Publications Project 04 -
Interactions of Ultrashort Field with Solid Surfaces and Nanostructures

Project Leader: Joachim Burgdörfer


Observing the ultrafast buildup of a Fano resonance in the time domain

A. Kaldun, A. Blättermann, V. Stooß, S. Donsa, H. Wei, R. Pazourek, S. Nagele, C. Ott, C. D. Lin, J. Burgdörfer, T. Pfeifer

  Although the time-dependent buildup of asymmetric Fano line shapes in absorption spectra has been of great theoretical interest in the past decade, experimental verification of the predictions has been elusive. Here, we report the experimental observation of the emergence of a Fano resonance in the prototype system of helium by interrupting the autoionization process of a correlated two-electron excited state with a strong laser field. The tunable temporal gate between excitation and termination of the resonance allows us to follow the formation of a Fano line shape in time. The agreement with ab initio calculations validates our experimental time-gating technique for addressing an even broader range of topics, such as the emergence of electron correlation, the onset of electron-internuclear coupling, and quasi-particle formation.

Published: 2016-11-07
Science 11 Nov 2016: Vol. 354, Issue 6313
DOI: 10.1126/science.aah6972

Press release TU Wien, 2016-11-11 [73/2016]


Attosecond correlation dynamics

M. Ossiander, F. Siegrist, V. Shirvanyan, R. Pazourek, A. Sommer, T. Latka, A. Guggenmos, S. Nagele, J. Feist, J. Burgdörfer, R. Kienberger & M. Schultze

  Photoemission of an electron is commonly treated as a one-particle phenomenon. With attosecond streaking spectroscopy we observe the breakdown of this single active-electron approximation by recording up to six attoseconds retardation of the dislodged photoelectron due to electronic correlations. We recorded the photon-energy-dependent emission timing of electrons, released from the helium ground state by an extreme-ultraviolet photon, either leaving the ion in its ground state or exciting it into a shake-up state. We identify an optical field-driven d.c. Stark shift of charge-asymmetric ionic states formed after the entangled photoemission as a key contribution to the observed correlation time shift. These findings enable a complete wavepacket reconstruction and are universal for all polarized initial and final states. Sub-attosecond agreement with quantum mechanical ab initio modelling allows us to determine the absolute zero of time in the photoelectric effect to a precision better than 1/25th of the atomic unit of time.

Published: 2016-11-07
Nature Physics (2016)
DOI: 10.1038/nphys3941

Press release TU Wien, 2016-11-07 [72/2016]


Coincidence spectroscopy of high-lying Rydberg states produced in strong field interaction

S. Larimian, S. Erattupuzha, R. Maurer, C. Lemell, S. Nagele, S. Yoshida, J. Burgdörfer, A. Baltuska, M. Kitzler, X. Xie

  We demonstrate the detection of high-lying Rydberg states produced in strong laser fields with coincidence spectroscopy. Electron emission after the interaction of strong laser pulses with atoms and molecules is measured together with the parent ions in coincidence measurements. These electrons originate from high-lying Rydberg states with quantum numbers from n∼20 up to n≲120 formed by frustrated field ionization. Ionization rates are retrieved from the measured ionization signal of these Rydberg states. Simulations show that both tunneling ionization by a weak dc field and photoionization by blackbody radiation contribute to delayed electron emission on the nano- to microsecond scale. Furthermore, the dependence of the Rydberg-state production on the ellipticity of the driving laser field indicates that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production in strong-field interaction.

Published: 2016-09-01
Phys. Rev. A 94, 033401 (2016)
DOI: 10.1103/PhysRevA.94.033401
(--> also listed in Project 03)


Time-dependent complete-active-space self-consistent-field method for atoms: Application to high-harmonic generation

T. Sato, K. L. Ishikawa, I. Brezinova, F. Lackner, S. Nagele, J. Burgdörfer

  We present a numerical implementation of the time-dependent complete-active-space self-consistent-field (TD-CASSCF) method [Phys. Rev. A 88, 023402 (2013)] for atoms driven by a strong linearly polarized laser pulse. The present implementation treats the problem in its full dimensionality and introduces a gauge-invariant frozen-core approximation, an efficient evaluation of the Coulomb mean field scaling linearly with the number of basis functions, and a split-operator method specifically designed for stable propagation of stiff spatial derivative operators. We apply this method to high-harmonic generation in helium, beryllium, and neon and explore the role of electron correlations.

Published: 2016-08-09
Phys. Rev. A 94, 023405 (2016)
DOI: 10.1103/PhysRevA.94.023405


Nonlinear response of graphene to a few cycle THz laser pulse: role of doping and disorder

L. Chizhova, J. Burgdörfer, and F. Libisch

  The nonlinear response of graphene to a THz laser pulse is studied by solving the time-dependent Dirac equation and the time-dependent Schrödinger equation within a tight-binding approximation applied to finite-sized structures. We compare predictions of these two approximations for the harmonic spectrum with the recent experiment by P. Bowlan et al. [Phys. Rev. B 89, 041408(R) (2014)]. We highlight the influence of short-range and long-range disorder which can be accounted for within the tight-binding description on a microscopic level. We find good agreement with the experiment. Most notably, the intensity of the second harmonic offers a quantitative indicator for the amount of short-range disorder.

Published: 2016-08-09
Phys. Rev. B 94, 075412 (2016)
DOI: 10.1103/PhysRevB.94.075412


Semiclassical two-step model for strong-field ionization

N. I. Shvetsov-Shilovski, M. Lein, L. B. Madsen, E. Räsänen, C. Lemell, J. Burgdörfer, D. G. Arbó, K. Tőkési

  We present a semiclassical two-step model for strong-field ionization that accounts for path interferences of tunnel-ionized electrons in the ionic potential beyond perturbation theory. Within the framework of a classical trajectory Monte Carlo representation of the phase-space dynamics, the model employs the semiclassical approximation to the phase of the full quantum propagator in the exit channel. By comparison with the exact numerical solution of the time-dependent Schrödinger equation for strong-field ionization of hydrogen, we show that for suitable choices of the momentum distribution after the first tunneling step, the model yields good quantitative agreement with the full quantum simulation. The two-dimensional photoelectron momentum distributions, the energy spectra, and the angular distributions are found to be in good agreement with the corresponding quantum results. Specifically, the model quantitatively reproduces the fanlike interference patterns in the low-energy part of the two-dimensional momentum distributions, as well as the modulations in the photoelectron angular distributions.

Published: 2016-07-19
Phys. Rev. A 94, 013415 (2016)
DOI: 10.1103/PhysRevA.94.013415


Size quantization of Dirac fermions in graphene constrictions

B. Terrés, L. A. Chizhova, F. Libisch, J. Peiro, D. Jörger, S. Engels, A. Girschik, K. Watanabe, T. Taniguchi, S. V. Rotkin, J. Burgdörfer, and C. Stampfer

  Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wavelength in high-quality bulk graphene can be tuned up to hundreds of nanometres, the observation of quantum confinement of Dirac electrons in nanostructured graphene has proven surprisingly challenging. Here we show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically defined graphene constrictions. At high carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity of ∼1.5 × 10⁶ m s-¹ in our constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.

Published: 2016-05-20
Nature Communications 7, 11528 (2016)
DOI: 10.1038/ncomms11528


Trap losses induced by near-resonant Rydberg dressing of cold atomic gases

J. A. Aman, B. J. DeSalvo, F. B. Dunning, T. C. Killian, S. Yoshida, and J. Burgdörfer

  The near-resonant dressing of cold strontium gases and Bose-Einstein condensates contained in an optical dipole trap (ODT) with the 5s30s³S1 Rydberg state is investigated as a function of the effective two-photon Rabi frequency, detuning, and dressing time. The measurements demonstrate that a rapid decrease in the ground-state atom population in the ODT occurs even for weak dressing and when well detuned from resonance. This decrease is attributed to Rydberg atom excitation, which can lead to direct escape from the trap and to population of very long-lived 5s5p³P0,2 metastable states. The effects of interactions between Rydberg atoms, including those populated by blackbody radiation, are analyzed. The work has important implications when considering the use of Rydberg dressing to control the interactions between dressed ground-state atoms.

Published: 2016-04-29
Phys. Rev. A 93, 043425 (2016)
DOI: 10.1103/PhysRevA.93.043425


Rydberg-blockade effects in Autler-Townes spectra of ultracold strontium

B. J. DeSalvo, J. A. Aman, C. Gaul, T. Pohl, S. Yoshida, J. Burgdörfer, K. R. A. Hazzard, F. B. Dunning, and T. C. Killian

  We present a combined experimental and theoretical study of the effects of Rydberg interactions on Autler-Townes spectra of ultracold gases of atomic strontium. Realizing two-photon Rydberg excitation via a long-lived triplet state allows us to probe the regime where Rydberg state decay presents the dominant decoherence mechanism. The effects of Rydberg interactions are observed in shifts, asymmetries, and broadening of the measured atom-loss spectra. The experiment is analyzed within a one-body density-matrix approach, accounting for interaction-induced level shifts and dephasing through nonlinear terms that approximately incorporate correlations due to the Rydberg blockade. This description yields good agreement with our experimental observations for short excitation times. For longer excitation times, the loss spectrum is altered qualitatively, suggesting additional dephasing mechanisms beyond the standard blockade mechanism based on pure van der Waals interactions.

Published: 2016-02-22
Phys. Rev. A 93, 022709 (2016)
DOI: 10.1103/PhysRevA.93.022709


Lifetimes of ultra-long-range strontium Rydberg molecules

Camargo, F., J. D. Whalen, R. Ding, H. R. Sadeghpour, S. Yoshida, J. Burgdörfer, F. B. Dunning, and T. C. Killian

  The lifetimes of the lower-lying vibrational states of ultra-long-range strontium Rydberg molecules comprising one ground-state 5s² ¹S0 atom and one Rydberg atom in the 5s38s³S1 state are reported. The molecules are created in an ultracold gas held in an optical dipole trap and their numbers determined using field ionization, the product electrons being detected by a microchannel plate. The measurements show that, in marked contrast to earlier measurements involving rubidium Rydberg molecules, the lifetimes of the low-lying molecular vibrational states are very similar to those of the parent Rydberg atoms. This results because the strong p-wave resonance in low-energy electron-rubidium scattering, which strongly influences the rubidium molecular lifetimes, is not present for strontium. The absence of this resonance offers advantages for experiments involving strontium Rydberg atoms as impurities in quantum gases and for testing of theories of molecular formation and decay.

Published: 2016-02-04
Phys. Rev. A 93, 022702 (2016)
DOI: 10.1103/PhysRevA.93.022702


Controlling ultrafast currents by the nonlinear photogalvanic effect

G. Wachter, S.A. Sato, I. Floss, C. Lemell, X.-M. Tong, K. Yabana, J. Burgdörfer

  We investigate the effect of broken inversion symmetry on the generation and control of ultrafast currents in a transparent dielectric (SiO2) by strong femtosecond optical laser pulses. Ab initio simulations based on time-dependent density functional theory predict ultrafast direct currents that can be viewed as a nonlinear photogalvanic effect. Most surprisingly, the direction of the current undergoes a sudden reversal above a critical threshold value of laser intensity of about Ic ~ 3 x 10¹³ W cm−². We trace this switching to the transition from nonlinear polarisation currents to the tunnelling excitation regime. The latter is found to be sensitive to the relative orientation between laser polarisation and chemical bonds. We demonstrate control of the ultrafast currents by the time delay between two laser pulses. While two temporally separated laser pulses lead to currents along one direction their temporal overlap can reverse the current. We find the ultrafast current control by the nonlinear photogalvanic effect to be remarkably robust and insensitive to the laser-pulse shape and the carrier-envelope phase.

Published: 2015-12-21
New J. Phys. 17, 123026 (2015)
DOI: 10.1088/1367-2630/17/12/123026


Protocol for observing molecular dipole excitations by attosecond self-streaking

G. Wachter, S. Nagele, S.A. Sato, R. Pazourek, M. Wais, C. Lemell, X.-M. Tong, K. Yabana, J. Burgdörfer

  We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene Ne@C60 is initiated by a moderately intense femtosecond UV-visible pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab initio simulations.

Published: 2015-12-18
Phys. Rev. A 92, 061403(R), 2015
DOI: 10.1103/PhysRevA.92.061403


Anomalous Fano Profiles in External Fields

A. Zielinski, V.P. Majety, S. Nagele, R. Pazourek, J. Burgdörfer, A. Scrinzi

  We show that the external control of Fano resonances in general leads to complex Fano q parameters. Fano line shapes of photoelectron and transient absorption spectra in the presence of an infrared control field are investigated. Computed transient absorption spectra are compared with a model proposed for a recent experiment [C. Ott et al., Science 340, 716 (2013)]. Control mechanisms for photoelectron spectra are exposed: control pulses applied during excitation modify the line shapes by momentum boosts of the continuum electrons. Pulses arriving after excitation generate interference fringes due to infrared two-photon transitions.

Published: 2015-12-10
Phys. Rev. Lett. 115, 243001 (2015)
DOI: 10.1103/PhysRevLett.115.243001


Rydberg blockade effects at n∼300 in strontium

X. Zhang, F.B. Dunning, S. Yoshida, J. Burgdörfer

  Rydberg blockade at n∼300, is examined using strontium nF31 Rydberg atoms excited in an atomic beam in a small volume defined by two tightly focused crossed laser beams. The observation of blockade for such states is challenging due to their extreme sensitivity to stray fields and the many magnetic sublevels associated with F states which results in a high local density of states. Nonetheless, with a careful choice of laser polarization to selectively excite only a limited number of these sublevels, sizable blockade effects are observed on an ∼0.1 mm length scale extending blockade measurements into the near-macroscopic regime and enabling study of the dynamics of strongly coupled many-body high-n Rydberg systems under carefully controlled conditions.

Published: 2015-11-23
Phys. Rev. A 92, 051402(R) (2015)
DOI: 10.1103/PhysRevA.92.051402


Application of norm-conserving pseudopotentials to intense laser-matter interactions

X.-M. Tong, G. Wachter, S. Sato, C. Lemell, K. Yabana, and J. Burgdörfer

  We investigate the applicability of norm-conserving pseudopotentials to intense laser-matter interactions by performing time-dependent density functional theory simulations with an all-electron potential and with norm-conserving pseudopotentials. We find pseudopotentials to be reliable for the simulation of above-threshold ionization over a broad range of laser intensities both for the total ionization probability and the photoelectron energy spectrum. For the simulation of high-order-harmonic generation, pseudopotentials are shown to be applicable for lower-order harmonics in the spectral range in which the one-photon recombination dipole-matrix element can be recovered by the pseudopotential calculation.

Published: 2015-10-29
Physical Review A 92, 043422 (2015)
DOI: 10.1103/PhysRevA.92.043422


Ultra-long-range Rydberg molecules in a divalent atomic system

B. DeSalvo, J. Aman, F.B. Dunning, T. Killian, H. Sadeghpour, S. Yoshida, and J. Burgdörfer

  We report the creation of ultra-long-range Sr2 molecules comprising one ground-state 5s2 1S0 atom and one atom in a 5sns 3S1 Rydberg state for n ranging from 29 to 36. Molecules are created in a trapped ultracold atomic gas using two-photon excitation near resonant with the 5s5p 3P1 intermediate state, and their formation is detected through ground-state atom loss from the trap. The observed molecular binding energies are reproduced with the aid of first-order perturbation theory that utilizes a Fermi pseudopotential with effective s-wave and p-wave scattering lengths to describe the interaction between an excited Rydberg electron and a ground-state Sr atom.

Published: 2015-09-28
Physical Review A 90, 031403(R) (2015)
DOI: 10.1103/PhysRevA.92.031403


Time delays in correlated photoemission processes

R. Pazourek, S. Nagele, and J. Burgdörfer

  We theoretically study time-resolved two-photon double ionization (TPDI) of helium as probed by attosecond streaking. We review recent advances in the understanding of the photoelectric effect in the time domain and discuss the differences between one- and two-photon ionization, as well as one- and two-electron emission. We perform exact ab-initio simulations for attosecond streaking experiments in the sequential TPDI regime and compare the results to the two-electron Eisenbud-Wigner-Smith delay for the process. Our calculations directly show that the timing of the emission process sensitively depends on the energy sharing between the two outgoing electrons. In particular, we identify Fano-like interferences in the relative time delay of the two emitted electrons when the sequential ionization channel occurs via intermediate excited ionic (shake-up) states. Furthermore, we find that the photoemission time delays are only weakly dependent on the relative emission angle of the ejected electrons.

Published: 2015-09-07
Journal of Physics: Conference Series 635, 012004 (2015)
DOI: 10.1088/1742-6596/635/1/012004


Signatures of tunneling and multiphoton ionization by short-laser pulses: The partial-wave distribution

D. Arbó, C. Lemell, and J. Burgdörfer

  We analyze the two-dimensional angular momentum-energy distribution of electrons emitted from argon by short laser pulses. We identify characteristic features of both multiphoton and tunneling ionization in the partial-wave distribution for Keldysh parameters close to unity. We observe a remarkable degree of quantum-classical correspondence in the photoinization process which becomes even more pronounced after intensity averaging over the focal volume. We derive an energy-dependent cut-off for the highest angular momentum accessible within the framework of the strong-field approximation, which accurately reproduces the partial wave distributions found from solutions of the time-dependent Schrödinger equation.

Published: 2015-09-07
Journal of Physics: Conference Series 635, 012003 (2015)
DOI: 10.1088/1742-6596/635/1/012003


Attosecond chronoscopy of photoemission

R. Pazourek, S. Nagele, and J. Burgdörfer

  Recent advances in the generation of well-characterized subfemtosecond laser pulses have opened up unpredicted opportunities for the real-time observation of ultrafast electronic dynamics in matter. Such attosecond chronoscopy allows a novel look at a wide range of fundamental photophysical and photochemical processes in the time domain, including Auger and autoionization processes, as well as photoemission from atoms, molecules, and surfaces, complementing conventional energy-domain spectroscopy. Attosecond chronoscopy raises fundamental conceptual and theoretical questions as to which novel information becomes accessible and which dynamical processes can be controlled and steered. Several of these questions, currently a matter of lively debate, are addressed in this review. The focus is placed on one prototypical case, the chronoscopy of the photoelectric effect by attosecond streaking. Is photoionization instantaneous or is there a finite response time of the electronic wave function to the photoabsorption event? Answers to this question turn out to be far more complex and multifaceted than initially thought. They touch upon fundamental issues of time and time delay as observables in quantum theory. Recent progress of our understanding of time-resolved photoemission from atoms, molecules, and solids is reviewed. Unresolved and open questions are highlighted and future directions are discussed addressing the observation and control of electronic motion in more complex nanoscale structures and in condensed matter.

Published: 2015-08-12
Reviews of Modern Physics 87, 765 (2015)
DOI: 10.1103/RevModPhys.87.765


Ionization of argon by two-color laser pulses with coherent phase control

D. Arbó, C. Lemell, S. Nagele, N. Camus, L. Fechner, A. Krupp, T. Pfeifer, S. Lopéz, R. Moshammer, and J. Burgdörfer

  We present a joint experimental and theoretical study of ionization of argon atoms by a linearly polarized two-color laser field (λ1=800 nm, λ2=400 nm). Changing the relative phase φ between the two colors, the forward-backward asymmetry of the doubly differential momentum distribution of emitted electrons can be controlled. We find excellent agreement between the measurements and the solution of the time-dependent Schrödinger equation in the single-active electron approximation. Surprisingly we also find good agreement between the quantum and classical calculations of electron momentum distributions generated by lasers at optical wavelengths.

Published: 2015-08-03
Physical Review A 92, 023402 (2015)
DOI: 10.1103/PhysRevA.92.023402


Large optical field enhancement for nanotips with large opening angles

S. Thomas, G. Wachter, C. Lemell, J. Burgdörfer, P. Hommelhoff

  We theoretically investigate the dependence of the enhancement of optical near-fields at nanometric tips on the shape, size, and material of the tip. We confirm the strong dependence of the field enhancement factor on the radius of curvature. In addition, we find a surprisingly strong increase of field enhancement with increasing opening angle of the nanotips. For gold and tungsten nanotips in the experimentally relevant parameter range (radius of curvature ≥5nm at 800 nm laser wavelength), we obtain field enhancement factors of up to ~35 for Au and ~12 for W for large opening angles. We confirm this strong dependence on the opening angle for many other materials featuring a wide variety in their dielectric response. For dielectrics, the opening angle dependence is traced back to the electrostatic force of the induced surface charge at the tip shank. For metals, the plasmonic response strongly increases the field enhancement and shifts the maximum field enhancement to smaller opening angles.

Published: 2015-06-10
New Journal of Physics 17, 063010 (2015)
DOI: 10.1088/1367-2630/17/6/063010


Real-time observation of collective excitations in photoemission

C. Lemell, S. Neppl, G. Wachter, K. Tökési, R. Ernstorfer, P. Feulner, R. Kienberger, J. Burgdörfer

  Ejection of an electron by absorption of an extreme ultraviolet (xuv) photon probes the many-electron response of a solid well beyond the single-particle picture. Photoemission spectra feature complex correlation satellite structures signifying the simultaneous excitation of single or multiple plasmons. The time delay of the plasmon satellites relative to the main line can be resolved in attosecond streaking experiments. Time-resolved photoemission thus provides the key to discriminate between intrinsic and extrinsic plasmon excitation. We demonstrate the determination of the branching ratio between intrinsic and extrinsic plasmon generation for simple metals.

Published: 2015-06-03
Physical Review B 91, 241101 (2015)
DOI: 10.1103/PhysRevB.91.241101


Coherent Electronic Wave Packet Motion in C-60 Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

H. Li, B. Mignolet, G. Wachter, S. Skruszewitcz, S. Zherebtsov, F. Suessmann, A. Kessel, S. Trushin, N. Kling, M. Kuebel, B. Ahn, D. Kim, I. Ben-Itzhak, C. Cocke, T. Fennel, J. Tiggesbaeumker, K. Meiwes-Broer, C. Lemell, J. Burgdörfer, R. Levine, F. Remacle, M. Kling

  Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C60 fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.

Published: 2015-03-27
Physical Review Letters 114, 123004 (2015)
DOI: 10.1103/PhysRevLett.114.123004


Probing time-ordering in two-photon double ionization of helium on the attosecond time scale

R. Pazourek, S. Nagele, and J. Burgdörfer

  We show that time ordering underlying time-dependent quantum dynamics is a physical observable accessible by attosecond streaking. We demonstrate the extraction of time ordering for the prototypical case of time-resolved two-photon double ionization of helium by an attosecond XUV pulse. The Eisenbud–Wigner–Smith time delay for the emission of a two-electron wavepacket and the time interval between subsequent emission events can be unambiguously determined by attosecond streaking. The delay between the two emission events sensitively depends on the energy, pulse duration, and angular distribution of the emitted electron pair. Our fully-dimensional ab initio quantum mechanical simulations provide benchmark data for experimentally accessible observables.

Published: 2015-03-04
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 061002 (2015)
DOI: 10.1088/0953-4075/48/6/061002


Propagating two-particle reduced density matrices without wave functions

F. Lackner, I. Brezinova, T. Sato, K. L. Ishikawa, and J. Burgdörfer

  Describing time-dependent many-body systems where correlation effects play an important role remains a major theoretical challenge. In this paper we develop a time-dependent many-body theory that is based on the two-particle reduced density matrix (2-RDM). We present a closed equation of motion for the 2-RDM by developing a reconstruction functional for the three-particle reduced density matrix (3-RDM) that preserves norm, energy, and spin symmetries during time propagation. We show that approximately enforcing N-representability during time evolution is essential for achieving stable solutions. As a prototypical test case which features long-range Coulomb interactions we employ the one-dimensional model for lithium hydride (LiH) in strong infrared laser fields. We probe both one-particle observables such as the time-dependent dipole moment and two-particle observables such as the pair density and mean electron-electron interaction energy. Our results are in very good agreement with numerically exact solutions for the N-electron wave function obtained from the multiconfigurational time-dependent Hartree-Fock method.

Published: 2015-02-11
Physical Review A 91, 023412 (2015)
DOI: 10.1103/PhysRevA.91.023412


Direct observation of electron propagation and dielectric screening on the atomic length scale

S. Neppl, R. Ernstorfer, A. Cavalieri, C. Lemell, G. Wachter, E. Magerl, E. Bothschafter, M. Jobst, M. Hofstetter, U. Kleineberg, J. Barth, D. Menzel, J. Burgdörfer, P. Feulner, F. Krausz, and R. Kienberger

  The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations—periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10−18 seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10−15 seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer—constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.

Published: 2015-01-15
Nature 517, 342 (2015)
DOI: 10.1038/nature14094


Formation of very-low-energy states crossing the ionization threshold of argon atoms in strong mid-infrared fields

B. Wolter, C. Lemell, M. Baudisch, M.G. Pullen, X.-M. Tong, M. Hemmer, A. Senftleben, C.D. Schröter, J. Ullrich, R. Moshammer, J. Biegert, J. Burgdörfer

  Atomic ionization by intense mid-infrared (mid-IR) pulses produces low-electron-energy features that the strong-field approximation, which is expected to be valid in the tunneling ionization regime characterized by small Keldysh parameters (γ≪1), cannot describe. These features include the low-energy structure (LES), the very-low-energy structure (VLES), and the more recently found zero-energy structure (ZES). They result from the interplay between the laser electric field and the atomic Coulomb field which controls the low-energy spectrum also for small γ. In the present joint experimental and theoretical study we investigate the vectorial momentum spectrum of photoelectrons emitted from an Ar gas target at very low energies. Using a reaction microscope optimized for the detection of very-low-energy electrons, we have performed a thorough study of the three-dimensional momentum spectrum well below 1 eV. Our measurements are complemented by quantum and classical simulations, which allow for an interpretation of the LES and VLES and of the ZES in terms of two-dimensional Coulomb focusing and recapture into Rydberg states, respectively.

Published: 2014-12-22
Phys. Rev. A 90, 063424 (2014)
DOI: 10.1103/PhysRevA.90.063424


Ionization of helium by slow antiproton impact: Total and differential cross sections

S. Borbély, J. Feist, K. Tökési, S. Nagele, L. Nagy, J. Burgdörfer

  We investigate theoretically the single and double ionization of the He atom by antiproton impact for projectile energies ranging from 3 keV up to 1000 keV. We obtain accurate total cross sections by directly solving the fully correlated two-electron time-dependent Schrödinger equation. The cross sections are in excellent agreement with the available experimental data. We also present fully ab initio doubly differential data for single ionization at 10 and 100 keV impact energies and compare to classical-trajectory Monte Carlo calculations. In these differential cross sections we identify the binary-encounter peak along with the anticusp minimum. Furthermore, we also point out the importance of the postcollisional electron-projectile interaction at low antiproton energies, which significantly suppresses electron emission in the forward direction.

Published: 2014-11-10
Phys. Rev. A 90, 052706 (2014)
DOI: 10.1103/PhysRevA.90.052706


Ab Initio Simulation of Electrical Currents Induced by Ultrafast Laser Excitation of Dielectric Materials

G. Wachter, C. Lemell, J. Burgdörfer

  We theoretically investigate the generation of ultrafast currents in insulators induced by strong few-cycle laser pulses. Ab initio simulations based on time-dependent density functional theory give insight into the atomic-scale properties of the induced current signifying a femtosecond-scale insulator-metal transition. We observe the transition from nonlinear polarization currents during the laser pulse at low intensities to tunnelinglike excitation into the conduction band at higher laser intensities. At high intensities, the current persists after the conclusion of the laser pulse considered to be the precursor of the dielectric breakdown on the femtosecond scale. We show that the transferred charge sensitively depends on the orientation of the polarization axis relative to the crystal axis, suggesting that the induced charge separation reflects the anisotropic electronic structure. We find good agreement with very recent experimental data on the intensity and carrier-envelope phase dependence [A. Schiffrin et al., Nature (London) 493, 70 (2013)].

Published: 2014-08-18
Physical Review Letters 113, 087401 (2014)
DOI: 10.1103/PhysRevLett.113.087401

Press release TU Wien, 2014-08-26

Attosecond streaking of Cohen-Fano interferences in the photoionization of H2+

Q.-C. Ning, L.-Y. Peng, S.-N. Song, W.-C. Jiang, S. Nagele, R. Pazourek, J. Burgdörfer, Q. Gong

  We present a numerical ab-initio simulation of the time delay in the photoionization of the simplest diatomic molecule H2+ as observed by attosecond streaking. We show that the strong variation of the Eisenbud-Wigner-Smith time delay tEWS as a function of energy and emission angle becomes observable in the streaking time shift tS provided laser field induced components are accounted for. The strongly enhanced photoemission time shifts are traced to destructive Cohen-Fano (or two-center) interferences. Signatures of these interferences in the streaking trace are shown to be enhanced when the ionic fragments are detected in coincidence.

Published: 2014-07-30
Physical Review A 90, 013423 (2014)
DOI: 10.1103/PhysRevA.90.013423


Efficient three-photon excitation of quasi-one-dimensional strontium Rydberg atoms with n~300

S. Ye, X. Zhang, F.B. Dunning, S. Yoshida, M. Hiller, J. Burgdörfer

  The efficient production of very-high-n, n∼300, quasi-one-dimensional (quasi-1D) strontium Rydberg atoms through three-photon excitation of extreme Stark states in the presence of a weak dc field is demonstrated using a crossed laser-atom beam geometry. Strongly polarized quasi-1D states with large permanent dipole moments ∼1.2n2 a.u. can be created in the beam at densities (∼106 cm−3) where dipole blockade effects should become important. A further advantage of three-photon excitation is that the product F states are sensitive to the presence of external fields, allowing stray fields to be reduced to very small values. The experimental data are analyzed using quantum calculations based on a two-active-electron model together with classical trajectory Monte Carlo simulations. These allow determination of the atomic dipole moments and confirm that stray fields can be reduced to ≤25 μV cm−1.

Published: 2014-07-01
Pysical Review A 90, 013401 (2014)
DOI: 10.1103/PhysRevA.90.013401


Interference of electron wave packets in atomic ionization by subcycle sculpted laser pulses

D.G. Arbó, S. Nagele, X.-M. Tong, X. Xie, M. Kitzler, J. Burgdörfer

  We present a theoretical analysis of the atomic photoelectron emission spectra produced by a linearly polarized sculpted laser pulse of two colors with frequencies ω and 2ω. The spectrum of the “direct” electrons with intermediate energies prominently features both intracycle and intercycle interferences. We derive a simple analytic expression for this spectral range based on a semiclassical approximation of the time-dependent distorted wave strong-field approximation generalized to strong-field ionization by a two-color pulse. We verify its applicability to approximately represent the intricate interference patterns by comparison with the exact solutions of the time-dependent Schr¨odinger equation and with the strong-field approximation.We show that the interference patterns can be tuned and its contrast enhanced by the additional “knob” available, the relative phase between the two frequency components. The present results confirm that two-color ionization allows resolving interference structures originating from trajectories launched within a time interval of less than 100 as [X. Xie et al., Phys. Rev. Lett. 108, 193004 (2012)].

Published: 2014-04-16
Physical Review A 89, 043414 (2014)
DOI: 10.1103/PhysRevA.89.043414
(--> also listed in Project 03)


Time-resolved photoemission using attosecond streaking

S. Nagele, R. Pazourek, M. Wais, G. Wachter, J. Burgdörfer

  We theoretically study time-resolved photoemission in atoms as probed by attosecond streaking. We review recent advances in the study of the photoelectric effect in the time domain and show that the experimentally accessible time shifts can be decomposed into distinct contributions that stem from the field-free photoionization process itself and from probe-field induced corrections. We perform accurate quantum-mechanical as well as classical simulations of attosecond streaking for effective one-electron systems and determine all relevant contributions to the time delay with attosecond precision. In particular, we investigate the properties and limitations of attosecond streaking for the transition from short-ranged potentials (photodetachment) to long-ranged Coulomb potentials (photoionization). As an example for a more complex system, we study time-resolved photoionization for endohedral fullerenes A@C60 and discuss how streaking time shifts are modified due to the interaction of the C60 cage with the probing infrared streaking field.

Published: 2014-04-10
Journal of Physics: Conference Series 488, 012004 (2014)
DOI: 10.1088/1742-6596/488/1/012004


Electron rescattering at metal nanotips induced by ultrashort laser pulses

G. Wachter, C. Lemell, J. Burgdörfer

  We theoretically investigate the interaction of moderate intensity near-infrared few cycle laser pulses with nano-scale metal tips. Local eld enhancement in a nanometric region around the tip apex triggers coherent electron emission on the nanometer length and femtosecond time scale. The quantum dynamics at the surface are simulated with timedependent density functional theory (TDDFT) and interpreted based on the simple man's model. We investigate the dependence of the emitted electron spectra on the laser wavelength.

Published: 2014-04-10
Journal of Physics: Conference Series 488, 012005 (2014)
DOI: 10.1088/1742-6596/488/1/012005


Time delays for attosecond streaking in photoionization of neon

J. Feist, O. Zatsarinny, S. Nagele, R. Pazourek, J. Burgdörfer, X. Guan, K. Bartschat, B.I. Schneider

  We revisit the time-resolved photoemission in neon atoms as probed by attosecond streaking. We calculate streaking time shifts for the emission of 2p and 2s electrons and compare the relative delay as measured in a recent experiment by Schultze et al. [Science 328, 1658 (2010)]. The B-spline R-matrix method is employed to calculate accurate Eisenbud-Wigner-Smith time delays from multielectron dipole transition matrix elements for photoionization. The additional laser field-induced time shifts in the exit channel are obtained from separate, time-dependent simulations of a full streaking process by solving the time-dependent Schr¨odinger equation on the single-active-electron level. The resulting accurate total relative streaking time shifts between 2s and 2p emission lie well below the experimental data. We identify the presence of unresolved shake-up satellites in the experiment as a potential source of error in the determination of streaking time shifts.

Published: 2014-03-14
Physical Review A 89, 033417 (2014)
DOI: 10.1103/PhysRevA.89.033417


What will it take to observe processes in real time?

S.R. Leone, C.W. McCurdy, J. Burgdörfer, L.S. Cederbaum, Z. Chang, N. Dudovich, J. Feist, C.H. Greene, M. Ivanov, R. Kienberger, U. Keller, M.F. Kling, Z.-H. Loh, T. Pfeifer, A.N. Pfeiffer, R. Santra, K. Schafer, A. Stolow, U. Thumm, M.J.J. Vrakking

  Even for simple systems, the interpretations of new attosecond measurements are complicated and provide only a glimpse of their potential. Nonetheless, the lasting impact will be the revelation of how short-time dynamics can determine the electronic properties of more complex systems.

Published: 2014-03
Nature Photonics 8 (2014)
DOI: 10.1038/nphoton.2014.48


Characterizing high-n quasi-one-dimensional strontium Rydberg atoms

M. Hiller, S. Yoshida, J. Burgdörfer, S. Ye, X. Zhang, F.B. Dunning

  The production of high-n, n∼300, quasi-one-dimensional (quasi-1D) strontium Rydberg atoms through two-photon excitation of selected extreme Stark states in the presence of a weak dc field is examined using a crossed laser-atom beam geometry. The dipolar polarization of the electron wave function in the product states is probed using two independent techniques. The experimental data are analyzed with a classical trajectory Monte Carlo simulation employing initial ensembles that are obtained with the aid of quantum calculations based on a two-active-electron model. Comparisons between theory and experiment highlight different characteristics of the product quasi-1D states, in particular, their large permanent dipole moments, ∼1.0 to 1.2n2ea0, where e is the electronic charge and a0 is the Bohr radius. Such states can be engineered using pulsed electric fields to create a wide variety of target states.
Published: 2014-02-21
Physical Review A 89, 023426 (2014)
DOI: 10.1103/PhysRevA.89.023426

Probing the influence of the Coulomb field on atomic ionization by sculpted two-color laser fields

X.H. Xie, S. Roither, S. Gräfe, D. Kartashov, E. Persson, C. Lemell, L. Zhang, M. S. Schöffler, A. Baltuška, J. Burgdörfer, M. Kitzler

  Interpretation of electron or photon spectra obtained with strong laser pulses that may carry attosecond dynamical and Ångström structural information about atoms or molecules usually relies on variants of the strongfield approximation (SFA) within which the influence of the Coulomb potential on the electron trajectory is neglected.We employ two-color sculpted laser fields to experimentally tune and probe the influence of the Coulomb field on the strong-field-driven wavepacket as observed by two-dimensional electron and ion momentum spectra. By comparison of measured spectra with predictions of the three-dimensional time-dependent Schrödinger equation as well as the quasiclassical limit of the SFA, the strong-field classical trajectory model, we are able to trace back the influence of the Coulomb field to the timing of the wavepacket release with sub-cycle precision.

Published: 2013-04-30
New Journal of Physics 15, 043050 (2013)
DOI: 10.1088/1367-2630/15/4/043050
(--> also listed in Project 03)


Production of very-high-n strontium Rydberg atoms

S. Ye, X. Zhang, T.C. Killian, F.B. Dunning, M. Hiller, S. Yoshida, S. Nagele, J. Burgdörfer

  The production of very-high-n (n∼300–500) strontium Rydberg atoms is explored using a crossed-laseratom-beam geometry. n1S0 and n1D2 states are created by two-photon excitation via the 5s5p 1P1 intermediate state using radiation with wavelengths of ∼461 and ∼413 nm. Rydberg atom densities as high as ∼3 × 105 cm−3 have been achieved, sufficient that Rydberg-Rydberg interactions can become important. The isotope shifts in the Rydberg series limits are determined by tuning the 461-nm light to preferentially excite the different strontium isotopes. Photoexcitation in the presence of an applied electric field is examined. The initially quadratic Stark shift of the n1P1 and n1D2 states becomes near-linear at higher fields and the possible use of n1D2 states to create strongly polarized, quasi-one-dimensional electronic states in strontium is discussed. The data are analyzed with the aid of a two-active-electron (TAE) approximation. The two-electron Hamiltonian, within which the Sr2+ core is represented by a semi-empirical potential, is numerically diagonalized allowing the calculation of the energies of high-n Rydberg states and their photoexcitation probabilities.

Published: 2013-10-28
Physical Review A 88, 043430 (2013)
DOI: 10.1103/PhysRevA.88.043430


Photoionization of helium by attosecond pulses: Extraction of spectra from correlated wave functions

L. Argenti, R. Pazourek, J. Feist, S. Nagele, M. Liertzer, E. Persson, J. Burgdörfer, E. Lindroth

  We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wave packet resulting from the integration of the time-dependent Schr¨odinger equation in a finite-element discrete variable representation basis. These techniques are projection on products of hydrogenic bound and continuum states, projection onto multichannel scattering states computed in a B-spline close-coupling basis, and a technique based on exterior complex scaling implemented in the same basis used for the time propagation. These methods allow one to monitor the population of continuum states in wave packets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wave packets for two-photon double ionization.

Published: 2013-05-13
Pysical Review A 87, 053405 (2013)
DOI: 10.1103/PhysRevA.87.053405


Time-resolved photoemission on the attosecond scale: opportunities and challenges

R. Pazourek, S. Nagele, J. Burgdörfer


The interaction of laser pulses of sub-femtosecond duration with matter opened up the opportunity to explore electronic processes on their natural time scale. One central conceptual question posed by the observation of photoemission in real time is whether the ejection of the photoelectron wavepacket occurs instantaneously, or whether the response time to photoabsorption is finite leading to a time delay in photoemission. Recent experimental progress exploring attosecond streaking and RABBIT techniques find relative time delays between the photoemission from different atomic substates to be of the order of ∼20 attoseconds. We present ab initio simulations for both one- and two-electron systems which allow the determination of both absolute and relative time delays with ∼1 attosecond precision. We show that the intrinsic time shift of the photoionization process encoded in the Eisenbud–Wigner–Smith delay time can be unambiguously disentangled from measurement-induced time delays in a pump-probe setting when the photoionized electronic wavepacket is probed by a modestly strong infrared streaking field. We identify distinct contributions due to initial-state polarization, Coulomb-laser coupling in the final continuum state as well as final-state interaction with the entangled residual ionic state. Extensions to multi-electron systems and to the extraction of time information in the presence of decohering processes are discussed.


Published: 2013-04-04
Faraday Discussions 163 (2013)
DOI: 10.1039/c3fd00004d


Classical-quantum correspondence in atomic ionization by midinfrared pulses: Multiple peak and interference structures

C. Lemell, J. Burgdörfer, S. Gräfe, K.I. Dimitriou, D.G. Arbo, X.-M. Tong

  Atomic ionization by strong and ultrashort laser pulses with frequencies in the midinfrared spectral region have revealed novel features such as the low-energy structures. We have performed fully three-dimensional quantum dynamical as well as classical trajectory Monte Carlo simulations for pulses with wavelengths from λ = 2000 to 6000 nm. Furthermore, we apply distorted-wave quantum approximations. This allows to explore the quantum-classical correspondence as well as the (non) perturbative character of the ionization dynamics driven by long-wavelength pulses. We observe surprisingly rich structures in the differential energy and angular momentum distribution which sensitively depend on λ, the pulse duration τp, and the carrier-envelope phase ΦCEP.

Published: 2013-01-22
Physical Review A 87, 013421 (2013)
DOI: 10.1103/PhysRevA.87.013421







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