the route toward the use of perovskites for the Bose-Einstein condensation of hybrid states of light and excitons.

Reference: Palmieri, T., Baldini, E., Steinhoff, A., Akrap, A., Kollár, M., Horváth, E., Forró, L., Jahnke, F., and Chergui, M. (2020). Mahan excitons in room-temperature methylammonium lead bromide perovskites. Nat Commun 11. (10.1038/s41467-020-14683-5)

February 13, 2020. More >>

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Low driving forces in high-efficiency organic solar cells

Probing ultrafast charge-transfer at donor:acceptor heterojunctions

Organic photovoltaic (OPV) devices containing non-fullerene acceptors (NFAs) blended with conjugated polymers have impressive efficiencies close to 18%. One of the reasons is the low driving force for charge separation in those systems, allowing high photovoltage. However, this low driving force is believed to slow down charge generation, leading to a tradeoff between voltage and current. The group of N. Banerji at the University of Bern has now shown that both the electron and hole transfer times at the donor:acceptor interface remain ultrafast (< 1 ps) even if the driving force approaches zero. Thus, the interfacial energy offset at the donor:acceptor interface can be minimized without concerns about a current-voltage tradeoff.

Reference: Zhong, Y., Causa’, M., Moore, G.J., Krauspe, P., Xiao, B., Günther, F., Kublitski, J., Shivhare, R., Benduhn, J., BarOr, E., Mukherjee, S., Yallum, K.M., Réhault, J., Mannsfeld, S.C.B., Neher, D., Richter, L.J., DeLongchamp, D.M., Ortmann, F., Vandewal, K., Zhou, E., and Banerji, N. (2020). Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers. Nat Commun 11, 833. (10.1038/s41467-020-14549-w)

February 11, 2020. More >>

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Towards jitter-free ultrafast electron diffraction technology

Stroboscopic visualization of nuclear or electron dynamics in atoms, molecules or solids requires ultrafast pump and probe pulses and a close to perfect synchronization between the two. We have developed a 3 MeV ultrafast electron diffraction (UED) probe technology that nominally reduces the electron bunch duration and the arrival time jitter to the sub-femtosecond level. This simple configuration uses a radiofrequency photogun and a 90° achromatic bend and is designed to provide effectively jitter-free conditions. Terahertz streaking measurements reveal an electron bunch duration of 25 fs, even for a charge as high as 0.6 pC, and an arrival time jitter of 7.8 fs, the latter limited by only the measurement accuracy. From pump–probe measurements of photoexcited bismuth films, the instrument response function was determined to be 31 fs. This pioneering jitter-free technique paves the way towards UED of attosecond phenomena in atomic, molecular and solid-state dynamics.


Reference: Hyun Woo Kim, Nikolay A. Vinokurov, In Hyung Baek, Key Young Oang, Mi Hye Kim, Young Chan Kim, Kyu-Ha Jang, Kitae Lee, Seong Hee Park, Sunjeong Park, Junho Shin, Jungwon Kim, Fabian Rotermund, Sunglae Cho, Thomas Feurer and Young Uk Jeong, “Towards jitter-free ultrafast electron diffraction technology”, Nature Photonics,
(https://doi.org/10.1038/s41566-019-0566-4 )

December 23, 2019. More >>

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A new look at thermally-induced chemical reactions

Toward time-resolved laser T-jump/X-ray probe spectroscopy in aqueous solutions.

Scientists at EPFL have been able to monitor the time-evolution of thermally-induced chemical reactions with element- and structural-sensitivity. The group of Majed Chergui, with Guilia Mancini, Olivero Cannelli and colleagues within the Lausanne Centre for Ultrafast Science (LACUS) demonstrate for the first time the use of an X-ray probe in a T-jump experiment to observe structural changes over the course of a chemical reaction.

Reference: Cannelli, O., Bacellar, C., Ingle, R.A., Bohinc, R., Kinschel, D., Bauer, B., Ferreira, D.S., Grolimund, D., Mancini, G.F., and Chergui, M. (2019). Toward time-resolved laser T-jump/X-ray probe spectroscopy in aqueous solutions. Struct Dynam 6, 064303. (10.1063/1.5129626)

December 16, 2019. More >>

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Attosecond timing of photons and electrons one by one

Free electrons can interact with light only via Compton scattering. In a Coulomb continuum, however, electrons can absorb or emit photons. Such continuum- continuum (cc) transitions [1] are commonly exploited in a variety of attosecond photoionization experiments in order to extract delays originating from the propagation of the liberated electron wave packet across the ionic potential [2]. In this work, we present a novel experimental protocol, which allows us to disentangle the contributions of multiple interfering quantum pathways in such experiments. This enables to retrieve a time delay arising purely due to the cc- transitions, which is a significant contribution to the total attosecond photoionization delay. We measured for the first time how the absorption and emission of a single photon alters the angular momentum dependent dynamics of an electron that is not bound to an atomic nucleus, but still feels its Coulomb potential.

Reference: J. Fuchs, N. Douguet, S. Donsa, F. Martin. J. Burgdörfer, L. Argenti, L. Cattaneo, U. Keller , "Time delays from one- photon transitions in the continuum, " Optica, vol. 7, No. 2, pp. 154- 161, 2020, (DOI: 10.1364/OPTICA.378639)


February 3, 2020 More >>

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The first sub-femtosecond study of the linear photon momentum transfer during an ionisation process

Unprecedented insight into the birth of photoelectrons

During multi-photon ionization of an atom it is well understood how the involved photons transfer their energy to the ion and the photoelectron. However, the transfer of the photon linear momentum is still not fully understood. The group of Ursula Keller presents a time-resolved measurement of linear momentum transfer along the laser pulse propagation direction. They were able to show that the linear momentum transfer to the photoelectron depends on the ionization time within the laser cycle using the attoclock technique. The measured linear momentum transfer can mostly be explained within a classical model for a free electron in a laser field. However, corrections are required due to the parent-ion interaction and due to the initial momentum when the electron enters the continuum. The parent-ion interaction induces a negative attosecond time delay between the appearance in the continuum of the electron with minimal linear momentum transfer and the point in time with maximum ionization rate.

Reference: Willenberg, B., J. Maurer, B. W. Mayer and U. Keller (2019). Sub-cycle time resolution of multi-photon momentum transfer in strong-field ionization. Nat. Commun. 10(1): 5548. (10.1038/s41467-019-13409-6)

December 6, 2019. More >>

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Controlling the optical properties of solids with acoustic waves

Exciton control in a room temperature bulk semiconductor with coherent strain pulses

Physicists from Switzerland, Germany, and France have found that large-amplitude acoustic waves, launched by ultrashort laser pulses, can dynamically manipulate the optical response of semiconductors. This has now just been achieved in the lab of Majed Chergui at EPFL within the Lausanne Centre for Ultrafast Science, in collaboration with the theory groups of Angel Rubio (Max-Planck Institute, Hamburg) and Pascal Ruello (Université de Le Mans). Publishing in Science Advances, the international team shows, for the first time, control of excitonic properties using acoustic waves. To do this, the researchers launched a high-frequency (hundreds of gigahertz), large-amplitude acoustic wave in a material using ultrashort laser pulses. This strategy further allows for the dynamical manipulation of the exciton properties at high speed.

Reference: Baldini, E., Dominguez, A., Palmieri, T., Cannelli, O., Rubio, A., Ruello, P., and Chergui, M. (2019). Exciton control in a room temperature bulk semiconductor with coherent strain pulses. Sci Adv 5, eaax2937 (10.1126/sciadv.aax2937)

See also: EPFL News,

November 29, 2019. More >>

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Extending orbital tomography to complex systems

A combined electron spectroscopy study for complete characterization of complex molecular catalyst

The frontier orbitals largely govern chemical reactivity. Researchers from the Physics and Chemistry Departments of the University of Zurich and the Forschungszentrum Jülich studied the hydrogen evolution catalysts Co-pyrphyrin by orbital tomography. Orbital tomography makes use of the simple relation between the molecular orbital initial state and the photoelectron momentum via Fourier transform, which is possible under certain conditions. Combining ARPES and electron diffraction data with high-level DFT calculations and simulations of the photoemission data enabled the complete determination of the adsorbate geometries and identification and characterization of five molecular valence states.

Reference: Kliuiev, P., Zamborlini, G., Jugovac, M., Gurdal, Y., Arx, K.v., Waltar, K., Schnidrig, S., Alberto, R., Iannuzzi, M., Feyer, V., Hengsberger, M., Osterwalder, J., and Castiglioni, L. (2019). Combined orbital tomography study of multi-configurational molecular adsorbate systems. Nature Commun 10, 5255. (10.1038/s41467-019-13254-7)

November 20, 2019. More >>

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Ultrafast chirality

 

Real-time probing of chirality during a chemical reaction

Chiral molecules interact and react differently, depending on their handedness (left vs. right). This chiral recognition is the principle governing most biomolecular interactions, such as the activity of drugs or our perception of scents. Denitsa Baykuscheva, Hans Jakob Wörner and co-workers report the real-time (femtosecond) observation of chirality during a chemical reaction, using a seemingly unlikely technique: high-harmonic generation (HHG) in tailored intense near-infrared laser fields. These results open the path to investigations of the chirality of molecular-reaction pathways, light-induced chirality in chemical processes, and the control of molecular chirality through tailored laser pulses.

Reference: Baykusheva, D., Zindel, D., Svoboda, V., Bommeli, E., Ochsner, M., Tehlar, A., and Wörner, H.J. (2019). Real-time probing of chirality during a chemical reaction. Proceedings of the National Academy of Sciences, 201907189. (www.pnas.org/content/early/2019/11/12/1907189116)

November 13, 2019. More >>

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Effective THz fields on nano-structured surfaces

Polarization-sensitive field reconstruction in a THz/XUV pump-probe experiment at FLASH

A team from the University of Zurich in collaboration with researchers from PSI, DESY and Augsburg university set out to study the feasibility of THz/XUV pump-probe experiments on surfaces at the FLASH free-electron laser facility at DESY. High THz fields are required to excite specific modes and drive a chemical reaction, for example. Employing THz streaking in conjunction with an angle-resolved analyzer, they were able to quantitatively reconstruct both parallel and perpendicular components of the THz field at the surface. Both bulk metal surfaces and nano-structured surfaces were investigated and distinctive differences in the dielectric response were observed.

Reference: Waltar, K., Haase, J., Pan, R., Golz, T., Kliuiev, P., Weinl, M., Schreck, M., Bajt, S., Stojanovic, N., van Bokhoven, J.A., Hengsberger, M., Osterwalder, J., and Castiglioni, L. (2019). Polarization-sensitive reconstruction of transient local THz fields at dielectric interfaces. Optica 6, 1431-1436. (doi.org/10.1364/OPTICA.6.001431)

November 13, 2019. More >>

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Processes in imaginary time

Semiclassical analysis of the quantum instanton approximation

The groups of Vanicek and Richardson teamed up to investigate the quantum tunnelling of atoms in chemical reactions.  These processes can be understood using ordinary classical mechanics where time is allowed to become imaginary.  By analysing alternative methods for estimating the rates of reactions in terms of these imaginary-time classical trajectories, the authors found that they might significantly overestimate the effect of tunnelling in exothermic reactions.  An improved method was suggested based on this analysis which is shown to give much more reliable predictions.

Reference: Vaillant, C.L., Thapa, M.J., Vaníček, J., and Richardson, J.O. (2019). Semiclassical analysis of the quantum instanton approximation. J Chem Phys 151, 144111 (doi.org/10.1063/1.5123800)

November 13, 2019. More >>

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Elucidating the Elusive: Nonlinear Optics Tracks Antiferromagnetism in Real Space

Tracking the ultrafast motion of an antiferromagnetic order parameter

A research collaboration between the Laboratory for Multifunctional Ferroic Materials (ETH Zurich) and the Satoh Lab (Tokyo Institute of Technology) tracks the “invisible” magnetism in an antiferromagnetic material on the move. Understanding the dynamics of antiferromag-nets is crucial for the development of electronic devices that are orders of magnitude faster than the existing ones.

Ferromagnets, such as iron, are omnipresent in our everyday life. A typical example is a fridge magnet. Its macroscopic magnetization allows the magnet to stick seemingly effortlessly to the refrigerator door. The same principle laid the foundation for current information technology, which now relies heavily on closely stacked, nanosized magnets to store logical bits in hard drives. When the north poles (or south poles) of two magnets ap- proach each other, however, they experience a repulsive force. The same repulsive force destabilizes bits in hard drives and increases the energy costs of their writing process.

Reference: Christian Tzschaschel, Takuya Satoh, Manfred Fiebig: “Tracking the ultra- fast motion of an antiferromagnetic order parameter”, Nature Communications 10, 3995 (2019). DOI:10.1038/s41467-019-11961-9


5 September, 2019 More >>

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Ultrafast Transient Increase of Oxygen Octahedral Rotations in a Perovskite

Via femtosecond hard x-ray diffraction we identified a perovskite system that complements the rare selection of materials in which a symmetry broken phase can be transiently stabilized by photoexcitation. EuTiO3 exhibits a second order purely structural phase transition (TC = 290 K) which is characterized by an antiferrodistortive rotation of the oxygen octahedra (see figure, left unit cell) with the rotation angle being the order parameter. We directly monitor this angle upon photoexcitation across the band gap (data points) via x-ray pulses obtained from a FEL. Within the first few-hundred femtoseconds after excitation we observe a transient increase of rotation angle. This stands in contrast to the situation of an increased temperature for which the order parameter decreases. We ascribe the surprising increase of the order parameter to a transient effective change of ionic sizes that transforms directly into an ultrafast change of the Goldschmidt tolerance factor. The ability to modify the Goldschmidt tolerance factor provides another tuning parameter to control electronic and magnetic properties of perovskites on ultrafast timescales.

Reference: M. Porer, M. Fechner, M. Kubli, M.J. Neugebauer, S. Parchenko, V. Espositio, A. Narayan, N. A. Spaldin, R. Huber, M. Radovic, E. M. Bothschafter, J. M. Glownia, T. Sato, S. Song, S. L. Johnson, and U. Staub.
Phys. Rev. Research 1. 012005 (R) (2019). DOI: 10.1103/PhysRevResearch.1.01005

9 August, 2019  More >>

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Velocity echoes in water provide hints about aquatic structure

 

Studying the echoes of disturbances in water can indicate inhomogeneities and local structures in the liquid

 

Much remains unknown about water and its structure. By studying velocity echoes of water in a very low-frequency, terahertz regime, scientists can learn about its intermolecular degrees of freedom. This has been investigated experimentally in the past, and a new paper by Peter Hamm provides a computational method to reproduce and confirm interpretations of previous experimental results.

Reference: Hamm, P. (2019). Velocity echoes in water. J Chem Phys 151, 054505. (https://doi.org/10.1063/1.5112163)

August 12, 2019. More >>

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How light steers electrons in metals

 

Physicists at ETH Zurich have measured how electrons in transition metals get redistributed within a fraction of an optical oscillation cycle

 

Mikhail Volkov, Ursula Keller and co-workers have measured how electrons in so-called transition metals get redistributed within a fraction of an optical oscillation cycle. They observed the electrons getting concentrated around the metal atoms within less than a femtosecond. This regrouping might influence important macroscopic properties of these compounds, such as electrical conductivity, magnetization or optical characteristics. The work therefore suggests a route to controlling these properties on extremely fast time scales.

Reference: Volkov, M., S. A. Sato, F. Schlaepfer, L. Kasmi, N. Hartmann, M. Lucchini, L. Gallmann, A. Rubio and U. Keller (2019). Attosecond screening dynamics mediated by electron localization in transition metals. Nature Physics. (10.1038/s41567-019-0602-9)

August 5, 2019. More >>

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Solvated electrons in neutral water cluster

The interest in the hydrated electron arises from its role in chemistry and radiation damage, and from the fact that it is one of the simplest quantum solutes. With a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations, the group of Ruth Signorell has now investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of electrons solvated in neutral water clusters. Surprisingly, the first results suggest that the hydrated electron seems to behave similarly in large neutral water clusters as in liquid bulk water.

Reference: Thomas E. Gartmann, Loren Ban, Bruce L. Yoder, Sebastian Hartweg, Egor Chasovskikh, and Ruth Signorell, Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters, doi.org/10.1021/acs.jpclett.9b01802

August 5, 2019. More >>

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Ultrafast dynamics in polycyclic aromatic hydrocarbons:

the key case of conical intersections at higher excited states and their role in the photophysics of phenanthrene monomer

We investigated the photophysics of a phenanthrene monomer by means of femtosecond transient absorption spectroscopy and non-adiabatic molecular dynamics simulation. Surprisingly, we found that a correct interpretation of photophysics and photochemistry of phenanthrene cannot be understood without taking into account higher-excited states and the presence of conical intersections among them. Indeed we observed a complex pathway including an ultrafast relaxation from the lowest bright state S2 towards S1 occurring through a conical intersection region involving ~80% of the excited phenanthrene. The remaining 20% is trapped in a hot equilibrated S2 for about 1 ps, before relaxing via a thermally activated incoherent internal conversion.

Reference: M. Nazari, C. D. Bösch, A. Rondi, A. Francés Monerris, M. Marazzi, E. Lognon, M. Gazzetto, S. M. Langenegger, R. Häner, T. Feurer, A. Monari, A. Cannizzo, “Ultrafast dynamics in polycyclic aromatic hydrocarbons: the key case of conical intersections in higher excited states and their role in the photophysics of phenanthrene monomer”, Physical Chemistry Chemical Physics 21 (2019) 16981. (DOI: 10.1039/c9cp03147b)

July 18, 2019. More >>

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Recording a molecular energy machine in action

 

PSI researchers have decoded energy production at cell membranes with the help of the Swiss Light Source SLS

 

Using the Swiss Light Source SLS, PSI researchers have recorded a molecular energy machine in action and thus revealed how energy production at cell membranes works. For this purpose, they developed a new investigative method that could make the analysis of cellular processes significantly more effective than before.

Reference: Weinert, T., P. Skopintsev, D. James, F. Dworkowski, E. Panepucci, D. Kekilli, A. Furrer, S. Brünle, S. Mous, D. Ozerov, P. Nogly, M. Wang and J. Standfuss (2019). Proton uptake mechanism in bacteriorhodopsin captured by serial synchrotron crystallography. Science 365: 61. (10.1126/science.aaw8634)

July 5, 2019. More >>

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Tunneling the Nucleobases - Energy Transfer in DNA-Organized Polyaromatic Chromophores

DNA-based light-harvesting antennae with varying arrangements of light-absorbing phenanthrene donor units and a pyrene acceptor dye were synthesized and tested for their light-harvesting properties. Excitation of phenanthrene is followed by rapid transfer of the excitation energy to the pyrene chromophore. A block of six light-absorbing phenanthrenes was separated from the site of the acceptor in a stepwise manner by an increasing number of intervening AT base pairs. Energy transfer occurs via interposed AT base pairs and is still detected when the phenanthrene antenna is separated by 5 AT base pairs.

Reference: C. D. Bösch, E. Abay, S. M. Langenegger, M. Nazari, A. Cannizzo, T. Feurer, R. Häner (2019), DNA-Organized Light-Harvesting Antennaer: Energy Transfer in Polyaromatic Stacks Proceeds through Interposed Nucleobase Pairs. Helv. Chim. Acta, 102, e1900148   (DOI: 10.1002/hlca.201900148)

June 19, 2019. More >>

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Modelling of air collisions during re-entry of a space vehicle

 

Exhaustive state-to-state cross sections for reactive molecular collisions from importance sampling simulation and a neural network representation

 

Reactive and nonreactive scattering at high impact velocities involves sub-picosecond dynamics of the atomic and molecular species participating in the process. During atmospheric reentry, a space vehicle is exposed to a collisionally dense environment which generates an immensely diverse population of ro-vibrational states of the collision partners. For reaction network modeling of this complex chemistry an exhaustive enumeration of all possible state-to-state collision cross sections is mandatory. However, due to the large number of states (≈ 10⁴) for each collision partner), there are of the order of 10⁸ such cross sections. Determining those from converged quasiclassical trajectory or even quantum wavepacket calculations is not possible. Here we show that training a neural network on a small subset of converged state-to-state cross sections from QCT simulations for the N + NO(v, j) → O + N₂(v' j') reaction provides such a quantitatively realistic description and a computationally extremely efficient model.

Reference: Koner, D., Unke, O.T., Boe, K., Bemish, R.J., and Meuwly, M. (2019). Exhaustive state-to-state cross sections for reactive molecular collisions from importance sampling simulation and a neural network representation. J Chem Phys 150, 211101.(10.1063/1.5097385)

June 13, 2019. More >>

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Optical control of vibrational dynamics from an ultrafast phase transition

 

Engineering decoherence via light

Using femtosecond x-ray diffraction, a collaboration of scientists at ETH Zurich, the Paul Scherrer Institute and the University of Mainz demonstrated the ability to use light to extend the coherent oscillation of a vibration triggered by the melting of a charge density wave. The phenomenon opens up a new avenue for lattice control of solids.

Reference: M. J. Neugebauer, T. Huber, M. Savoini, E. Abreu, V. Esposito, M. Kubli, L. Rettig, E. Bothschafter, S. Grübel, T. Kubacka, J. Rittmann, G. Ingold, P. Beaud, D. Dominko, J. Demsar, and S. L. Johnson, Optical control of vibrational coherence triggered by an ultrafast phase transition, Phys. Rev. B 99, 220302(R) (DOI: https://doi.org/10.1103/PhysRevB.99.220302)

 

June 9, 2019. More >>

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Twisting whirlpools of electrons

 

Generating ultrashort vortex electron beams and actively switching its vorticity on the attosecond

 

Scientists from the lab of Fabrizio Carbone at EPFL are challenging the idea that it takes a device called a "passive phase mask" to  make vortex beams of electrons and neutrons. Demonstrating for the first time that it is possible to use light to dynamically twist an individual electron's wave function, the researchers were able to generate an ultrashort vortex electron beam and actively switching its vorticity on the attosecond (10-18 seconds) timescale. In experimental terms, the scientists fired circularly polarized, ultrashort laser pulses through a nano-hole fabricated onto a metallic film. This induced a strong, localized electromagnetic field (the chiral plasmon), and individual electrons were made to interact with it.

Reference: Vanacore, G.M., Berruto, G., Madan, I., Pomarico, E., Biagioni, P., Lamb, R.J., McGrouther, D., Reinhardt, O., Kaminer, I., Barwick, B., Larocque, H., Grillo, V., Karimi, E., García de Abajo, F.J., and Carbone, F. (2019). Ultrafast generation and control of an electron vortex beam via chiral plasmonic near fields. Nat Mater. (10.1038/s41563-019-0336-1)

May 6, 2019. More >>

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Nonlinear XUV-optical transient grating spectroscopy at the Si L2,3–edge

Nonlinear XUV-optical transient grating spectroscopy at the Si L2,3-edge: Time-resolved transient grating spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. We demonstrated the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron densities has been determined.

Reference: R. Bohinc, G. Pamfilidis, J. Rehault, P. Radi, C. Milne, J. Szlachetko, F. Bencivenga, F. Capotondi, R. Cucini, L. Foglia, C. Masciovecchio, R. Mincigrucci, E. Pedersoli, A. Simoncig, N. Mahne, A. Cannizzo, H.M. Frey, Z. Ollmann, T. Feurer, A.A. Maznev, K. Nelson, and G. Knopp “Nonlinear XUV-optical transient grating spectroscopy at the Si L2,3-edge”, Applied Physics Letters (https://doi.org/10.1063/1.508541)


May 6, 2019. More >>

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Attoclock revisited on electron tunneling time

 

Quantum tunneling time is a highly debated topic – we explain why.

 

This is the latest update on the electron tunneling time measured with the attoclock technique. Quantum tunneling time is a highly debated topic – we explain why. We discuss the attoclock technique to extract tunneling delays with regards to the typical approximations such as the dipole approximation, non-adiabatic effects, photoelectron momenta at the tunnel exit, electron correlation and exit coordinate. We can confirm that the He attoclock measurement is in agreement with finite tunneling time models. However, the adiabatic approximation gives the wrong field strength calibration which effectively increases the tunnel barrier width (Fig. 13). Unresolved is the issue of the starting time of the tunneling process. Some results indicate a starting time before the peak of the electric field which would increase the tunneling time shown in Fig. 12 and 13. Single active electron time-dependent Schrödinger equation (TDSE) calculations overlap with the non-adiabatic data (Fig. 6) - mostly within the error bars.

Reference: Hofmann, C., Landsman, A.S., and Keller, U. (2019). Attoclock revisited on electron tunnelling time. J Modern Opt 66, 1052-1070. (https://doi.org/10.1080/09500340.2019.1596325)

April 25, 2019. More >>

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2-D THz spectroscopy of electronic dynamics in narrow-band semiconductors

 

Probing ballistic electrons and band curvatures with THz spectroscopy

The so-called “band structure” of solid-state materials determines how electrons move through them, and is often critical in understanding an even manipulating their conductive properties.  Scientists at ETH Zurich have demonstrated a new method applying low-frequency electromagnetic pulses to drive electrons in a “ballistic” regime, using them to measure subtle features of the band structure.

Reference: S. Houver, L. Huber, M. Savoini, E. Abreu, and S. L. Johnson, 2D THz spectroscopic investigation of ballistic conduction-band electron dynamics in InSb, Vol. 27, No. 8, 15 Apr 2019, OPTICS EXPRESS, 10854 (https://doi.org/10.1364/OE.27.010854)

April 15, 2019. More >>

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New model suggests source of spectral broadening in liquid water

 

Feynman diagram description of 2D-Raman-THz spectroscopy applied to water

 

Liquid water is awash in molecular chaos. Individual water molecules constantly rotate and rearrange as they forge and break hydrogen bonds with their neighbors. This bustle blurs the intermolecular modes that recent spectroscopic measurements have struggled to resolve, and microscopic theories based on molecular dynamics simulations have failed to predict the precise source of this spectroscopic smearing. A new paper by Sidler and Hamm presents a model that suggests an origin for the observed mode broadening, arguing that it is the inhomogeneities in the stretching of intermolecular hydrogen bonds that are most likely responsible.

Reference: Sidler, D. and P. Hamm (2019). Feynman diagram description of 2D-Raman-THz spectroscopy applied to water. J. Chem. Phys. 150: 044202 (10.1063/1.5079497) Sidler-2019

January 29, 2019. More >>

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Impact of nuclear quantum effects on the structural inhomogeneity of liquid water

 

The degree to which water is structured is an extremely intriguing problem and a matter of ongoing debate.

 

The authors studied the 2D Raman–terahertz response of liquid water in dependence of temperature and isotope substitution, considering H₂O, D₂O, H¹⁸₂O. A very short-lived echo is observed, whose lifetime slows down with decreasing temperature. It differs for H2O versus D2O, whereas that of H¹⁸₂O versus H₂O is the same. The comparison of the three isotopologues allows the authors to disentangle nuclear quantum effects from trivial dynamical mass effects. The former dominate the echo lifetime, hence, it is concluded that it is a measure of the heterogeneity of the hydrogen bond networks of water.

Reference: Berger, A., G. Ciardi, D. Sidler, P. Hamm and A. Shalit (2019). Impact of nuclear quantum effects on the structural inhomogeneity of liquid water. Proc. Natl. Acad. Sci. U.S.A.: 201818182 (10.1073/pnas.1818182116) Berger-20191

January 28, 2019. More >>

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Terahertz-driven phonon upconversion in SrTiO₃

 

Creating new states of matter in complex materials.

 

Recent developments in mid-infrared laser sources have enabled a new route to control material properties: resonant excitation of phonon modes to dynamically alter the lattice structure and phonon–phonon coupling are exploited to coherently control inaccessible (that is, non-infrared-active) phonon modes. This approach, dubbed `nonlinear phononics’, requires precisely tuned mid-infrared radiation and impulsive excitation.

Reference: Kozina, M., M. Fechner, P. Marsik, T. van Driel, J. M. Glownia, C. Bernhard, M. Radovic, D. Zhu, S. Bonetti, U. Staub and M. C. Hoffmann (2019). Terahertz-driven phonon upconversion in SrTiO₃. Nat. Phys. (10.1038/s41567-018-0408-1) Kozina-2019

January 21, 2019. More >>

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Chirality in “real-time”

Ultrafast broadband circular dichroism in the deep ultraviolet

Distinguishing between left-handed and right-handed (“chiral”) molecules is crucial in chemistry and the life sciences, and is commonly done using a method called circular dichroism. However, during biochemical reactions the chiral character of molecules may change. EPFL scientists have for the first time developed a method that uses ultrashort deep-ultraviolet pulses to accurately probe such changes in real-time in (bio)molecular systems.

Reference: Oppermann, M., B. Bauer, T. Rossi, F. Zinna, J. Helbing, J. Lacour and M. Chergui (2019). Ultrafast broadband circular dichroism in the deep ultraviolet. Optica 6: 56-60. (10.1364/OPTICA.6.000056) Oppermann-2019

January 10, 2019. More >>

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The ultrafast Einstein–de Haas effect

A new twist on a mesmerising story

The Einstein–de Haas effect, first demonstrated more than a century ago, provides an intriguing link between magnetism and rotation in ferromagnetic materials. An international team led by ETH physicist Steven Johnson now established that the effect has also a central role in ultrafast processes that happen at the sub-picosecond timescale — and thus deliver fresh insight into materials that might form the basis for novel devices.

Reference: Dornes, C., Y. Acremann, M. Savoini, M. Kubli, M. J. Neugebauer, E. Abreu, L. Huber, G. Lantz, C. A. F. Vaz, H. Lemke, E. M. Bothschafter, M. Porer, V. Esposito, L. Rettig, M. Buzzi, A. Alberca, Y. W. Windsor, P. Beaud, U. Staub, D. Zhu, S. Song, J. M. Glownia and S. L. Johnson (2019). The ultrafast Einstein–de Haas effect. Nature 565: 209-212 (10.1038/s41586-018-0822-7) Dornes-2019

January 2, 2019. More >>

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A “structural switch,” allowing for maximum efficiency in the visual system

 

Shedding light on the principles of G protein selectivity

G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptors (GPCRs) are a large family of membrane receptors that interact with heterotrimeric G proteins to transform extracellular signals into cellular responses. Selective coupling of GPCRs to specific Gα-protein subtypes is a critical step that determines their physiology and their response to natural ligands and clinical drugs. Details can be elucidated by examining and comparing the structure of GPCR–G protein complexes, which lie at the center of this signal transduction event.

Reference: Tsai, C. J., F. Pamula, R. Nehme, J. Muhle, T. Weinert, T. Flock, P. Nogly, P. C. Edwards, B. Carpenter, T. Gruhl, P. Ma, X. Deupi, J. Standfuss, C. G. Tate and G. F. X. Schertler (2018). Crystal structure of rhodopsin in complex with a mini-G(o) sheds light on the principles of G protein selectivity. Sci. Adv. 4. (10.1126/sciadv.aat7052) Tsai-2018.

September 19, 2018. More >>

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Titanium dioxide as a nanoscale sensor of mechanical stress

Potential applications as a medium for room-temperature sensors of mechanical stress at the nanoscale and with an optical read-out

Measuring mechanical stress in the nano-world is a major challenge in materials science and engineering. Key to this advancement is the ability to combine cheap nano-sized materials that react to mechanical stress and simple detection schemes. A promising route would involve the development of sensors with an optical read-out. However, there are no known nano-materials that change their light-absorbing properties upon application of mechanical stress in a simple and predictable way, especially at room temperature. Such materials would be extremely useful in a number of sensing applications, ranging from bioscience to metrology.

Reference: Baldini, E., T. Palmieri, A. Dominguez, P. Ruello, A. Rubio and M. Chergui (2018). Phonon-Driven Selective Modulation of Exciton Oscillator Strengths in Anatase TiO2 Nanoparticles. Nano Lett. (10.1021/acs.nanolett.8b01837) Baldini-2018.

July 24, 2018. More >>

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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

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

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.aat0094,  Nogly-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

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

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?

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

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

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

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

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 6^th 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

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

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

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 Pr_1−xCa_xMnO₃ 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

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

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

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

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

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

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

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 $E_{2g}$ phonons in the multiband superconductor ${\mathrm{MgB}}_2$ can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the $\sigma$-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 $\sigma \text{−}\pi$ interband scattering mechanism in ${\mathrm{MgB}}_2$.

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

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.

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 Pr_0.5Ca_0.5MnO₃ 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

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

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

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

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

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

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

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

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

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?

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

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

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

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

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

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

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

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

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)

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

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

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

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?

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?

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

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

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?

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

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

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. February 6, 2015. More >>

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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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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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November 17, 2014. More >>

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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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July 27, 2014. More >>

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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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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). 

More >>    (external link opening a new window)

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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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