Dr. Klaus Sokolowski-Tinten

Ultrafast Phenomena in Solids and at Interfaces
University of Duisburg-Essen

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  • Generation of ultrashort keV Ar+ ion pulses via femtosecond laser photoionization
    Golombek, A. and Breuer, L. and Danzig, L. and Kucharczyk, P. and Schleberger, M. and Sokolowski-Tinten, K. and Wucher, A.
    New Journal of Physics 23 (2021)
    Ion beams with energies in the keV regime are widely utilized in solid-state physics, but the ultrafast dynamics triggered by an ion impact onto a solid surface is to date exclusively accessible via simulations based on many untested assumptions and model parameters. A possible experimental access rests on the availability of a laser-synchronized ion source delivering sufficiently short ion pulses for time resolved pump-probe experiments. Here, we demonstrate a new miniaturized ion optical bunching setup for the creation of rare gas ion pulses using strong-field femtosecond laser photoionization. Neutral Ar gas atoms at room temperature are intercepted by a 50 fs, 800 nm laser pulse focused to ∼10 μm spot size. We demonstrate the generation of monoenergetic 2 keV Ar+ ion pulses with 180 ps duration (FWHM) at laser peak intensities around 1014 W cm−2 and of multiply charged Arq+ ions (q = 1-5) at higher laser intensities. The results are in good agreement with detailed ion trajectory simulations, which show that the temporal resolution is essentially limited by the initial (thermal) velocity spread of the generated photo-ions, indicating the possibility to achieve even better time resolution by cooling the gas prior to ionization. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft
    view abstract10.1088/1367-2630/abe443
  • Acoustic response of a laser-excited polycrystalline Au-film studied by ultrafast Debye-Scherrer diffraction at a table-top short-pulse x-ray source
    Lu, W. and Nicoul, M. and Shymanovich, U. and Brinks, F. and Afshari, M. and Tarasevitch, A. and Von Der Linde, D. and Sokolowski-Tinten, K.
    AIP Advances 10 (2020)
    The transient acoustic response of a free-standing, polycrystalline thin Au-film upon femtosecond optical excitation has been studied by time-resolved Debye-Scherrer x-ray diffraction using ultrashort Cu Kα x-ray pulses from a laser-driven plasma x-ray source. The temporal strain evolution has been determined from the transient shifts of multiple Bragg diffraction peaks. The experimental data are in good agreement with the results of calculations based on the two-temperature model and an acoustic model assuming uniaxial strain propagation in the laser-excited thin film. © 2020 Author(s).
    view abstract10.1063/1.5142220
  • Erratum: Delayed Onset of Nonthermal Melting in Single-Crystal Silicon Pumped with Hard X Rays (Physical Review Letters (2018) 120 (265701) DOI: 10.1103/PhysRevLett.120.265701)
    Pardini, T. and Alameda, J. and Aquila, A. and Boutet, S. and Decker, T. and Gleason, A.E. and Guillet, S. and Hamilton, P. and Hayes, M. and Hill, R. and Koglin, J. and Kozioziemski, B. and Robinson, J. and Sokolowski-Tinten, K. and Soufli, R. and Hau-Riege, S.P.
    Physical Review Letters 124 (2020)
    A reanalysis of the crater morphology on the pillars exposed to the Linac Coherent Light Source (LCLS) beam during this experimental campaign revealed an effect we had previously overlooked. This effect is concerned with beam intensity fluctuations, and the loss of pump-probe beam overlap during one of the data runs. Unfortunately, the combined nature of these effects makes the data inconclusive as far as setting a lower limit for the time scale of the delayed onset of nonthermal melting in silicon pumped with hard x rays. For the reasons outlined below, we therefore retract the statement that the onset of this order-disorder transition is delayed by 150 ± 40 ? ? fs . In our Letter, we reported data capturing the ultrafast order-disorder transition in single crystal silicon pumped with 25 fs, 5.95 keV x-ray pulses. The intensity of the Si(333) Bragg peak measured 220 fs after pumping the system is significantly lower than that measured on unperturbed silicon, well within our measurement uncertainties. This is a clear sign of a disorder transition in progress. The femtosecond nature of the transition is in agreement with previous experimental and theoretical work on the subject. Our data also indicated that 150 fs after pumping the system, the Si(333) Bragg reflection is indistinguishable from that of unperturbed silicon. We interpreted these data as an indication that the onset of nonthermal melting must belong to the range 150t 220 ? ? fs . With this in mind, we set a lower limit for the delayed onset of the transition at 110 fs, given our uncertainty on the delay time between pump and probe. However, the recent discovery of beam intensity fluctuations noticeable in the probe beam during the run pertinent to the data point at t=150 ? ? fs forces us to retract our statement pertaining to the transition onset. A reanalysis of the size of selected craters pertaining to this run shows that craters left by the probe beam have areas consistently smaller than those in other runs by approximately a factor of 3. Our reanalysis also revealed that, later in the same run, overlap between pump and probe beam was lost. Although it is difficult to pinpoint exactly at which point this happened, we estimate that data from ~80 shots were recorded with good overlap. If we limit our analysis to these shots only, we obtain the ratio (I/I0)=0.96 ± 0.09 for the data point at t=150 ? ? fs . Using the formula (I/I0)=[(Ipp+CIpr)/(Ipp0+Ipr0)] described in the Letter, we derive that a ratio of 0.87 would be obtained for a complete loss of reflectivity with a pump-to-probe beam intensity ratio of (Ipp/Ipr)=41. The constant C=0.35 takes into account the area of the sample that is probed while not undergoing melting. Since, based on our reanalysis of the crater size, we cannot rule out a pump-to-probe beam intensity ratio for this run of the order of 4:1 (instead of the nominal ~4:2), we conclude that our experimental data point (I/I0)=0.96 ± 0.09 is inconclusive as far as measuring the reflectivity of silicon 150 fs after pumping. We therefore conclude that our data do not support our original claim setting a temporal limit for the delayed onset of nonthermal melting in silicon pumped with hard x rays. While it is clear that at 220 fs after pumping the transition is under way, the experimental determination of its onset remains unknown. © 2020 American Physical Society. All rights reserved.
    view abstract10.1103/PhysRevLett.124.129903
  • Time-resolved diffraction with an optimized short pulse laser plasma X-ray source
    Afshari, M. and Krumey, P. and Menn, D. and Nicoul, M. and Brinks, F. and Tarasevitch, A. and Sokolowski-Tinten, K.
    Structural Dynamics 7 (2020)
    We present a setup for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the setup to the specific requirements (e.g., X-ray optics and sample environment) of particular applications. The configuration discussed here has been optimized toward high angular/momentum resolution and uses Kα-radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization, and focusing of the emitted radiation. 2 × 10 5 Ti-Kα1 photons per pulse with 10 - 4 relative bandwidth are delivered to the sample at a repetition rate of 10 Hz. This allows for the high dynamic range (104) measurements of transient changes in the rocking curves of materials as for example induced by laser-triggered strain waves. © 2020 Author(s).
    view abstract10.1063/1.5126316
  • Tracking the ultrafast nonequilibrium energy flow between electronic and lattice degrees of freedom in crystalline nickel
    Maldonado, P. and Chase, T. and Reid, A.H. and Shen, X. and Li, R.K. and Carva, K. and Payer, T. and Horn Von Hoegen, M. and Sokolowski-Tinten, K. and Wang, X.J. and Oppeneer, P.M. and Dürr, H.A.
    Physical Review B 101 (2020)
    Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by "https://www.kb.se/samverkan-och-utveckling/oppen-Tillgang-och-bibsamkonsortiet/bibsamkonsortiet.html" Bibsam.
    view abstract10.1103/PhysRevB.101.100302
  • A concept to generate ultrashort ion pulses for pump-probe experiments in the keV energy range
    Breuers, A. and Herder, M. and Kucharczyk, P. and Schleberger, M. and Sokolowski-Tinten, K. and Wucher, A.
    New Journal of Physics 21 (2019)
    The impact of an energetic particle onto a solid surface generates a strongly perturbed and extremely localized non-equilibrium state, which relaxes on extremely fast time scales. In order to facilitate a time-resolved observation of the relaxation dynamics using established ultrafast pump-probe techniques, it is necessary to pinpoint the projectile impact in time with sufficient accuracy. In this paper, we propose a concept to generate ultrashort ion pulses via femtosecond photoionization of rare gas atoms entrained in a supersonic jet, combined with ion optical bunching of the resulting ion package. We calculate the photoion cloud generated by an intense focused laser pulse and show that Arq+ ions with q = 1-5 can be generated with a standard table-top laser system, which are then accelerated to energies in the keV range over a very short distance and bunched to impinge onto the target surface in a time-focused manner. Detailed ion trajectory simulations show that single ion pulses of sub-picosecond duration can be generated this way. The influence of space charge broadening is included in the simulations, which reveal that flight time broadening is insignificant for pulses containing up to 10-20 ions and starts to increase the pulse width above ∼50 ions/pulse. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab1775
  • Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials
    Zalden, P. and Quirin, F. and Schumacher, M. and Siegel, J. and Wei, S. and Koc, A. and Nicoul, M. and Trigo, M. and Andreasson, P. and Enquist, H. and Shu, M.J. and Pardini, T. and Chollet, M. and Zhu, D. and Lemke, H. and Ronneberger, I. and Larsson, J. and Lindenberg, A.M. and Fischer, H.E. and Hau-Riege, S. and Reis, D.A. and Mazzarello, R. and Wuttig, M. and Sokolowski-Tinten, K.
    Science 364 (2019)
    In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics. 2017 © The Authors, some rights reserved
    view abstract10.1126/science.aaw1773
  • Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. and Bovensiepen, U. and Sokolowski-Tinten, K. and Eschenlohr, A.
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.174301
  • Precision Plasmonics with Monomers and Dimers of Spherical Gold Nanoparticles: Nonequilibrium Dynamics at the Time and Space Limits
    Schumacher, L. and Jose, J. and Janoschka, D. and Dreher, P. and Davis, T.J. and Ligges, M. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X. and Meyer Zu Heringdorf, F. and Sokolowski-Tinten, K. and Schlücker, S.
    Journal of Physical Chemistry C 123 (2019)
    Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). Recent investigations in precision plasmonics have largely focused on static properties using conventional techniques such as transmission electron microscopy and optical dark-field microscopy. In this Feature Article, we first highlight the application of femtosecond time-resolved electron diffraction for monitoring the nonequilibrium dynamics of spherical gold NPs after ultrafast optical excitation. The analysis of the transient diffraction patterns allows us to directly obtain quantitative information on the incoherent excitation of the lattice, that is, heating upon electron-lattice equilibration, as well as on the development of strain due to lattice expansion on picosecond time scales. The controlled assembly of two spherical gold NPs into a dimer with a few nanometers gap leads to unique optical properties. Specifically, extremely high electric fields (hot spot) in the gap are generated upon resonant optical excitation. Conventional optical microscopy cannot spatially resolve this unique hot spot due to the optical diffraction limit. We therefore employed nonlinear photoemission electron microscopy to visualize hot spots in single dimers of spherical gold NPs. A quantitative comparison of different single dimers confirms the homogeneity of the hot spots on the single-particle level. Overall, these initial results are highly encouraging because they pave the way to investigate nonequilibrium dynamics in highly uniform plasmonic nanostructures at the time and space limits. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b01007
  • Delayed Onset of Nonthermal Melting in Single-Crystal Silicon Pumped with Hard X Rays
    Pardini, T. and Alameda, J. and Aquila, A. and Boutet, S. and Decker, T. and Gleason, A.E. and Guillet, S. and Hamilton, P. and Hayes, M. and Hill, R. and Koglin, J. and Kozioziemski, B. and Robinson, J. and Sokolowski-Tinten, K. and Soufli, R. and Hau-Riege, S.P.
    Physical Review Letters 120 (2018)
    In this work, we monitor the onset of nonthermal melting in single-crystal silicon by implementing an x-ray pump-x-ray probe scheme. Using the ultrashort pulses provided by the Linac Coherent Light Source (SLAC) and a custom-built split-and-delay line for hard x rays, we achieve the temporal resolution needed to detect the onset of the transition. Our data show no loss of long-range order up to 150±40 fs from photoabsorption, which we interpret as the time needed for the electronic system to equilibrate at or above the critical nonthermal melting temperature. Once such equilibration is reached, the loss of long-range atomic order proceeds inertially and is completed within 315±40 fs from photoabsorption. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.120.265701
  • Dynamics of Electron-Phonon Coupling in Bicontinuous Nanoporous Gold
    Zheng, Q. and Shen, X. and Sokolowski-Tinten, K. and Li, R.K. and Chen, Z. and Mo, M.Z. and Wang, Z.L. and Weathersby, S.P. and Yang, J. and Chen, M.W. and Wang, X.J.
    Journal of Physical Chemistry C 122 (2018)
    The nanosize effect on electron-phonon interactions in free-electron noble metals has been a subject of intense discussion because of their important applications in physics, chemistry, and biomedicine. However, the interference from supports of dispersive nanoparticulate metals has led to controversial observations. We utilize freestanding, bicontinuous nanoporous gold (NPG) films to investigate electron-phonon interaction dynamics using ultrafast MeV electron diffraction. Compared to solid gold films, NPG shows faster electron-phonon interaction and thus noticeably higher electron-phonon coupling constant. The results demonstrate that surface states of electrons and phonons play an important role in electron-phonon coupling of nanostructured materials. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b03299
  • Heterogeneous to homogeneous melting transition visualized with ultrafast electron diffraction
    Mo, M.Z. and Chen, Z. and Li, R.K. and Dunning, M. and Witte, B.B.L. and Baldwin, J.K. and Fletcher, L.B. and Kim, J.B. and Ng, A. and Redmer, R. and Reid, A.H. and Shekhar, P. and Shen, X.Z. and Shen, M. and Sokolowski-Tinten, K. and Tsui, Y.Y. and Wang, Y.Q. and Zheng, Q. and Wang, X.J. and Glenzer, S.H.
    Science 360 (2018)
    The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega–electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds. © 2017 The Authors
    view abstract10.1126/science.aar2058
  • Reply to "comment on 'Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3 ' "
    Chen, F. and Zhu, Y. and Liu, S. and Qi, Y. and Hwang, H.Y. and Brandt, N.C. and Lu, J. and Quirin, F. and Enquist, H. and Zalden, P. and Hu, T. and Goodfellow, J. and Sher, M.-J. and Hoffmann, M.C. and Zhu, D. and Lemke, H. and Glownia, J. and Chollet, M. and Damodaran, A.R. and Park, J. and Cai, Z. and Jung, I.W. and Highland, M.J. and Walko, D.A. and Freeland, J.W. and Evans, P.G. and Vailionis, A. and Larsson, J. and Nelson, K.A. and Rappe, A.M. and Sokolowski-Tinten, K. and Martin, L.W. and Wen, H. and Lindenberg, A.M.
    Physical Review B 97 (2018)
    In this reply to S. Durbin's comment on our original paper "Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3," we concur that his final equations 8 and 9 more accurately describe the change in diffracted intensity as a function of Ti displacement. We also provide an alternative derivation based on an ensemble average over unit cells. The conclusions of the paper are unaffected by this correction. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.226102
  • Electron-lattice energy relaxation in laser-excited thin-film Au-insulator heterostructures studied by ultrafast MeV electron diffraction
    Sokolowski-Tinten, K. and Shen, X. and Zheng, Q. and Chase, T. and Coffee, R. and Jerman, M. and Li, R.K. and Ligges, M. and Makasyuk, I. and Mo, M. and Reid, A.H. and Rethfeld, B. and Vecchione, T. and Weathersby, S.P. and Dürr, H.A. and Wang, X.J.
    Structural Dynamics 4 (2017)
    We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels. © Author(s) 2017.
    view abstract10.1063/1.4995258
  • Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter
    Mo, M.Z. and Shen, X. and Chen, Z. and Li, R.K. and Dunning, M. and Sokolowski-Tinten, K. and Zheng, Q. and Weathersby, S.P. and Reid, A.H. and Coffee, R. and Makasyuk, I. and Edstrom, S. and McCormick, D. and Jobe, K. and Hast, C. and Glenzer, S.H. and Wang, X.
    Review of Scientific Instruments 87 (2016)
    We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime. © 2016 Author(s).
    view abstract10.1063/1.4960070
  • Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3
    Chen, F. and Zhu, Y. and Liu, S. and Qi, Y. and Hwang, H.Y. and Brandt, N.C. and Lu, J. and Quirin, F. and Enquist, H. and Zalden, P. and Hu, T. and Goodfellow, J. and Sher, M.-J. and Hoffmann, M.C. and Zhu, D. and Lemke, H. and Glownia, J. and Chollet, M. and Damodaran, A.R. and Park, J. and Cai, Z. and Jung, I.W. and Highland, M.J. and Walko, D.A. and Freeland, J.W. and Evans, P.G. and Vailionis, A. and Larsson, J. and Nelson, K.A. and Rappe, A.M. and Sokolowski-Tinten, K. and Martin, L.W. and Wen, H. and Lindenberg, A.M.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.180104
  • Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory
    Weathersby, S.P. and Brown, G. and Centurion, M. and Chase, T.F. and Coffee, R. and Corbett, J. and Eichner, J.P. and Frisch, J.C. and Fry, A.R. and Gühr, M. and Hartmann, N. and Hast, C. and Hettel, R. and Jobe, R.K. and Jongewaard, E.N. and Lewandowski, J.R. and Li, R.K. and Lindenberg, A.M. and Makasyuk, I. and May, J.E. and McCormick, D. and Nguyen, M.N. and Reid, A.H. and Shen, X. and Sokolowski-Tinten, K. and Vecchione, T. and Vetter, S.L. and Wu, J. and Yang, J. and Dürr, H.A. and Wang, X.J.
    Review of Scientific Instruments 86 (2015)
    Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4926994
  • Thickness-dependent electron-lattice equilibration in laser-excited thin bismuth films
    Sokolowski-Tinten, K. and Li, R.K. and Reid, A.H. and Weathersby, S.P. and Quirin, F. and Chase, T. and Coffee, R. and Corbett, J. and Fry, A. and Hartmann, N. and Hast, C. and Hettel, R. and Horn-Von Hoegen, M. and Janoschka, D. and Lewandowski, J.R. and Ligges, M. and Meyer zu Heringdorf, F.-J. and Shen, X. and Vecchione, T. and Witt, C. and Wu, J. and Dürr, H.A. and Wang, X.J.
    New Journal of Physics 17 (2015)
    Electron-phonon coupling processes determine electronic transport properties of materials and are responsible for the transfer of electronic excess energy to the lattice. With decreasing device dimensions an understanding of these processes in nanoscale materials is becoming increasingly important. Here we use time-resolved electron diffraction to directly study energy relaxation in thin bismuth films after optical excitation. Precise measurements of the transient Debye-Waller-effect for various film thicknesses and over an extended range of excitation fluences allow to separate different contributions to the incoherent lattice response. While phonon softening in the electronically excited state is responsible for an immediate increase of the r.m.s. atomic displacement within a few hundred fs, 'ordinary' electron-phonon coupling leads to subsequent heating of the material on a few ps time-scale. The data reveal distinct changes in the energy transfer dynamics which becomes faster for stronger excitation and smaller film thickness, respectively. The latter effect is attributed to a cross-interfacial coupling of excited electrons to phonons in the substrate. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/17/11/113047
  • Visualization of nanocrystal breathing modes at extreme strains
    Szilagyi, E. and Wittenberg, J.S. and Miller, T.A. and Lutker, K. and Quirin, F. and Lemke, H. and Zhu, D. and Chollet, M. and Robinson, J. and Wen, H. and Sokolowski-Tinten, K. and Lindenberg, A.M.
    Nature Communications 6 (2015)
    Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. These functional properties often involve large-amplitude strains and structural modifications, and thus require an understanding of the dynamics of these processes. Here we use femtosecond X-ray scattering techniques to visualize, in real time and with atomic-scale resolution, light-induced anisotropic strains in nanocrystal spheres and rods. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrier-induced stress. These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. This work represents the first direct real-time probe of the dynamics of these large-amplitude strains and shape changes in few-nanometre-scale particles. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms7577
  • Real-time visualization of nanocrystal solid-solid transformation pathways
    Wittenberg, J.S. and Miller, T.A. and Szilagyi, E. and Lutker, K. and Quirin, F. and Lu, W. and Lemke, H. and Zhu, D. and Chollet, M. and Robinson, J. and Wen, H. and Sokolowski-Tinten, K. and Alivisatos, A.P. and Lindenberg, A.M.
    Nano Letters 14 (2014)
    Measurement and understanding of the microscopic pathways materials follow as they transform is crucial for the design and synthesis of new metastable phases of matter. Here we employ femtosecond single-shot X-ray diffraction techniques to measure the pathways underlying solid-solid phase transitions in cadmium sulfide nanorods, a model system for a general class of martensitic transformations. Using picosecond rise-time laser-generated shocks to trigger the transformation, we directly observe the transition state dynamics associated with the wurtzite-to-rocksalt structural phase transformation in cadmium sulfide with atomic-scale resolution. A stress-dependent transition path is observed. At high peak stresses, the majority of the sample is converted directly into the rocksalt phase with no evidence of an intermediate prior to rocksalt formation. At lower peak stresses, a transient five-coordinated intermediate structure is observed consistent with previous first principles modeling. © 2014 American Chemical Society.
    view abstract10.1021/nl500043c
  • Silicon Mirrors for High-Intensity X-Ray Pump and Probe Experiments
    Pardini, T. and Boutet, S. and Bradley, J. and Döppner, T. and Fletcher, L.B. and Gardner, D.F. and Hill, R.M. and Hunter, M.S. and Krzywinski, J. and Messerschmidt, M. and Pak, A.E. and Quirin, F. and Sokolowski-Tinten, K. and Williams, G.J. and Hau-Riege, S.P.
    Physical Review Applied 1 (2014)
    An all-x-ray pump and probe capability is highly desired for the free-electron laser community. A possible implementation involves the use of an x-ray mirror downstream of the sample to backreflect the pump beam onto itself. We expose silicon single crystals, a candidate for this hard-x-ray mirror, to the hard-x-ray beam of the Linac Coherent Light Source (SLAC National Acceleration Laboratory) to assess its suitability. We find that silicon is an appropriate mirror material, but its reflectivity at high x-ray fluences is somewhat unpredictable. We attribute this behavior to x-ray-induced local damage in the mirror, which we have characterized post mortem via microdiffraction, scanning electron microscopy, and Raman spectroscopy. We demonstrate a strategy to reduce local damage by using a structured silicon-based mirror. Preliminary results suggest that the latter yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray backreflectors. © 2014 American Physical Society.
    view abstract10.1103/PhysRevApplied.1.044007
  • Experimental set-up and procedures for the investigation of XUV free electron laser interactions with solids
    Sobierajski, R. and Jurek, M. and Chalupský, J. and Krzywinski, J. and Burian, T. and Dastjani Farahani, S. and Hájková, V. and Harmand, M. and Juha, L. and Klinger, D. and Loch, R.A. and Ozkan, C. and Pełka, J.B. and Sokolowski-Tinten, K. and Sinn, H. and Toleikis, S. and Tiedtke, K. and Tschentscher, T. and Wabnitz, H. and Gaudin, J.
    Journal of Instrumentation 8 (2013)
    In this article, we describe the experimental station and procedures for investigating the interaction of short-wavelength free-electron lasers (FELs) pulses with solids. With the advent of these sources, a unique combination of radiation properties (including wavelength range from tens of nanometers down to sub-Angstroms, femtosecond pulse duration, and high pulse energy reaching milli-Joules level) creates new research possibilities for the systematic studies of radiation-induced structural changes in solids. However, the properties of the intense FEL radiation generate, apart from the new experimental opportunities, extreme demands on the experimental set-up (mostly in terms of radiation hardness of detectors and their saturation levels). Thus, radiation-induced phase transitions in solids, beyond the fundamental scientific interest, are of importance for the design of FEL beamlines and instruments which interact with the direct beam. In this report, we focus on the instrumentation and experimental techniques used in the recent studies performed at the FLASH facility in Hamburg. © 2013 IOP Publishing Ltd and Sissa Medialab srl.
    view abstract10.1088/1748-0221/8/02/P02010
  • Fourier-transform inelastic X-ray scattering from time- and momentum-dependent phonon-phonon correlations
    Trigo, M. and Fuchs, M. and Chen, J. and Jiang, M.P. and Cammarata, M. and Fahy, S. and Fritz, D.M. and Gaffney, K. and Ghimire, S. and Higginbotham, A. and Johnson, S.L. and Kozina, M.E. and Larsson, J. and Lemke, H. and Lindenberg, A.M. and Ndabashimiye, G. and Quirin, F. and Sokolowski-Tinten, K. and Uher, C. and Wang, G. and Wark, J.S. and Zhu, D. and Reis, D.A.
    Nature Physics 9 (2013)
    The macroscopic characteristics of a material are determined by its elementary excitations, which dictate the response of the system to external stimuli. The spectrum of excitations is related to fluctuations in the density-density correlations and is typically measured through frequency-domain neutron or X-ray scattering. Time-domain measurements of these correlations could yield a more direct way to investigate the excitations of solids and their couplings both near to and far from equilibrium. Here we show that we can access large portions of the phonon dispersion of germanium by measuring the diffuse scattering from femtosecond X-ray free-electron laser pulses. A femtosecond optical laser pulse slightly quenches the vibrational frequencies, producing pairs of high-wavevector phonons with opposite momenta. These phonons manifest themselves as time-dependent coherences in the displacement correlations probed by the X-ray scattering. As the coherences are preferentially created in regions of strong electron-phonon coupling, the time-resolved approach is a natural spectroscopic tool for probing low-energy collective excitations in solids, and their microscopic interactions. © 2013 Macmillan Publishers Limited.
    view abstract10.1038/nphys2788
  • Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers
    Riedel, R. and Al-Shemmary, A. and Gensch, M. and Golz, T. and Harmand, M. and Medvedev, N. and Prandolini, M.J. and Sokolowski-Tinten, K. and Toleikis, S. and Wegner, U. and Ziaja, B. and Stojanovic, N. and Tavella, F.
    Nature Communications 4 (2013)
    The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si 3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms2754
  • Ultrafast laser-induced melting and ablation studied by time-resolved diffuse X-ray scattering
    Nicoul, M. and Quirin, F. and Lindenberg, A.M. and Barty, A. and Fritz, D.M. and Zhu, D. and Lemke, H. and Chollet, M. and Reis, D.A. and Chen, J. and Ghimire, S. and Trigo, M. and Fuchs, M. and Gaffney, K.J. and Larsson, J. and Becker, T. and Meyer, S. and Payer, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M. and Jerman, M. and Sokolowski-Tinten, K.
    EPJ Web of Conferences 41 (2013)
    Time-resolved diffuse X-ray scattering with 50 fs, 9.5 keV X-ray pulses from the Linear Coherent Light Source was used to study the structural dynamics in materials undergoing rapid melting and ablation after fs laser excitation. © Owned by the authors, published by EDP Sciences, 2013.
    view abstract10.1051/epjconf/20134104013
  • Structural dynamics in FeRh during a laser-induced metamagnetic phase transition
    Quirin, F. and Vattilana, M. and Shymanovich, U. and El-Kamhawy, A.-E. and Tarasevitch, A. and Hohlfeld, J. and Von Der Linde, D. and Sokolowski-Tinten, K.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Time-resolved x-ray diffraction with ultrashort x-ray pulses from a laser-produced plasma is used to study the lattice response of FeRh during a femtosecond laser-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition. Pump-probe measurements at initial sample temperatures below as well as above the AFM-to-FM transition temperature and for different laser pump fluences allowed to disentangle the various contributions driving lattice expansion. In particular, the data reveal that the structural changes associated with the magnetic phase transition occur on a time scale of a hundred picoseconds. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.020103
  • Ultrafast photovoltaic response in ferroelectric nanolayers
    Daranciang, D. and Highland, M.J. and Wen, H. and Young, S.M. and Brandt, N.C. and Hwang, H.Y. and Vattilana, M. and Nicoul, M. and Quirin, F. and Goodfellow, J. and Qi, T. and Grinberg, I. and Fritz, D.M. and Cammarata, M. and Zhu, D. and Lemke, H.T. and Walko, D.A. and Dufresne, E.M. and Li, Y. and Larsson, J. and Reis, D.A. and Sokolowski-Tinten, K. and Nelson, K.A. and Rappe, A.M. and Fuoss, P.H. and Stephenson, G.B. and Lindenberg, A.M.
    Physical Review Letters 108 (2012)
    We show that light drives large-amplitude structural changes in thin films of the prototypical ferroelectric PbTiO 3 via direct coupling to its intrinsic photovoltaic response. Using time-resolved x-ray scattering to visualize atomic displacements on femtosecond time scales, photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion of photogenerated free charges within the ferroelectric and can be simply explained by a model including both shift and screening currents, associated with the displacement of electrons first antiparallel to and then parallel to the ferroelectric polarization direction. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.087601
  • Ultrafast transitions from solid to liquid and plasma states of graphite induced by X-ray free-electron laser pulses
    Hau-Riege, S.P. and Graf, A. and Döppner, T. and London, R.A. and Krzywinski, J. and Fortmann, C. and Glenzer, S.H. and Frank, M. and Sokolowski-Tinten, K. and Messerschmidt, M. and Bostedt, C. and Schorb, S. and Bradley, J.A. and Lutman, A. and Rolles, D. and Rudenko, A. and Rudek, B.
    Physical Review Letters 108 (2012)
    We used photon pulses from an x-ray free-electron laser to study ultrafast x-ray-induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg-peak degradation is smaller and the ion-heating rate is faster than current x-ray-matter interaction models predict. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.217402
  • Picosecond acoustic response of a laser-heated gold-film studied with time-resolved x-ray diffraction
    Nicoul, M. and Shymanovich, U. and Tarasevitch, A. and Von Der Linde, D. and Sokolowski-Tinten, K.
    Applied Physics Letters 98 (2011)
    We apply time-resolved x-ray diffraction using ultrashort x-ray pulses from a laser-produced plasma to probe the picosecond acoustic response of a thin laser-heated gold film. Measurements of the temporal changes in the angular distribution of diffracted x-rays provide direct quantitative information on the transient evolution of lattice strain. This allows to disentangle electronic and thermal pressure contributions driving lattice expansion after impulsive laser excitation. The electron-lattice energy equilibration time τE = (5±0.3) ps as well as the electronic Grüneisen parameter γe = (1.48±0.3) have been determined. © 2011 American Institute of Physics.
    view abstract10.1063/1.3584864
  • Coherent acoustic and optical phonons in laser-excited solids studied by ultrafast time-resolved X-ray diffraction
    Shymanovich, U. and Nicoul, M. and Lu, W. and Tarasevitch, A. and Kammler, M. and Horn-Von Hoegen, M. and Von Der Linde, D. and Sokolowski-Tinten, K.
    AIP Conference Proceedings 1278 (2010)
    We apply ultrafast time-resolved X-ray diffraction to directly study coherent acoustic and optical phonons in laser-excited materials. In Au the ps acoustic response has been investigated with the particular goal to clarify the interplay of electronic and thermal pressure contributions. In Bi the extreme softening of the coherently excited A1g optical phonon presents strong indication that upon intense laser-excitation the Peierls-transition which determines the equilibrium structure of Bi can be reversed. In FeRh we studied the lattice response after a fast laser-induced anti-ferromagnetic to ferromagnetic phase transition. © 2010 American Institute of Physics.
    view abstract10.1063/1.3507146
  • Short-pulse laser induced transient structure formation and ablation studied with time-resolved coherent XUV-scattering
    Sokolowski-Tinten, K. and Barty, A. and Boutet, S. and Shymanovich, U. and Chapman, H. and Bogan, M. and Marchesini, S. and Hau-Riege, S. and Stojanovic, N. and Bonse, J. and Rosandi, Y. and Urbassek, H.M. and Tobey, R. and Ehrke, H. and Cavalleri, A. and Düsterer, S. and Redlin, H. and Frank, M. and Bajt, S. and Schulz, J. and Seibert, M. and Hajdu, J. and Treusch, R. and Bostedt, C. and Hoener, M. and Möller, T.
    AIP Conference Proceedings 1278 (2010)
    The structural dynamics of short-pulse laser irradiated surfaces and nano-structures has been studied with nm spatial and ultrafast temporal resolution by means of single-shot coherent XUV-scattering techniques. The experiments allowed us to time-resolve the formation of laser-induced periodic surface structures, and to follow the expansion and disintegration of nano-objects during laser ablation. © 2010 American Institute of Physics.
    view abstract10.1063/1.3507123
  • Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure
    Khorsand, A.R. and Sobierajski, R. and Louis, E. and Bruijn, S. and Van Hattum, E.D. and Vande Kruijs, R.W.E. and Jurek, M. and Klinger, D. and Pelka, J.B. and Juha, L. and Burian, T. and Chalupsky, J. and Cihelka, J. and Hajkova, V. and Vysin, L. and Jastrow, U. and Stojanovic, N. and Toleikis, S. and Wabnitz, H. and Tiedtke, K. and Sokolowski-Tinten, K. and Shymanovich, U. and Krzywinski, J. and Riege Hau, S. and London, R. and Gleeson, A. and Gullikson, E.M. and Bijkerk, F.
    Optics Express 18 (2010)
    We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference- polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources. © 2010 Optical Society of America.
    view abstract10.1364/OE.18.000700
  • femtosecond X-ray science

  • laser ablation

  • Lasers

  • nanostructures

  • structural dynamics

  • structural phase transition

  • ultrafast dynamics

  • ultrafast electron diffraction

  • ultrafast phase transitions

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