QUTIF Workshop 2018

14 Feb 2018, 12:00 → 16 Feb 2018, 14:00 Europe/Berlin

CFEL (blgd. 99) SR 1–3 (Center for Free-Electron Laser Science, DESY and Universität Hamburg)

Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg), Manfred Lein (Leibniz Universität Hannover)

Annual Meeting of the DFG priority program Quantum Dynamics in Tailored Intense Fields (QUTIF) at the Center for Free-Electron Laser Science (CFEL), DESY and Universität Hamburg, Hamburg, Germany.

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Please send your poster/talk information (abstract, max. ~250 words) by email: qutif2018@cfel.de

Accommodation will be reserved for you if you request it in the registration form. In this case you do not have to book a hotel. For QUTIF members and invited speakers, the cost of meals and housing will be covered by QUTIF central funds.

Scientific organizers of the workshop are Jochen Küpper and Manfred Lein. Questions regarding the organization of the meeting should be directed to the organizers at  qutif2018@cfel.de.

With best regards, the Coordination Team

Wednesday, 14 February

12:00 → 12:50 Registration: (snacks and refreshments served)

Convener: Barbora Vagovic (FS-CFEL-1)

12:50 → 13:00 Welcome

Conveners: Prof. Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg), Prof. Manfred Lein (Leibniz Universität Hannover)

13:00 → 14:20 Chirality 1

Convener: Prof. Jamal Berakdar (Martin-Luther-Universität Halle)

13:00 Twists and turns, ups and downs of a walk through chiral photoionization

The study of photoelectron circular dichroism (PECD) per se offers unique insights into fundamental molecular photoionization dynamics spanning the full range of physical conditions. Equally, PECD has unique capabilities as a technique for studying structure, interactions, and by extension chemical and biological properties in chiral molecular systems. Due to homochirality these are topics of ever increasing interest. But while some such applications may be developed in an empirical manner, eg by calibrating against known standards, it will be shown that fully and securely exploiting the sensitivity of PECD to molecular structure, will continue to place demands on understanding and modelling the fundamental physics that underpin PECD phenomena.

Speaker: Ivan Powis (University of Nottingham)

13:40 Control of spatially resolved spin polarization of photoelectrons produced from nitric oxide

We theoretically study the electron dynamics of \pi_\pm orbitals of nitric oxide (NO) driven by strong elliptically polarized (EP) laser pulses. Our theoretical analysis shows that the interactions of \pi_+ and \pi_- orbitals with strong EP pulses will lead to the tilted polarization of the orbital with respect to the major axis of the EP field. As a result, the most probable ionization times of \pi_+ and \pi_- orbitals are after and before and the field maximum, respectively. According to the attoclock configuration, the spin-up and spin-down photoelectrons from \pi_- and \pi_+ orbitals, respectively, will be spatially separated due to different ionization times

Speaker: Kunlong Liu (Max Planck Institute of Microstructure Physics, Halle (Saale))

14:00 Interaction of vortex fields with atoms

The interaction of spatially tailored laser pulses (such as photonic vortices) with matter is receiving an increasing attention recently. This is mainly due to the possibility of imparting to matter a well defined amount of orbital angular momenta (OAM) which is associated with winding number of the optical vortex [1]. This renders possible non-dipolar transitions or transitions with well defined polarity [2], and provides a new tool to control the spin in spin-orbital coupled systems [3]. Furthermore, appropriately tailored optical vortices result in strong mechanical forces [4]. In this contribution we will consider low-energy electron emission from atoms via vortex beams, particularly Laguerre-Gaussian beams. The low energy electron emission probabilities of atoms in vortex fields are calculated as a function of the OAM of the laser and the position of the target inside the beam. A modified threshold behavior of the corresponding ionization cross sections is found. We will also present and discuss the low-energy photoelectron angular distributions and contrast with recent experimental measurements [5]. [1] L. Allen et al, Phys. Rev. A 45, 8185 (1992) [2] J. Wätzel and J. Berakdar, Phys. Rev. A 94, 033414 (2016) [3] K. Köksal and J. Berakdar, Phys. Rev. A 86, 63812 (2012) [4] D. Schulze et al, Ann. Phys. 529, 1600379 (2017) [5] T. Kaneyasu et al, Phys. Rev. A 95, 023413 (2017)

Speaker: Carlos Granados (Martin-Luther Universität Halle-Wittenberg)

14:20 → 15:10 Coffee break

14:20 → 14:40 Young Scientists Meeting

Convener: Nicolas Eicke (Leibniz Universität Hannover)

14:40 → 15:10 Poster setup: (mount posters)

15:10 → 17:00 Attosecond 1

Convener: Dr Arnaud Rouzée

15:10 Attosecond reversible and irreversible electron dynamics in strong optical fields

Attosecond pump-probe spectroscopy has opened up the possibility to study light-matter interaction with unprecedented time resolution. In this talk, I will present the results of our experiment-theory collaboration, where we apply attosecond transient absorption spectroscopy to follow the sub-laser-cycle electron dynamics of atomic xenon during strong-field tunnel ionization. The electron dynamics of ‘fuzzy’ xenon is found to exhibit two opposite types of electron motion induced by an intense, nearinfrared laser: one corresponds to an irreversible, monotonic ion build-up process as predicted by widely used tunnel ionization models; another one is associated with a reversible, periodic electron displacement due to transient ground-state polarization. Albeit well known in the weak-field regime, the role of polarization has so far been largely unnoticed, both experimentally and theoretically, in the strong-field regime. Such a dichotomy of electron dynamics in strong optical fields is expected to have an even larger impact on the strong-field-driven behavior of spatially extended, complex physical systems.

Speaker: Yi-Jen Chen (CFEL, DESY)

15:40 Laser induced inter-site spin transfer

Laser pulses induce spin-selective charge flow that we show generates dramatic changes in the magnetic structure of materials, including a switching of magnetic order from anti-ferromagnetic (AFM) to transient ferromagnetic (FM) in multi-sub-lattice systems. The microscopic mechanism underpinning this ultra-fast switching of magnetic order is dominated by spin-selective charge transfer from one magnetic sub-lattice to another. As this spin modulation is purely optical in nature (i.e. not mediated indirectly via the spin-orbit interaction) this is one of the fastest means of manipulating spin by light. We further demonstrate this mechanism to be universally applicable to AFM, FM and ferri-magnets, in both multilayer and bulk geometry, and provide three rules that encapsulate early time magnetization dynamics of multi-sub-lattice systems.

Speaker: Sangeeta SHARMA (Max Planck Institute of Microstructure Physics, Halle (Saale))

16:00 Strong-field ionization of laser-aligned molecules

The interaction of strong laser fields with matter intrinsically enables the imaging of transient dynamics with extremely high spatiotemporal resolution. This paradigm of photophysics has grown into new emerging research areas, ranging from attosecond science to laser-induced electron di diffraction, providing new insight into atoms, molecules and, more recently, condensed matter. Also, the earliest moments of strong-field interactions have attracted attention for capturing the intrinsic nature of strong-field physics. While pioneering attosecond science experiments and molecular-frame measurements revealed non-trivial spatiotemporal features in electron tunneling, these initial conditions are generally considered a weak perturbation. We investigated strong- field ionization in the molecular frame. Carbonyl sulfide (OCS) molecules were quantum- state selected, strongly laser aligned, and ionized using short near-infrared laser pulses. We analyzed the dynamics of the electrons and discuss the obtained molecular-frame photoelectron-angular distributions. Our findings have strong impact in the interpretation of laser induced electron diffraction, where the photoelectron momentum distribution is used to retrieve molecular structures. Furthermore, the encoding of the time-energy relation in the photoelectron momenta provides new ways of probing electron tunneling and the molecular potential with sub-femtosecond resolution.

Speaker: Andrea Trabattoni (CFEL, DESY Hamburg)

16:20 Control of attosecond light polarization in two-color bicircular fields

Circular or highly elliptical femtosecond and attosecond pulses in the extreme ultraviolet spectral range present numerous applications in chiral-sensitive light-matter interactions1,2. Until recently, such radiation has only been available at large-scale facilities, where the time resolution is above 100 femtoseconds. Generation of coherent light sources with attosecond duration and controllable ellipticity will enable complementary studies on ultrafast time-scales. An elegant approach to the generation of such pulses consists of combining a circularly polarized fundamental field with a counter-rotating second harmonic3. This scheme leads to harmonic peaks at (3N+1) and (3N+2) lines, with the helicity of the fundamental field and the second harmonic, respectively, while 3N harmonics are absent due to symmetry. Recent theoretical studies4,5 have shown that when neon is ionized by such bicircular fields, a considerable amount of suppression of the (3N+2)-lines is observed in the high harmonic spectrum. The reason behind this effect is still not well understood and thus, while these works opened the way to the generation of elliptically polarized attosecond pulses, control over such ellipticity is still to be achieved. In this work, we provide an in-depth analytical analysis of the high harmonic generation process in two-color counter-rotating circular fields6,7. We do so using an analytical model based on the strong field approximation and by solving the time-dependent Schrödinger equation in the single-active electron approximation for both helium and neon, comparing our predictions to experiment. In particular, we derive three propensity rules which are responsible for the suppression of the (3N+2)-lines in the high harmonic spectrum of neon. By changing the relative intensity between the two fields, we show that we can coherently control the ellipticity of the generated attosecond bursts. [1] C. Lux et al., Angew. Chem. Int. Ed. , 51: 5001-5005 (2012) [2] R. Cireasa et al., Nat. Phys. 11 654-658 (2015) [3] D.B. Milosevic et al., Phys. Rev. A 61 063403 (2000) [4] L. Medisauskas et al., Phys. Rev. Lett. 115 153001 (2015) [5] D.B. Milosevic, Opt. Lett. 40 2381 (2015) [6] E. Pisanty and Á. Jiménez-Galán, Phys. Rev. A 96 063401 (2016) [7] Á. Jiménez-Galán et al., Phys. Rev. A (accepted)

Speaker: Álvaro Jiménez-Galán (Max Born Institut, Berlin)

16:40 Analysis and control of attosecond electron dynamics with near-fields

When compared to atoms and solids, the collective and correlated electron dynamics in finite systems under intense light fields can be substantially modified, enhanced and controlled by plasmonic near-fields [1]. Localized near-fields result from electronic polarization and charge separation, may unfold on femtosecond or even attosecond time scales, and can have various implications on the strong-field physics of nanostructures, nanoparticles, and clusters [2]. Examples include waveform-controlled electron acceleration through resonant plasmonic field amplification [3], directionally controlled surface backscattering via field propagation effects [4], or near-field induced attosecond streaking [5]. In this talk, three new aspects of controlling near-field induced strong-field dynamics with bichromatic laser fields will be discussed. The first scenario addresses the plasmon enhanced forward acceleration in metal clusters [3]. The second case is the directional control achieved through spectrally selective field amplification at isolated nanospheres. The third scenario concerns two-color control of electron backscattering from metallic nanotips. References: [1] T. Fennel et al., Rev. Mod. Phys. 82, 1793 (2010) [2] M. F. Ciappina et al., Rep. Prog. Phys. 80, 054401 (2017) [3] J. Passig et al., Nat. Commun. 8, 1181 (2017) [4] F. Süßmann et al., Nat. Commun. 6, 7944 (2015) [5] Seiffert et al., Nat. Phys. 13, 766 (2017) Email of corresponding author: fennel@mbi-berlin.de / thomas.fennel@uni-rostock.de

Speaker: Thomas Fennel (Max-Born-Institute, Berlin and University of Rostock)

17:00 → 19:00 Poster session 1: (snacks and refreshments served)

Conveners: Prof. Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg), Prof. Manfred Lein (Leibniz Universität Hannover)

17:00 A non-linear mapping from photo-electron spectra to pulse shape

Strong field quantum dynamics are very sensitive to the shape of the interacting field. Finding a suitable pulse shape to reach a predefined target lies in the heart of quantum control. The forward non-linear mapping between the interacting pulse and the objective (photo-electron spectra(PES)) can be achieved by solving the time-dependent Schrödinger equation. However, the non-linear inverse mapping i.e. a mapping from the PES to the pulse shape is not straightforward. In this work, we have explored the non-linear inverse mapping using the artificial neural network. As a test system, we have studied quasi-resonant two-photon ionization of a helium atom for the interaction with different pulse shapes of the same energy and same frequency content.

Speaker: Sajal Kumar Giri (Max Planck Institute for the Physics of Complex Systems, Dresden)

17:00 Attosecond delays in the photoemission from the layered, centrosymmetric Bi2Te3 and non-centrosymmetric BiTeCl crystals

Attosecond time-resolved photoemission based on the photoelectron streaking in a time-correlated strong IR field allows investigating temporal delays in the photoemission from different initial states with unprecedented resolution [1]. The physical origin of these delays is not yet fully understood and various theoretical models coexist demonstrating our still limited understanding of the fundamentals of the photoemission process. Here we report on attosecond time-resolved photoemission from the layered crystals Bi2Te3 and non-centrosymmetric BiTeCl. For the latter the lack of inversion symmetry allows studying relative photoemission delays for differently terminated but well-defined and inert surfaces. In addition, photoelectron propagation effects such as the inelastic mean free path can be determined experimentally because of the reversed layer stacking for differently terminated surfaces. This reduces the ambiguities for classical electron trajectory calculations performed to model the observed relative photoemission delays. However, electron propagation alone cannot explain the measured relative delays. Taking into account local atomic effects and many body corrections reduce the discrepancy between experimental observations and theoretical predictions but still yield no satisfactory explanation. This shows that additional mechanism in the photoemission process significantly influence the photoelectron dynamic and hence a refined model of photoemission is needed. [1] A. L. Cavalieri, et al., Nature 449, 1029 (2007)

Speaker: Sergej Neb (University of Bielefeld)

17:00 Attosecond streaking with twisted X waves

Attosecond streaking is an established technique to measure timing information in the interaction of ultrashort laser pulses with atoms or molecules. This technique is based on the photoionization by an attosecond laser pulse in the presence of a strong linearly polarized near infrared (NIR) laser pulse. We investigate the attosecond streaking with an X wave pulse carrying orbital angular momentum and a strong linearly polarized near infrared (NIR) laser pulse. In contrast to plane wave pulses, X waves have a spatially dependent temporal profile, which modifies the ionization process. In this contribution we theoretically explore the influence of this complex pulse structure on the streaking of photoelectrons for both localized and macroscopically extended targets. On the basis of the strong-field approximation (SFA), we find that the streaking spectra of localized targets sensitively depend on the opening angle of the X wave and the position of the atomic target relative to the beam axis. For macroscopically extended targets, we find that the streaking spectra do not depend on the parameters characterizing the twist of the X wave.

Speaker: Birger Böning (Friedrich-Schiller-Universität Jena)

17:00 Attosecond timing with spectral resolution near resonances

To directly observe the ultrafast motion of electrons in atomic or molecular systems is an aspect of fundamental physics and is achievable thanks to the generation of attosecond pulses in the XUV spectral range. A key advantage of attosecond pulse trains over isolated attosecond pulses is their high spectral resolution, while high temporal resolution is still retained [1]. I will introduce attosecond measurements using attosecond pulse trains near resonances and discuss the role of continuum-continuum transitions in attosecond time delay measurement within a perturbative approach. In the future, these experiments will benefit substantially by the advent of high-repetition rate attosecond experiments based on optical parametric amplifier systems driving high-order harmonic generation [2]. [1] Isinger et al. Science 358, 893 (2017) [2] Harth et al. Journal of Optics 20, 014007 (2018)

Speaker: Anne Harth (Max-Planck-Institut für Kernphysik, Heidelberg)

17:00 Charge migration in propiolic acid and its dephasing by the coupling to the nuclear motion

A particular charge migration dynamics, driven only by the electron correlation, occurs in the propiolic acid molecule after the ionization of its HOMO orbital. This dynamics consist of an oscillation of the charge between its carbon triple bond and its carbonyl oxygen with a period of 6.2 fs. Performing fully quantum coupled electron-nuclear dynamics calculation, taking into account all 26 valence electrons and all 15 nuclear degrees of freedom, we show that the charge migration survives the decoherence induce by the nuclear motion long enough to be observed and controlled.

Speaker: Victor Despré (Universität Heidelberg)

17:00 Coherent control of photoemission from nanostructures with synthesized two-color fields - an update

We demonstrated two-color coherent control of multi-photon photoemission from a tungsten nanotip with the optical phase between two light fields [1] and now continue to investigate the coherent nature of two-color above-threshold photoemission from nanotip sources. As discussed in [1], by focusing 74 fs drive pulses at 1560 nm and their second harmonic at 780 nm onto the tip and changing the optical phase between the two colors, we observed an emission current modulation of up to 97.5 % due to interference between two different quantum channels in the material. We argued that the extremely high degree of coherence evidenced by this near-unity current modulation depth is due to the confining nature of local field enhancement at the nanotip. In addition, the pointy solid-state nature of the nanotip enables us to apply large DC fields, offering an additional degree of freedom to investigate the modulation contrast of the photoemitted electron yield [2]. As an experimental outlook an in-situ resharpening technique will be presented which produces single nanometer sized tips. This technique can be used to adjust apex radius and opening angle of the nanotip, which in turn strongly influences the local near-field distribution [3]. Furthermore, a highly nonlinear fiber (HNF) will be implemented to broaden the output spectrum of the fiber laser, achieving laser pulses with nearly single-cycle duration. Ultimately, both approaches aim to reach into the strong-field regime and observe field-driven two-color electron emission dynamics in future studies. [1] M. Förster et al., Phys. Rev. Lett., 117, 217601 (2016). [2] T. Paschen et al., J. Mod. Opt. 64, 10-11, 1054-1060 (2017). [3] S. Thomas et al., New J. Phys., 17, 063010 (2015). [4] M. Krüger et al., J. Phys. B., 47, 124022 (2014).

Speaker: Timo Paschen (Friedrich-Alexander-Universität Erlangen-Nürnberg)

17:00 Controlling the directionality of photoemission from M- vs. N-photon ionization with CEP stable polarization-tailored bichromatic fields

Polarization-tailored bichromatic laser fields with commensurable center frequencies have emerged as a new twist to steer ultrafast electron dynamics. Recently, we introduced a novel approach to the generation of polarization-tailored bichromatic fields, based on ultrafast pulse shaping of an octave-spanning Carrier Envelope Phase (CEP)-stable white light supercontinuum [1-2]. The shaper-based approach allows us to combine control strategies based on bichromatic multipath interference with CEP-sensitive excitation and, in addition, to utilize the full repertoire of femtosecond pulse shaping. In this contribution, bichromatic pulse shaping is applied to study CEP-sensitive lateral asymmetries in the photoelectron angular distribution from 7- vs. 8-photon ionization of Xenon atoms. The physical mechanism is discussed in terms of the interference of states with opposite parity, addressed by the corresponding quantum pathways. In addition to the CEP studies, we vary the polarization state of both colors from linear to circular, generating a CEP-sensitive 3D-photoelectron wave packets with 1-arm-vortex shape. The 3D photoelectron wave packets is detected using a velocity map imaging spectrometer in combination with tomographic reconstruction techniques. [1] S. Kerbstadt, L. Englert, T. Bayer and M. Wollenhaupt, J. Mod. Opt. 64 (2017) 1010. [2] S. Kerbstadt, D. Timmer, L. Englert, T. Bayer and M. Wollenhaupt, Opt. Expr. 25 (2017) 12518.

Speaker: Stefanie Kerbstadt (Carl von Ossietzky Universität Oldenburg, Institut für Physik)

17:00 Effects of the Coulomb potential in strong-field holography with photoelectrons

Strong-field photoelectron holography (SFPH) is a new method for time-resolved molecular imaging [1]. The SFPH technique is based on the interference between rescattered electrons that are driven back to their parent ions and rescatter on them, and direct electrons that do not recollide with the ions. The semiclassical three-step model [2] accounting only for the laser field predicts four different types of subcycle interference structure [3]. Despite the appealing physical picture of the SFPH provided by the three-step model, neglecting the Coulomb potential may be severe. We investigate modification of the interference structures emerging in the SFPH due to the Coulomb potential of the atomic core using the semiclassical two-step model for strong-field ionization [4]. For each kind of interference pattern predicted by the three-step model we calculate the corresponding structure emerging in the presence of the Coulomb potential. We show that the Coulomb potential can manifest itself in three main effects. These are: the shift of the interference pattern as a whole, the filling of the parts of the interference structure that are missing when the Coulomb potential is neglected, and the characteristic kink of the interference stripes at zero momentum. [1] Y. Huismans et al., Science 331, 61 (2011). [2] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993). [3] X.-B. Bian et al., Phys. Rev. A 84, 043420 (2011). [4] N. I. Shvetsov-Shilovski et al., Phys. Rev. A 94, 013415 (2016).

Speaker: Nikolay SHVETSOV-SHILOVSKIY (Institute for Theoretical Physics, Leibniz University Hannover)

17:00 Efficient time evolution of thermal ensembles by Hilbert space sampling

Coherent control of bond making in chemical reactions has recently been successfully demonstrated under thermal conditions. In this experiment, pairs of magnesium atoms were made to collide at a temperature of around 1000 K and were subsequently excited into electronically excited states by a femtosecond laser pulse. The resulting UV emission signal proved to be a clear indicator of the formation of magnesium dimers. The current theoretical framework to describe this experiment models the thermal ensemble of magnesium atoms via random phase wavefunctions. Although this proves to yield significantly reduced numerical effort compared to a full description of the thermal density matrix, time propagation still proves to be the crucial computational bottleneck. We aim to alleviate this bottleneck by investigating the possibility to sample the Hilbert space not randomly but systematically via eigenstates of the Hamiltonian. It turns out that using this eigenstate-based sampling minimizes the worst-case error among arbitrary observables if no a priori information on the dynamics is available. We systematically benchmark the computational effort to obtain accurate estimations on observables both via random-phase sampling and via eigenstate-based sampling.

Speaker: Marec Heger (Universität Kassel)

17:00 Field-free Alignment of Asymmetric-Top Molecules

Fixing molecules in space enables a variety of investigations into molecular structure and dynamics, such as high harmonic spectroscopy or ultrafast diffractive imaging of molecular dynamics. Molecules can be aligned and oriented to the laboratory frame using suitable ac and dc electric fields [1,2]. State-selection can lead to a significant enhancement of the degrees of alignment and orientation [2,3]. We follow both, experimental and theoretical, paths to investigate techniques to improve 1D and 3D alignment and orientation. Experiments have resulted in both, in-field as well as laser-field-free, alignment and orientation of the linear carbonyl sulfide (OCS) molecule [4,5,6]. Many of these techniques are also applicable to more complex molecules and clusters [2,3,7]. Here, we discuss how laser-pulse shaping was utilized to show field-free 3D alignment of indole experimentally. More complex shaping, combined with learning-loop approaches, is planned to further enhance the degree of alignment and orientation with the goal of near-perfect 3D alignment and orientation of complex asymmetric top molecules. Theoretical simulations of the rotational behavior of such systems interacting with electric fields will be used to guide experiments. [1] Stapelfeldt and Seideman, Rev. Mod. Phys. 75, 543–557 (2003). [2] Chang, Horke, Trippel, and Küpper Int. Rev. Phys. Chem. 34, 557-590 (2015) [3] Holmegaard, Nielsen, Nevo, Stapelfeldt, Filsinger, Küpper, and Meijer PRL 102, 023001 (2009) [4] Trippel, Mullins, Müller, Kienitz, Omiste, Stapelfeldt, González-Férez, and Küpper Phys. Rev. A 89, 051401R (2014) [5] Trippel, Mullins, Müller, Kienitz, González-Férez, and Küpper Phys. Rev. Lett. 114, 103003 (2015) [6]Omair Ghafur, Arnaud Rouzée, Arjan Gijsbertsen, Wing Kiu Siu, Steven Stolte and Marc J. J. Vrakking Nature Physics 5, 289–293 (2009) [7] Kierspel et al. (30 authors) J. Phys. B 48, 204002 (2015)

Speaker: Terry Mullins (CFEL)

17:00 Free-Electron Quantum State Reconstruction by SQUIRRELS

Programmable phase-shaping of free-electron wavefunctions is expected to significantly enhance the capabilities of electron microscopy. In particular, temporal shaping by optical fields promises time-resolved electron diffraction and imaging with attosecond precision. Here, we demonstrate the generation, coherent control and characterization of free-electron momentum superposition states by optical phase-modulation using multiple near-fields [1-3]. Our recently developed “SQUIRRELS” algorithm (Spectral QUantum Interference for the Regularized Reconstruction of free-ELectron States) reconstructs the free-electron Wigner function from experimental spectrograms, which are obtained by recording electron energy spectra for a range of relative phase delays between two optical near-fields, either at different frequencies or at spatially separated regions along the electron beam path. Free-space propagation over a few millimetre distance leads to the formation of an attosecond electron pulse train by dispersive reshaping of the electron density [1]. The temporal shape of the electron quantum state is contained in the Wigner function, and we successfully applied SQUIRRELS to experimentally demonstrate the generation of electron density spikes of only 655 attosecond duration (full-width at half-maximum) [3]. [1] A. Feist et al., Nature 521, 200-203 (2015) [2] K. Echternkamp et al., Nature Physics 12, 1000-1004 (2016) [3] K. Priebe et al., Nature Photonics 11, 793-797 (2017)

Speaker: Katharina Priebe (4th Physical Institute – Solids and Nanostructures, University of Göttingen)

17:00 Imprints of the molecular electronic structure in the photoelectron spectra of strong-field ionized asymmetric triatomic model molecules

We examine the circular dichroism in the photoelectron momentum distribution of triatomic model systems ionized by strong-field ionization. We demonstrate how the symmetry and electronic structure of the system is imprinted into the photoelectron momentum distribution. We use classical trajectories to reveal the origin of the three-folded pattern in the photoelectron momentum distribution, and show how an asymmetric nuclear configuration of the triatomic system effects the CDAD.

Speaker: Matthias Paul (Institut für Physikalische Chemie, Universität Jena)

17:00 Interaction of light carrying orbital angular momentum with matter

Recent advances in generating structured light fields for a wide range of pulse parameters has led to important discoveries and fascinating applications ranging from optical tweezers for microscale objects to electronics, life sciences, quantum information or optical telecommunications. Especially light pulses carrying orbital angular momentum (OAM) enable photomechanics for moving, trapping and rotating nanostructured objects molecular matter. OAM laser beams may also result in a novel type of photovoltaic effects [1,4]. In this poster, which supplements the talk of C. Granados, we will present recent discoveries and studies of our research group. Our findings reveal the immense potential of optical vortices for applications in photovoltaics as well as laser-induced magnetism in nano-structures [2,3,5]. Even on the atomic scale the application of light carrying orbital angular momentum instead of conventional laser pulses changes has large consequences [6,7]. For instance, the change of the internal angular momentum state of an (photo)-ionized electron opens the door to the identification of its origin with respect to the energy, space and initial magnetic sublevel. In mesoscopic systems, the abovementioned photomechanic effect on the charge carriers gives access to a totally different concept of photovoltaics: The transferred orbital angular momenta are transformed in directed circulating currents which are fully controllable by the winding number of the applied optical vortex without increasing the light field intensity. These photo-induced current loops open the door to optomagnetism [2] and photogalvanic applications [1]. [1] J. Wätzel and J. Berakdar, Sci. Rep. 6, 21476 (2016). [2] J. Wätzel, A. Schäffer, Y. Pavlyukh and J. Berakdar, Carbon 99, 439 (2016). [3] J. Wätzel and J. Berakdar, Opt. Expr. 25, 27857 (2017). [4] J. Wätzel, I. Barth and J. Berakdar, J. Mod. Opt. 64, 10-11 (2017) [5] A. Schäffer, J. Wätzel and J. Berakdar, Spintronics IX, 9931 (2016) [6] J. Wätzel and J. Berakdar, Phys. Rev. A 94, 033414 (2016) [7] D. Schulze, A. Thakur, AS Moskalenko and J. Berakdar, Ann. Phys. 529, 5 (2017)

Speaker: Jonas Wätzel (Martin-Luther Universität Halle-Wittenberg)

17:00 Iterative Time Ordering for Optimal Control of Open Quantum Systems

An explicit time-dependence of the Hamiltonian, for example due to an external driving field, introduces an additional challenge for dynamical simulations. The most commonly used propagation approaches usually rely on dividing the overall propagation time into small steps, in which the time-dependence of the Hamiltonian is approximately constant and the time evolution operator becomes a matrix exponential. This inevitably introduces inaccuracies due to neglection of time ordering. In contrast, the iterative time ordering (ITO) approach al- lows to fully account for any explicit time-dependence of the Hamiltonian. It was originally constructed for numerically exact propagation in Hilbert space for state vectors. Here, we generalize it to density matrices and use the driven quantum harmonic oscillator for bench- marking. Furthermore we discuss the combination of this algorithm with quantum optimal control theory and apply it to a strongly driven superconducting circuit. Keywords: propagation scheme, optimal control, quantum dynamics

Speaker: Lutz Marder (Institut für Physik, Universität Kassel)

17:00 Non-adiabatic ponderomotive shift in strong-field photoemission from nanostructures

The interaction of light with nanostructures shows unique properties such as enhancement and confinement of the electric near field down to a few nanometer. In strong-field photoemission from nanotips, as a consequence of the latter, electrons can experience the decay of the electric field amplitude and therefore the ponderomotive potential already during the time scale of a laser cycle. We experimentally study strong-field electron emission from metal nanotips in mid-infrared few-cycle laser fields. We identify a low-energy peak in the kinetic energy spectrum and investigate its shift to higher energies with increasing laser intensities from 1.7 to 8.9 × 10^11 W/cm2. Comparison to a simple model and numerical simulations shows that the decay of the near field on a nanometer scale leads to a nonadiabatic transfer of the ponderomotive potential to the kinetic energy of emitted electrons which results in the observed shift of the peak. We derive an analytic expression for the nonadiabatic ponderomotive shift of the low-energy peak. After the previously found quenching of the quiver motion, this completes the understanding of the role of inhomogeneous fields in strong-field photoemission from nanostructures.

Speaker: Johannes Schötz (Max-Planck-Institute for Quantum Optics, Garching)

17:00 Noncollinear UV Pulse Generation

Pulses with durations short enough to probe the electronic timescale can be generated in the XUV and in the IR-VIS regime. At photon energies around 4-20 eV, pulses with such extremely short durations have not yet been demonstrated. Frequency conversion in a gas cell or filamentation yields comparatively short pulses, but the complex interplay of nonlinear light-matter interaction and significant linear dispersion has prevented to approach the femtosecond barrier, even when extremely short (<4 fs) driving pulses are used. For spectroscopic applications such as transient absorption spectroscopy, broadband and sub-femtosecond pulses in the deep-UV would be very useful, because they allow the direct probing of the bandgap in many materials. Laser pulses (centre wavelength 700 nm, pulse duration 5 fs) are focused into a generation medium (sapphire, MgO, fused silica, BK7 glass) with a beam waist of 85 μm and a crossing angle 2α = 1°. A spectrometer has been constructed with resolution in the emission angle φ behind the generation medium. Data stacks dependent on 3 parameters (wavelength λ, pulse delay τ, emission angle φ) are recorded. The cascaded processes of third harmonic generation (THG) and self-diffraction yield a multifaceted emission pattern in the deep-UV with pulse durations < 3 fs at selected emission angles. Hence it is demonstrated that the generation of short deep-UV pulses by THG can be improved by using a noncollinear geometry.

Speaker: Jan Reislöhner (Institute of Optics and Quantum Electronics, FSU Jena)

17:00 Optimization of field-free alignment of molecules for imaging experiments

Field-free alignment and orientation of molecules is an important prerequisite for many gas phase imaging experiments. It allows to access information directly in the molecular frame and extract observables such as internuclear distances, angles and the electronic density through a measurement of the photoelectron angular distribution. We present new experimental results and simulations on strongly field-free aligned molecules, ranging from linear to complex asymmetric rotor molecules. Additionally, photoelectron momentum distributions of strongly aligned OCS molecules, recorded experimentally at different angles between the laser polarization axis and the molecular axis will be presented. The photoelectron angular distributions display large modification with the molecular axis distribution, both at low and high kinetic energies, which encode the molecular structure and electronic density of the molecule. A discussion of the experimental results will be presented.

Speaker: Evangelos Karamatskos (CFEL, Universität Hamburg)

17:00 Single shot velocity map imaging of electrons from dopand-induced helium nanoplasmas in strong near-infrared laser pulses

A doped helium nanodroplet irradiated by intense near-infrared (NIR) laser pulses forms a highly ionized nanoplasma even at laser intensities where the helium is not directly ionized. The dopant atoms provide first seed electrons which start the electron impact ionization avalanche of the whole droplet. The dynamics of ignition and explosion of the nanoplasma depends not only on the number and the kind of dopants but also on the droplet size and laser intensity. We present single shot velocity map imaging (VMI) measurements of electrons produced by irradiation of pure and xenon doped helium nanodroplets with intense NIR femtosecond laser pulses at various laser intensities for different helium and dopant cluster sizes. The salient structures of the electron spectra are discussed and compared to molecular dynamics simulations. Additionally, ion time of flight (TOF) spectra are recorded in parallel to the electron VMI.

Speaker: Dominik Schomas (Albert-Ludwigs Universität, Freiburg)

17:00 Spatial separation of chiral molecules with electric fields

Chirality is now one of the top-class subjects in physical, chemical and biological academic research as well as industrial pharmacology. Accurate experimental characterisation of the enantiomeric excess and absolute handiness in mixtures of chiral molecules, efficient chiral purification and discrimination remain very challenging and highly demanding tasks for a broad scope of applications. A number of novel experiments have recently been developed for measuring the enantiomeric excess [1,2] and absolute handiness [3] as well as preparation of the enantiomerically enriched samples [4]. Here we propose a new robust scheme for enantiomeric enrichment in a cold beam of chiral molecules. First, we employ an off-resonant optical centrifuge pulse combined with a strong static dc-field to create a difference in the rotational state population distributions of two enantiomers. Molecules then continue into an electrostatic deflector, where different rotational states are spatially separated. Our realistic theoretical calculations for the propylene-oxide molecule predict up to 30% enantiomeric enrichment. Challenges and perspectives for future experimental realisation are discussed. [1] D. Patterson, M. Schnell, J. M. Doyle, Nature 497 (2013) 475–477. [2] M. H. M. Janssen, I. Powis, Phys. Chem. Chem. Phys. 16 (2014) 856–871. [3] M. Pitzer, M. Kunitski, A.S. Johnson, T. Jahnke, H. Sann, F. Sturm, L. Ph. H. Schmidt, H. Schmidt-Böcking, R. Dörner, J. Stohner, J. Kiedrowski, M. Reggelin, S. Marquardt, A. Schießer, R. Berger, M. S. Schöffler, Science 341 (2013) 1096. [4] C. Pérez, A. L. Steber, S. R. Domingos, A. Krin, D. Schmitz, M. Schnell, Angew. Chem. Int. Ed. 56 (2017) 12512–12517.

Speaker: Andrey Yachmenev (CFEL, DESY, CUI Hamburg)

17:00 Spatiotemporal filament characterization - Towards transient atom dynamics

Filaments are an interesting phenomenon caused by high-intense laser pulses in various media. If a femtosecond laser pulse exceeds a certain critical peak power, the laser beam will experience ongoing self-focusing due to the Kerr effect, causing an intensity-dependent nonlinear refractive index n2, as the self-focusing enhances itself. Eventually, the self-focusing beam exceeds the ionization threshold of the medium and plasma is generated. The plasma leads to a defocusing of the beam, so that the intensity of the beam is lowered until the ionization threshold is not exceeded anymore. With enough energy, the beam can self-focus and then be defocused by the plasma again in many more iterations. If the self-focusing and the defocusing balance each other, the beam stays focused over a distance much longer than the Rayleigh range with a clamping intensity of around 50 TW/cm2 and is called a filament. Because of its generation process the filament inherently shows strongly nonlinear spatial and temporal dynamics which lead to interesting effects like intensity spiking, where the clamping intensity is exceeded by far, or self-shortening of few-cycle laser pulses, in which the spectrum of the pulse broadens significantly with a nearly flat spectral phase. We present the first experimental approach to investigate the spatial and temporal dynamics of a filament simultaneously using the common d-scan pulse characterization technique and a 2D- interferometer. Later, this technique can also be applied to investigate transient atom dynamics in strong light fields.

Speaker: Christoph Jusko (Institut für Quantenoptik, Leibniz Universität Hannover)

17:00 Strong-field Ionization of HeH⁺

The helium hydride molecular ion, HeH+, is the simplest heteronuclear polar molecule and serves as a benchmark system for the investigation of multi-electron molecules and molecules with a permanent dipole. We specifically address the question: How does the permanent dipole of HeH+ affect the fragmentation dynamics in intense ultrashort laser pulses? We study the laser induced laser-induced fragmentation; including non-ionizing dissociation, single ionization and double ionization; of an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. These results are interpreted using reduced dimensionality solutions to the time-dependent Schrödinger equation and with simulations based on Dressed surface hopping.

Speaker: Florian Oppermann (Uni Hannover)

17:00 Strong-field polarization-state control of higher harmonics generated in crystalline solids

We analyse the polarization states of high-harmonics generated with strong elliptical laser pulses in silicon. We find that the polarization states are dependent both on crystal symmetries and nonperturbative dynamics. Circular harmonics can be generated with circular and non-circular drivers which could pave the way to relatively simple sources of circular XUV pulses.

Speaker: Nicolai Klemke (CFEL, DESY Hamburg)

17:00 Strong-field-induced dynamical rotation and ionization of HeH+

Strong-field-induced rotation and ionization of HeH+ are investigated. The nuclei are treated as classical trajectories moving on field-dressed potential energy surfaces, while single and double ionization are simulated as stochastical jumps to the ionic surfaces, using the internuclear-distance- and orientation-dependent static ionization rates obtained from many-electron weak-field-asymptotic theory (ME-WFAT). The laser-induced dynamical rotation is found to be crucial for the detected angular distribution of the nuclear fragments, with the orientation-dependent ionization rate taking a less important role. Interestingly, there is a critical internuclear distance where dynamical rotation is especially important. Temperature, isotope and pulse duration effects are investigated, and comparison with experiment is performed.

Speaker: Lun Yue (Friedrich Schiller Universität Jena)

17:00 Subcycle Interference upon Tunnel Ionization by Counterrotating Circularly Polarized Two-Color Laser Fields

We report on our results on studying single ionization of helium in counterrotating circular two-color (CRTC) laser fields (780 nm & 390 nm) with overall intensities of up to 8*10^14 W/cm^2. The three dimensional momenta of the fragments are recorded using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) as experimental technique. We present our new results regarding single photoelectron interferences that are investigated in three dimensional momentum space. Further we discuss new insights regarding the low energy structure of the photoelectron in CRTC laser fields.

Speaker: Sebastian Eckart (Goethe-Universität Frankfurt am Main)

17:00 Terahertz radiation out of an optical parametric oscillator

Ultrafast light sources with a wavelength around two micrometers are interesting for several experiments like high harmonic generation and investigation of Brunel harmonics. We want to present a doubly resonant optical parametric oscillator (DROPO) for intracavity experiments. Our system is pumped by a home built kerr lens mode locked Yb:YAG thin disk laser with a repetition rate of 34 MHz. The DROPO is operating in a bowtie configuration and uses a BBO as the nonlinear medium. The wavelength can be rapidly adjusted between the degeneracy point up to 1900 nm + 2300 nm by cavity length tuning alone. An additional focus point inside the cavity is suitable for an gallium phophide wafer in which we want to excitate Brunel harmonics with the signal and the pump wave.

Speaker: Christian Markus Dietrich (Institute of Quantum Optics, Leibniz Universität Hannover)

17:00 The bicircular attoclock

The attoclock technique [1] in which the emission time of a photoelectron is mapped to its detection angle is an important tool in strong-field ionization of atoms. However, the use of close-to-circularly polarized laser fields is inherent to the method and all conclusions drawn from the attoclock are stricly valid only in this kind of orthogonal tunneling geometry, which is characterized by the eletric field being perpendicular to its derivative. In this work we show how counter-rotating bicircular laser fields can be used in a new approach to attoclock experiments to probe the ionization process in parallel tunneling geometry, as it is the case for linear polarization. This is possible because the ratio of the two fields can be chosen in a way that the vector potential has aspects of the attoclock in the sense that time is mapped directly onto the photoelectron momentum distribution, but the shape of the electric field corresponds to three repitions of close-to-linearly polarized fields. We present photoelectron momentum distributions calculated via solutions of the time-dependent Schrödinger equation and investigate shifts in these distributions with respect to a possible delay in ionization time using a trajectory-free method developed recently. References: [1] P. Eckle et al, Nat. Phys. 4, 565–570 (2008)

Speaker: Nicolas Eicke (Universität Hannover)

17:00 The ionization times of harmonic emission from asymmetric molecules

We study the ionization times of electrons in high-order harmonic generation from oriented HeH+ through numerical solution of the time-dependent Schrödinger equation. With using orthogonally polarized two-color laser pulses composed of an intense fundamental field and a time-delayed weak second-harmonic field, we probe the sub-cycle electron dynamics as the polarization of the fundamental field is parallel or antiparallel to the internuclear vector pointing from the heavy nucleus to the light nucleus. Our results show that, for low harmonic orders, the ionization time of electrons in the parallel case is tens of attoseconds earlier than the antiparallel case, reflecting a subtle temporal difference between tunneling on the H side versus the He side. From this difference, we can retrieve effective orientation-dependent ionization potentials of the asymmetric system, the interpretation of which is unclear so far.

Speaker: Bing Zhang (Harbin Institute of Technology, China; Leibniz Universität Hannover)

17:00 Ultra long-range ab initio calculations

We propose a generalization of the Bloch state which involves an additional sum over a finer grid in reciprocal space around each k-point. This allows for ab-initio calculations of ultra long-range modulations in the density which may involve millions of unit cells but with an efficiency rivaling that of single unit cell. This is due to a new algorithm developed specifically for solving the particular eigenvalue problem that this ansatz requires. Thus physical effects on the micron length scale, which nevertheless depend on details of the electronic structure on nanometer length scales, can be computed exactly within density functional theory. As a specific example, we will apply our method to treat a tailored spatially extended field in a solid.

Speaker: Tristan Müller (MPI of Microstructure Physics, Halle)

Thursday, 15 February

09:00 → 10:40 Attosecond 2

Convener: Prof. Adrian Pfeiffer

09:00 Coherent Control and Multicolor Synthesis at FERMI

The generation of intense, multicolor fields in the extreme ultraviolet spectral range at Free Electron Lasers (FELs) opens new perspectives for the characterization and control of nonlinear processes in atoms and molecules. In particular, these sources give access to the high intensities required for the observation and investigation of nonlinear processes, and, using suitable delay lines, they can be used for the implementation of XUV-pump-XUV-probe experiments. The seeded FEL FERMI (Trieste, Italy) offers the possibility to synthetize multicolor coherent fields, whose amplitudes and relative phases can be independently controlled. Recently the first experiment demonstration of the coherent control of the photoionization process in neon atoms was reported [1]. In the temporal domain, the coherent superposition of two or more coherent harmonics leads to a complex temporal structure, whose characteristics depend on the relative phases between the harmonics. I will describe future perspectives on the synthesis and temporal characterization of multicolor fields in the XUV and X-ray spectral range using FELs. References [1] K. Prince et al Nature Phot. 10, 176-179 (2016).

Speaker: Giuseppe Sansone (Albert-Ludwigs-Universität Freiburg)

09:40 Signatures of attosecond-scale electron dynamics in low-frequency harmonics by photoionization of noble gases

Brunel radiation appears during ionization of electron and subsequent dynamics, and is independent on the return to the atomic core. Surprisingly, although the time scale of the ionization process is much faster than the period of the most intense Brunel harmonics, the signatures of the ionization dynamics can be still found in the Brunel harmonic spectrum.

Speaker: Ihar Babushkin (Leibniz Universität Hannover)

10:00 Coupled electron-nuclear dynmaics following ionization of propiolic acid

Due to the electron correlation, the removal of an electron from a molecular orbital can trigger ultrafast, pure electron dynamics. The created by the ionization hole charge can migrate throughout the molecule on a few-femtoseconds timescale even at frozen nuclei. The slower nuclear dynamics is expected to dephase at a later stage the pure electronic coherence and trap the charge. A full-dimensional quantum calculation of the concerted electron-nuclear dynamics following the outer-valence ionization of the propiolic acid molecule will be presented, showing that the charge created upon ionization of the HOMO will oscillate between the carbon triple bond and the carbonyl oxygen for more than 10 fs before getting trapped by the nuclear motion. Contrary to recently reported calculations showing an ultrafast dephasing of the charge migration by the nuclear motion, the present results suggest that longer-lived electron coherences can exist even in polyatomics.

Speaker: Alexander Kuleff (Heidelberg University)

10:20 Terahertz-induced ultrafast symmetry control in condensed matter: Water and Silicon

The combination of strong terahertz (THz) with tailored optical fields for spectroscopy of solids lies at the heart of SOLSTICE. New research avenues emerge for THz-induced symmetry control in condensed matter, as illustrated for liquid water and silicon in this talk. Water dynamics in the THz region is dominated by the intermolecular forces of the hydrogen-bond network, which is held responsible for many of water’s anomalous properties. While there is recent consensus that these dynamics consist of several relaxation mechanisms, their microscopic origin is still debated. Most commonly, they are associated with the rotation and/or libration of water molecules. Within a larger collaboration, we studied the molecular polarizability anisotropy of liquid water revealed by THz-induced transient orientation, that breaks the susceptibility’s isotropy [1]. The observed ultrafast orientation of water permits deeper insights into the transient structure of water. In silicon, we explore new opportunities of THz-dressing-based symmetry control manifesting in high-order harmonic generation from crystals [2]. Terahertz-dressing of crystals extends the toolbox for ultrafast information manipulation for petahertz electronics and, more generally, for controlling nonperturbative light-matter interactions. [1] P. Zalden, L. Song, X. Wu, H. Huang, F. Ahr, O. D. Mücke, J. Reichert, M. Thorwart, P. K. Mishra, R. Welsch, R. Santra, F. X. Kärtner, and C. Bressler, “Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation,” submitted (2017). [2] H. Huang, L. Song, N. Tancogne-Dejean, N. Klemke, A. Rubio, F. X. Kärtner, and O. D. Mücke, “High-Order Harmonic Generation from Solids Dressed by an Intense Terahertz Field,” submitted to CLEO 2018.

Speaker: Oliver Mücke (CFEL, DESY, Universität Hamburg)

10:40 → 11:10 Coffee break

11:10 → 13:00 Chirality 2

Convener: Prof. Sascha Schäfer (Universität Oldenburg)

11:10 Climbing the rotational ladder to chirality

Chirality is conventionally associated with a chemical or optical property of a molecule being in either of its two enantiomeric (mirror-image) forms. In a more general sense, chirality is determined by the time and space inversion (PT) symmetry of the system. Chiral molecules are ‘born’ to be so, owing to their quasi-rigid spatially enantiomorphic geometrical structures with high potential energy barriers between the enantiomers. That said, it is possible to induce and modulate chirality in statically non-chiral molecules. For example, by forcing a molecule to rotate coherently in one direction, i.e., to possess a well-defined helicity, we can create a chiral entity. Phosphine (PH3) is an excellent example: at high rotational excitation it forms well separated near degenerate rotational cluster states where the molecule undergoes stable rotation around one of its P-H bonds in a clockwise or anti-clockwise manner [1,2]. This is analogous to a system with static chirality: oppositely rotating forms are energetically indistinguishable from each other and are separated by a high (kinetic) energy barrier. We will present robust, quantum mechanical simulations of the experimental methods for creating rotational cluster states in PH3 [3] (e.g., using an optical centrifuge [4,5]), techniques for spatial separation [6] of the dynamically chiral enantiomers, as well as perspectives for detecting chirality using modern experiments [7,8,9]. [1] P. R. Bunker, P. Jensen, J. Mol. Spectrosc. 228, 640 (2004). [2] S. N. Yurchenko, W. Thiel, S. Patchkovskii, P. Jensen, Phys. Chem. Chem. Phys. 7, 573 (2005). [3] A. Owens, A. Yachmenev, J. Kupper, in preparation. [4] J. Karczemarek, J. Wright, P. Corkum, M. Ivanov, Phys. Rev. Lett. 82, 3420 (1999). [5] A. Korobenko, V. Milner, Phys. Rev. Lett. 116, 183001 (2016). [6] Y.-P. Chang, D. A. Horke, S. Trippel, J. Kupper, Int. Rev. Phys. Chem. 34, 557 (2015). [7] A. A. Lutman et al., Nat. Photon. 10, 468 (2016). [8] A. Yachmenev, S. N. Yurchenko, Phys. Rev. Lett. 117, 033001 (2016). [9] M. H. M. Janssen, I. Powis, Phys. Chem. Chem. Phys. 16, 856 (2014).

Speaker: Alec Owens (CFEL, Universität Hamburg, The Hamburg Centre for Ultrafast Imaging)

11:40 Electron vortices

Recently, the emergence of vortex structures in the momentum distribution of free electron wave packets from photoionization of atoms with sequences of two time-delayed counterrotating circularly polarized (CRCP) ultrashort laser pulses was predicted [1] and demonstrated experimentally [2]. Electron vortices arise from the superposition of two time-delayed free electron wave packets with different magnetic quantum numbers. In our experiment three-dimensional electron vortices are generated by multiphoton ionization of potassium atoms using CRCP femtosecond laser pulses from a polarization-shaped supercontinuum source [3] and reconstructed tomographically from velocity map imaging (VMI) measurements [4]. Absorption of another photon in the continuum changes the c6 azimuthal symmetry of the threshold vortex into c8 for above threshold ionization (ATI) [5]. Electron vortices from non-perturbative excitation show c4 azimuthal symmetry and a pi-phase jump in the polar direction. Currently, we study electron vortices generated by bichromatic polarization-shaped CEP-stable supercontinua [3]. [1] J. M. Ngoko Djiokap et al., Phys. Rev. Lett. 115, 113004 (2015). [2] D. Pengel et al., Phys. Rev. Lett. 118, 053003 (2017). [3] S. Kerbstadt et al., Opt. Express 25, 12518 (2017). [4] S. Kerbstadt et al., New J. Phys. 19, 103017 (2017). [5] D. Pengel et al., Phys. Rev. A 96, 043426 (2017).

Speaker: Matthias Wollenhaupt (Carl von Ossietzky Universität, Oldenburg)

12:00 Chiral dichroism in high-order harmonic generation

High-harmonic generation is a nonlinear process that converts intense infrared radiation into high-frequency light. It can be understood as a sequence of three steps: tunnel ionization, laser-driven acceleration of the electron in the continuum, and recombination with the core resulting in the emission of harmonic light. Since there is a well-defined relationship between the duration of the electron round-trip and the energy released during recombination, the harmonic spectrum provides snapshots of the dynamics in the ion. The recent application of elliptically polarized fields to the generation of high-order harmonics in chiral molecules has allowed to probe chiral electron dynamics with sub-femtosecond time resolution [1], opening new directions in high-harmonic spectroscopy [2]. In general, the chiral response of a system increases with the ellipticity (chirality) of the driving field, usually maximizing for circularly polarized light. However, the harmonic signal quickly drops with ellipticity as the liberated electron misses recollision with the core. In this context, the use of two-color counter-rotating bi-circular fields [3] constitutes a promising tool for exploring time-resolved chiral dynamics as they allow recombination while maximizing chiral responses. We have calculated the high-order harmonic spectra of the chiral molecule propylene oxide interacting with different configurations of laser fields. The chiral response of the system arises due to the interplay between electric and magnetic effects. Our calculations have been performed using the method described in [4], including accurate photorecomination matrix elements evaluated in the framework of density functional theory [5,6]. The resulting harmonic dipole has been averaged coherently on a Lebedev grid in order to account for the experimental condition of randomly oriented molecules. In this presentation, I will discuss the most relevant features arising in the high-order harmonic spectra of propylene oxide, showing that high-harmonic spectroscopy constitutes a promising tool for probing chiral electron dynamics with sub-femtosecond time resolution. [1] R. Cireasa et al Nat. Phys. 11, 654-8 (2015). [2] O. Smirnova et al, J. Phys. B 48 234005 (2015). [3] D. B. Miloševic et al, Phys. Rev. A 62 011403 (2000). [4] O. Smirnova and M. Ivanov, Chap. 7 in Attosecond and XUV physics, Wiley (2013). [5] D. Toffoli, M. Stener, G. Fronzoni, and P. Decleva, Chemical Physics 276, 25 (2002). [6] H. Bachau, E. Cormier, P. Decleva, J. E. Hansen, and F. Martín, Reports on Progress in Physics 64, 1815 (2001).

Speaker: David Ayuso (Max-Born-Institut, Berlin)

12:20 Chiral recognition in the gas phase using polarization-tailored two-color ionization

The asymmetry of photoelectron angular distributions (PADs) from randomly oriented enantiomers of chiral molecules in the ionization with circularly polarized light arises in forward/backward direction with respect to the light propagation. This effect is known as Photoelectron Circular Dichroism (PECD) and has so far been investigated using synchrotron radiation [1]. By employing resonance enhanced multi-photon ionization, we observed highly structured asymmetries in the range of ± 10% on bicyclic Ketones and were able to study dependency on laser parameters [2, 3, 4, 5]. Two-color fields can be used to drive electrons into trajectories usually not accessible by using a single-color field with elliptical or linear polarization. Our goal is to study to what extent two-color field geometries can be used to control electron dynamics in chiral molecules. In this talk, we give an overview over the experimental results using a phase-locked, two-color ionization scheme on noble gases as well as chiral specimen with different laser intensities covering the multi-photon up to the region close to the tunneling regime. [1] Laurent Nahon, Gustavo A. Garcia and Ivan Powis, Journal of Electron Spectroscopy and Related Phenomena, 204, 322-334 (2015) [2] Christian Lux, Matthias Wollenhaupt, Tom Bolze, Qingqing Liang, Jens Köhler, Cristian Sarpe, and Thomas Baumert, Angewandte Chemie Int. Ed. 51, 5001-5005 (2012) [3] Christian Lux, Matthias Wollenhaupt, Cristian Sarpe, and Thomas Baumert, Chem. Phys. Chem, 16, 115-137 (2015) [4] Alexander Kastner, Christian Lux, Tom Ring, Stefanie Züllighoven, Cristian Sarpe, Arne Senftleben, and Thomas Baumert, Chem. Phys. Chem, 17, 1119-1122 (2016) [5] Alexander Kastner, Tom Ring, Bastian C. Krüger, G. Barratt Park, Tim Schäfer, Arne Senftleben, and Thomas Baumert, J. Chem. Phys. 147, 013926 (2017)

Speaker: Alexander Kastner (University of Kassel)

12:40 Strong field physics with twisted light

We apply Quantum Trajectory Monte Carlo (QTMC) computations in order to model strong field ionization of atoms by twisted Bessel pulses and calculate photoelectron momentum distributions (PEMD). Since Bessel beams can be considered as an infinite superposition of circularly polarized plane waves with the same helicity, whose wave vectors lie on a cone, we compared the PEMD of such Bessel pulses to those of cicurlarly polarized pulse. We focus on the momentum distributions in propagation direction of the pulse and show how these momentum distributions are affected by experimental accessible parameters, such as the opening angle of the beam or the impact parameter of the atom with regard to the beam axis. In particular, we show that we can find higher momenta of the photoelectrons, if the opening angle is increased.

Speaker: Willi Paufler (Friedrich Schiller Universität Jena)

13:00 → 14:20 Lunch

Light lunch buffet served for all QUTIF2018 participants

14:20 → 16:00 Nanoscale

Convener: Prof. Manfred Lein (Leibniz Universität Hannover)

14:20 Do tailored light fields match tailored careers for female scientists? Light landscapes vs. career landcapes in Photonics

Complex tailored light fields are a disruptive field of photonics that foster basic research areas as singular optics, optical angular momentum or entanglement, and advance applications of photonic lattices, plasmonic excitation, optical trapping or optomechanics. In contrast, tailored careers in the field of light seem to be less succesful for female researchers, lacking behind in comparison to male colleagues as well as on an international scale. This presentation attempts to combine an introduction into pathbreaking tailored light field research with facts, data, and recent gender research on career development in photonics. In this latter area, we will discuss career obstacles, gender inequality in academia as well as approaches to overcome these hurdles. By contrasting the development in a research field to its associated careers, I will try to exemplify today's gender issues in science.

Speaker: Cornelia Denz (Die Westfälische Wilhelms-Universität in Münster)

15:20 Femtosecond spin-dependent charge transfer at Co/Cu(001) interfaces

Controlling ultrafast charge and spin dynamics in solids with ultrashort laser pulses requires us to first identify the quantum processes relevant to light-matter interaction on femtosecond timescales. We disentangle the elementary processes behind ultrafast spin transfer at epitaxial Co/Cu(001) interfaces due to optical excitation with 1.5 eV photon energy by combining femtosecond time-resolved interface-sensitive magnetization-induced second harmonic generation and ab initio time-dependent density functional theory. Finding a convincing agreement between the observables in theory and experiment, we directly identify spin-dependent charge transfer between Co and Cu active at < 30 fs, and spin-flips mediated by the spin-orbit interaction, which lead to a loss of spatially integrated spin polarization and dominate at > 30 fs.

Speaker: Andrea Eschenlohr (Uni Duisburg-Essen)

15:40 Space-, time-, and energy-resolved observation of charge dynamics in a single nanostructure

Observing the motion of charges in solid state nanostructures during their interaction with strong light fields is a fundamental goal and an open challenge of contemporary ultrafast science. It requires ultrahigh combined spatial and temporal resolution, and in order to reveal the interaction, the charges‘ energy gain or loss are vital information. Here we present a first experimental demonstration of space-, time-, and energy-resolved measurements, where we observe the motion of charges photoemitted from a plasmonic nanoresonator. Merging ultrafast nanoplasmonics and point-projection electron microscopy achieves 20 nm spatial resolution and 25 fs temporal resolution. Probing electrons are emitted from a sharp nanotaper and propagate through a light-driven plasmonic nanoresonator. In order to have direct access to the energy gain or loss they experience, the probing electrons are detected with a time-of-flight delay-line detector. We study the effect of the real-space motion of charge carriers photoemitted in the plasmonic nanoresonator on the energy and momentum of the probing electrons. We will discuss applications of this new microscope for probing strong, ultrafast fields in plasmonic nanostructures.

Speaker: Petra Gross (Carl von Ossietzky Universität, Oldenburg)

16:00 → 18:00 Poster session 2 (coffee served 16:00 h)

see Contributions in Poster session 1

Conveners: Prof. Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg), Prof. Manfred Lein (Leibniz Universität Hannover)

18:15 → 18:45 Transfer CFEL to restaurant

19:00 → 22:00 Conference dinner

Convener: Prof. Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg)

Friday, 16 February

09:00 → 10:40 Molecular Dynamics

Convener: Terry Mullins (CFEL, DESY Hamburg)

09:00 Strong laser field control of the dynamics and stereodynamics of molecular photodissociation

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place [1]. In recent experiments, we have studied strong laser field control scenarios of ultrafast molecular photodissociation dynamics. The control has been exerted on different observables of the photochemical reaction, such as quantum yields [2,3] and lifetimes [2] or even on fragment translational energies [3]. The case study involves photodissociation of the polyatomic prototype methyl iodide (CH3I), whose ultrafast photodissociation dynamics has been studied in our laboratory for some years both in the A-band [4,5] and B-band [6], under strong femtosecond or picosecond near-IR laser pulses [2,3]. The control is achieved by opening new strong-field-induced reaction channels [2] or by creating light-induced conical intersections and modulating the potentials around them by light-induced potentials [3]. In particular, control of the fragment spatial distribution (the stereodynamics) in the predissociation of methyl iodide has been achieved by using strong picosecond laser pulses [7] and the results will be presented at the Conference. References [1] I. R. Solá, J. González-Vázquez, R. de Nalda, and L. Bañares, Phys. Chem. Chem. Phys. 17, 13183 (2015). [2] M. E. Corrales, G. Balerdi, V. Loriot, R. de Nalda, and L. Bañares, Faraday Discuss. 163, 447 (2013).
 [3] M. E. Corrales, J. González-Vázquez, G. Balerdi, I. R. Solá, R. de Nalda, L. Bañares, Nature Chem. 6, 785 (2014). [4] R. de Nalda, J. Durá, A. García-Vela, J. G. Izquierdo, J. González-Vázquez, and L. Bañares, J. Chem. Phys. 128, 244309 (2008). [5] A. García-Vela, R. de Nalda, J. Durá, J. González-Vázquez, and L. Bañares, J. Chem. Phys. 135, 154306 (2011). [6] G. Gitzinger, M. E. Corrales, V. Loriot, R. de Nalda, and L. Bañares, J. Chem. Phys. 136, 074303 (2012). [7] M. E. Corrales, R. de Nalda, and L. Bañares, Nature Comm. 8, 1345 (2017).

Speaker: Luis Bañares (Universidad Complutense de Madrid)

09:40 A closer look at bond softening and Lochfrass effects

About a decade ago, the so-called lochfrass effect was predicted that creates a vibrational wavepacket in the non-ionized neutral molecule upon strong-field ionization. It was shortly thereafter experimentally observed in molecular deuterium. However, as was pointed out in those works, the vibrational wavepacket could, in principle, also be generated by bond softening (the dressed-state description of stimulated Raman scattering). Using the surprising stability of the formed wavepacket it was possible to distinguish the two processes and to confirm lochfrass to be (predominantly) responsible. We have now revisited the processes in order to investigate the astonishing robustness of the two effects and why it was possible to distinguish the two processes by the absolute phase of the wavepacket, despite the fact that the experiment used laser pulses with no control over the carrier-envelope phase. The result reveals that a much more general control mechanism is responsible that is expected to be an extremely robust and thus useful alternative for the standard generation of coherent wavepackets using light fields.

Speaker: Alejandro Saenz (Humboldt-Universität zu Berlin)

10:00 Coherent control of ionic yields after tailored multiphoton excitation in atoms and molecules

When excited by a pulse sequence generated by sinusoidal spectral phase modulation a resonantly driven atom or molecule may experience the control mechanism SPODS. The Selective Population of Dressed States enables selective and efficient transient as well as final state control by precisely tailoring the phase of the laser field to the induced dynamics and making use of the dynamical Stark effect. The same pulse sequences can be employed to excite atoms or molecules non-resonantly. In several species we could observe a strong modulation of the ionic yield with respect to the relative interpulse optical phase. We used the prototype systems xenon as well as isopropyl alcohol, ethanol and acetone and excited these substances with the shaped femtosecond laser pulses of constant pulse energy centred at 785 nm. The first absorption band of all examined species lies well above 3 eV, it is therefore driven by at least 2 infrared photons, 6 in the case of xenon. Surprisingly, already a simple level approach is sufficient to model the experimental results. We use our simulations to analyze the underlying population dynamics. Our calculations indicate them as a general behaviour of systems undergoing multiphoton excitation followed by resonantly driven Rabi-oscillations among excited electronic states.

Speaker: Tom Ring (Universität Kassel)

10:20 Fragmentation dynamics of HeH⁺ in ultrashort intense laser fields

The helium hydride molecular ion, HeH+, is the simplest heteronuclear polar molecule and serves as a benchmark system for the investigation of multi-electron molecules and molecules with a permanent dipole. We specifically address the question: How does the permanent dipole of HeH+ affect the fragmentation dynamics in intense ultrashort laser pulses? We study the laser induced laser-induced fragmentation; including non-ionizing dissociation, single ionization and double ionization; of an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. These results are interpreted using reduced dimensionality solutions to the time-dependent Schrödinger equation and with simulations based on Dressed surface hopping.

Speaker: Philipp Wustelt (Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität Jena)

10:40 → 11:10 Coffee break

11:10 → 12:40 Strong-field ionization

Convener: Prof. Claus Ropers

11:10 Molecular Orbital Imprint in Laser-Driven Electron Recollision

Electrons released by strong-field ionization from atoms, molecules, or in solids can be accelerated in the oscillating laser field and driven back to their ion core. The ensuing interaction, phase-locked to the optical cycle, initiates the central processes underlying attosecond science. A key long-standing assumption regards the returning electron wavepacket as a plane wave. Here we study laser-induced electron rescattering associated with two different ionization continua in the same, spatially aligned, polyatomic molecule. We show by experiment and theory that the electron return probability is in fact molecular-frame dependent and carries structural information on the ionized orbital. Pronounced deviations of the returning wavepacket from plane-wave character have to be accounted for in analyzing attosecond experiments based on strong laser fields.

Speaker: Jochen Mikosch (Max-Born-Institut, Berlin)

11:40 Strong-field control with minimally tailored laser pulses

It is shown that two-photon strong-field ionization of atoms can be extremely well controlled -- basically switching between no and complete ionization -- with minor modification of the laser pulse. The underlying mechanism will be explained for a model system and realistic calculations for single-active-electron atoms are presented.

Speaker: Ulf Saalmann (Max Planck Institute for the Physics of Complex Systems, Dresden)

12:00 Single-shot electron imaging of NIR-induced He nanoplasmas

Nanoplasmas formed from doped helium nanodroplets by irradiation with intense near-infrared fs-pulses feature peculiar properties. A helium ionization avalanche is triggered by tunnel ionization of the dopant cluster at comparatively low light intensities (~1014 W/cm2). Subsequent light absorption is enhanced by resonances due to nanoplasma anisotropies [PRL 107, 173402 (2011)] and expansion [NJP 14 075016 (2012)]. Consequently, dopant and helium atoms charge up to high charge states [J. Mod. Opt. 64, 1061 (2017)], and energetic ions and electrons are emitted by Coulomb explosion. Surprisingly, our single-shot velocity-map images of nanoplasma electrons display sharp electron energies peaked at ~eV energies. We discuss the systematics of electron and ion spectra, as well as possible interpretations based on classical MD simulations.

Speaker: Marcel Mudrich (Aarhus University)

12:20 Subcycle dynamics of strong-field ionization

In previous studies of the attosecond scale temporal structure of strong-field ionization in atoms driven by few-cycle pulses using the "attoclock" scheme, a one-to-one correspondence between photoelectron momentum and time of ionization on the basis of outgoing electron trajectories has been used. We introduce a purely quantum mechanical and trajectory-free definition of the ionization time. For circular polarization, we show that the strongest ionization takes place without delay at the time of highest field strength. In contrast, the bicircular attoclock using two counter-rotating circular fields exhibits non-negligible ionization delays. We also show simulations results for rescattering beyond the electric dipole approximation in linearly polarized light, demonstrating considerable forward momentum shifts of the high-energy photoelectrons due to radiation pressure.

Speaker: Manfred Lein (Leibniz Universität Hannover)

12:40 → 12:50 Conclusions

Conveners: Prof. Jochen Küpper (Center for Free Electron Laser Science, DESY and Universität Hamburg), Prof. Manfred Lein (Leibniz Universität Hannover)

12:50 → 13:30 Lunch & departure