Quantum Dynamics in Tailored Intense Fields

Annual Workshop 26th February – 28th February 2020

Venue: Max-Born-Institut (MBI) Berlin Max-Born-Saal Max-Born-Straße 2A 12489 Berlin

Wednesday, 26th February 2020

10:15 – 11:00 Tutorial Tobias Witting 11:00 – 11:45 Tutorial Mikhail Ivanov 11:45 – 12:35 Registration / lunch 12:35 – 12:45 Welcome

Nirit Dudovich 12:45 – 13:20 Attosecond interferometry

Walter Pfeiffer 13:25 – 13:45 Attosecond delays in solid state photoemission Armin Feist 13:50 – 14:05 Controlling free-electron wavefunctions by traveling optical waves and whispering-gallery modes Niklas Müller 14:10 – 14:25 Broadband coupling of fast electrons with high-Q whispering gallery mode resonators 14:30 – 15:00 Coffee break Matthias Wollenhaupt 15:00 – 15:20 Dynamic quantum state holography using CEP-stable bichromatic polarization-tailored laser pulses Sajjad Azizi 15:25 – 15:45 Controlling non-adiabatic ionization with ultra-short pulses Matthias Kübel 15:50 – 16:10 Carrier-envelope phase measurements at 3µm wavelength 16:15 – 17:30 Labtours 17:30 – 19:00 Poster session Thursday, 27th February 2020

Fernando Martin 09:00 – 09:35 Attosecond pump-probe spectroscopy of molecular electron dynamics Emil Zak 09:40 – 10:00 Controlling the rotation axis in polyatomic molecules with an optical centrifuge Jochen Mikosch 10:05 – 10:25 Molecular frame studies of channel-resolved laser-driven electron recollision 10:30 – 11:00 Coffee break Philipp Wustelt 11:00 – 11:15 Dissociation of HeH⁺ by long wavelength ultrashort laser pulses Florian Oppermann 11:20 – 11:35 Ionization and dissociation of HeH⁺ in strong two-color fields

Adrian Pfeiffer 11:40 – 12:00 Observation of dynamical bloch oscillations in dielectrics

Peter Elliott 12:05 – 12:25 Ab-initio transient XMCD spectroscopy

12:30 – 13:25 Lunch break

Thursday, 27th February 2020

Alejandro Saenz 13:25 – 14:00 A more detailed look into enhanced ionization in intense laser fields Alexander Kuleff 14:05 – 14:20 Ultrafast non-adiabatic relaxation in XUV-excited molecules Victor Despré 14:25 – 14:40 Ultrafast electron dynamics and its control in the presence of non-adiabatic effects

Álvaro Jiménez Gálan 14:45 – 15:05 Lightwave topology for strong-field valleytronics

Jin Zhang 15:10 – 15:25 Theoretical investigation of HHG/SHG from hBN rotators

Ihar Babushkin 15:30 – 15:45 Ionization dynamics of electrons from the lowest Brunel harmonics 15:50 – 16:20 Coffee break 16:30 – 17:30 Bus transfer – departure at 16:30! 17:30 – 19:30 Excursion – Reichstag 19:30 – 22:00 Dinner at Hackescher Hof 22:00 Bus transfer

Friday, 28th February 2020

Markus Gühr 09:00 – 09:35 Photophysics in the gas phase illuminated by ultrafast x-rays and electrons

Anne Harth 09:40 – 10:00 Phase information of continuum-continuum couplings

Andres Ordonez 10:05 – 10:25 Multiphoton ionization of chiral molecules: what can we control and where's the button to control it? 10:30 – 11:00 Coffee break

David Ayuso 11.00 – 11:20 Polarization of chirality

Nicolai Klemke 11:25 – 11:40 Circularly polarized high-harmonics from solids originating from intraband dynamics Timo Paschen 11:45 – 12:05 Rescattering effects in two-color photoemission from tungsten needle tips Markus Debatin 12:10 – 12:30 X ray-induced helium nanoplasmas - ultrafast charge migration delays Coulomb explosion 12:35 – 13:30 Lunch break 13:30 – 14:30 Discussion

Location map Abstracts Attosecond interferometry Nirit Dudovich

One of the most important aspects of attosecond spectroscopy lies in its coherent nature. Resolving the internal coherence is a primary challenge in this field, serving as a key step in our ability to reconstruct the internal dynamics. As in many other branches in physics, coherence is resolved via interferometry. In my talk, I will describe advanced schemes for attosecond interferometry. The application of these schemes provides direct insights into a range of fundamental phenomena in nature, from tunneling and photoionization in atomic systems to ultrafast chiral phenomena in molecules.

Attosecond delays in solid state photoemission Walter Pfeiffer

S. Neb, A. Gebauer, W. Enns, N. Müller, U. Heinzmann, and W. Pfeiffer E.E. Krasovskii, V. M. Silkin, N. M. Kabachnik, A. K. Kazansky, P. M. Echenique

In recent years, time-resolved spectroscopy of electron dynamics in solids advanced to the attosecond regime, i.e. on the time scale on which electron motion occurs on an atomic scale. One example of such a technique is attosecond time- resolved streaking spectroscopy. This technique allows investigating relative temporal delays in the photoemission from different initial states with a resolution down to a few attoseconds. Up to now there is not yet a unified theoretical model for describing the observed photoemission delays in such experiments. In this presentation the present status in the development of such a theoretical model is presented. Based on ab-initio electronic structure calculations, solutions of the time- dependent Schrödinger equation, photoemission delays were determined and are compared to experimentally measured delays for two different materials, i.e., the van-der-Waals crystals WSe2 and BiTeCl.

Controlling free-electron wavefunctions by traveling optical waves and whispering-gallery modes Armin Feist

In recent years, the manipulation and interaction of electrons with optical fields has seen significant progress, enabling schemes of near-field electron acceleration, attosecond electron pulse generation and capabilities for imaging nano-localized optical modes. Here, we present two novel concepts for the coherent control of free-electron beams in an ultrafast transmission electron microscope (UTEM). In the first experiment, we demonstrate the quantized transfer of photon energy and transverse momentum to a high- coherence electron beam. The three-dimensional optical phase modulation at a laser-illuminated graphite thin film constitutes a coherent inelastic beam splitter for free electrons. In a second study, the phase-matched interaction of electrons with optical whispering gallery modes (WGMs) of dielectric microresonators facilitates a drastically enhanced electron-light coupling and traces the intrinsic temporal cavity ring-down. These results provide for the elementary components of optically programmable electron phase plates and may facilitate a continuous-wave electron acceleration or attosecond structuring by cavity-enhanced and phase-matched optical interactions.

Broadband coupling of fast electrons with high-Q whispering-gallery mode resonators Niklas Müller

Niklas Müller, Vincent Hock, Christopher Rathje, Holger Koch and Sascha Schäfer

The inelastic interaction of fast electrons with spatially confined intense light fields has recently enabled new techniques in ultrafast transmission electron microscopy (UTEM) [1,2,3], enabling the coherent control of free-electron states. Whereas previous work focused on coherent light states as driving fields, advanced quantum control scenarios, including electron-light entanglement and non-trivial electron/photon counting statistics, become accessible if non-classical (quantum optical) light states are applied [4,5,6]. However, to mitigate the reduced coupling strength when considering few-photon-states, novel concepts for coupling electrons to high-Q optical resonators are required. Here, we demonstrate the excitation of high-Q whispering gallery modes in a silica microfiber taper in a transmission electron microscope by relativistic electrons (200 keV electron energy) passing close to the fiber surface. The evanescent electric field of the passing electron induces a femtosecond electric polarization in the silica, which can be decomposed into optical whispering gallery modes (WGM) within the taper geometry. Utilizing a home-built TEM sample holder, fiber- guided light field components are detected in a high-resolution spectrometer. The coherent cathodoluminescence spectra consist of octave-spanning frequency combs with narrow- bandwidth peaks. By probing the WGM resonances for different distances from the taper apex, we demonstrate that the peaks within the comb exhibit a frequency spacing inversely scaling with the local fiber circumference. The experimental results are further supported by simulations of the electron energy loss probability using a transition matrix method [7]. Further it is shown that the bandwidth of the WGM peaks strongly depends on the local taper angle of the fiber, with Q-factors up to 700.

[1] De Abajo et al., New J. Phys. 10, 073035 (2008) [2] Barwick et al., Nature 462, 902-906 (2009) [3] Feist et al., Nature 521, 200-203 (2015) [4] Meuret et al., Phys. Rev. Lett. 114, 197401 (2015) [5] Hyun et al., Appl. Phys. Lett. 93, 243106 (2008) [6] Kfir, Phys. Rev. Lett. 123, 103602 (2019) [7] Yalunin et al., Phys. Rev. B 93, 115408 (2016)

Dynamic quantum state holography using CEP-stable bichromatic polarization-tailored laser pulses Matthias Wollenhaupt

Three-dimensional photoelectron momentum distributions (PMDs) with unusual symmetry properties are generated by multiphoton ionization (MPI) of atoms using polarization- tailored ultrashort laser fields. The PMDs are manipulated by the pulse parameters including the carrier-envelope phase (CEP). In the experiment, we combine supercontinuum pulse shaping with photoelectron tomography for 3D reconstruction of the PMD. A 4f polarization pulse shaper is used to sculpture bichromatic fields from a CEP-stable white light supercontinuum by spectral amplitude and phase modulation [1]. MPI of atoms with single-color sequences of counterrotating circularly polarized (CRCP) femtosecond laser pulses produces vortex-shaped PMDs with even-numbered rotational symmetry [2]. In contrast, bichromatic CEP-stable counter- (CRCP) and corotating (COCP) femtosecond laser pulses generate odd-numbered rotationally symmetric or asymmetric PMDs [3]. In this contribution we focus on a pulse- shaper-based holographic technique for the time-resolved and phase-sensitive observation of ultrafast quantum dynamics [4]. The interference of continuum states with different angular momenta yields a crescent-shaped photoelectron wave packet rotating in the laser polarization plane due to the interplay of the optical phase and the accumulated quantum phase. CEP- control of the rotation provides access to the photoelectron asymmetry, enabling background-free detection of the crescent’s angular motion which maps the bound Rydberg wave packet dynamics.

Tomographic reconstruction of the photoelectron density for a series of time delays covering about one period of the slow 7f-oscillation. Green frames in the top row display 3D PMDs for selected time delays covering about one period of the fast 8f-oscillation. Orange frames in the bottom row display selected 3D PMDs from the 7f-oscillation Adapted from [4].

[1] S. Kerbstadt, D. Timmer, L. Englert, T. Bayer, M. Wollenhaupt, Ultrashort polarization-tailored bichromatic fields from a CEP-stable white light supercontinuum, Opt. Express 25 (2017) 12518. [2] D. Pengel, S. Kerbstadt, D. Johannmeyer, L. Englert, T. Bayer, M. Wollenhaupt, Electron Vortices in Femtosecond Multiphoton Ionization, Phys. Rev. Lett. 118 (2017) 053003. [3] S. Kerbstadt, K. Eickhoff, T. Bayer, M. Wollenhaupt, Odd electron wave packets from cycloidal ultrashort laser fields, Nat. Comm. 10 (2019) 658. [4] K. Eickhoff, T. Bayer, K. Kerbstadt, M. Wollenhaupt, Phys. Rev. A, (2020) accepted.

Controlling non-adiabatic ionization with ultra-short pulses Sajjad Azizi

Non-adiabatic ionization, a new channel in the photo-ionization with high-frequency lasers, occurs for strong and short pulses due to large gradients of the *pulse envelope*. This unusual dependence on the envelope derivative can be explored by manipulating the time profile of the pulse envelope. It is shown that the non-adiabatic ionization yield can be enhanced with particular shaped pulses in comparison to the yield of the corresponding Fourier-limited pulse (FLP). This is surprising since the FLP is the strongest and shortest pulses for a given spectral representation.

Carrier-envelope phase measurements at 3µm wavelength Matthias Kübel

Matthias Kübel, Dominik Hoff, Philipp Wustelt, Slawomir Skruszewicz, Yinyu Zhang, Huipeng Kang, Daniel Würzler, Richard Hollinger, Christian Spielmann, Balint Kiss, Sergei Kühn, A. Max Sayler, and Gerhard G. Paulus

Realizing the full potential of mid-IR lasers in strong-field and attosecond physics will require efficient methods for the measurement of the carrier-envelope phase (CEP), be it for stabilization schemes or single-shot phase tagging. In this talk, we will discuss two independent approaches to measure the CEP in the mid-IR. The first approach is based on the Stereo-ATI technique, where the asymmetry of photoelectron spectra in both directions along the polarization axis are recorded. We will present measurements of both Xe and Cs, using the CEP-stable MIR laser at the ELI-ALPS user facility. The results represent the basis for designing a stereo-ATI phase meter for the mid-IR. In light of the unfavorable scaling of the electron recollision probability, we present a second method based on high- harmonic generation (HHG) in solids. In the HHG spectra generated from ZnO, successive harmonic orders overlap, owing to the large bandwidth of the MIR laser. The resulting interference pattern allows for the measurement of the CEP of the driving laser, in very good agreement with the conventional f-2f method. We show that this approach represents a convenient and economic route to measuring the CEP of mid-IR lasers, which scales favorably towards longer wavelengths.

Attosecond pump-probe spectroscopy of molecular electron dynamics Fernando Martín

Attosecond and few femtosecond light pulses allow one to probe the inner workings of atoms, molecules and solids on the timescale of the electronic motion. In molecules, sudden ionization by such pulses is followed by charge redistribution on a time scale ranging from a few femtoseconds down to hundreds of attoseconds, and usually leads to fragmentation of the remaining molecular cation. Such complex dynamics arises from the coherent superposition of electronic states populated by the broadband attosecond pulse and from rearrangements in the electronic structure of the molecular cation due to electron correlation. To investigate these ultrafast processes, attosecond pump-probe spectroscopy has been shown to be a very valuable tool. In this talk I will present the results of recent attosecond pump-probe simulations in which several atoms and molecules, from hydrogen to the amino acid tryptophan, are ionized with a single or a train of attosecond pulses and are subsequently probed by an infrared or an XUV pulse (see [1] for a recent review on the subject).

[1] M. Nisoli, P. Decleva, F. Calegari, A. Palacios, and F. Martín, Chem. Rev. 117, 10760 (2017)

Controlling the rotation axis in polyatomic molecules with an optical centrifuge Emil Zak

E. J. Zak, A. Yachmenev, J. Küpper

It has been theoretically shown [1] that it is possible to populate rotational states of gas phase asymmetric-top molecules which correspond to classical rotation around its a- and c- principal axis of inertia. This can be achieved through an optical centrifuge with time-modulated electric field intensity envelope. Here we propose a method for tailoring the rotational wavepackets without modulating the field intensity (i.e. only with the standard optical centrifuge setup), in which the acceleration rate for the rotating electric field plane is appropriately chosen. We computationally show, on the example of D2S and 2H-imidazole molecules, a protocol for creating arbitrary coherences between states in which the molecule rotates around the a-, b- or c-axis.

[1] A. Owens, A. Yachmenev, J. Küpper, Phys. Chem. Lett. 9, 15, 4206-4209 (2018)

Molecular frame studies of channel-resolved laser- driven electron recollision Jochen Mikosch

Federico Branchi, Horst Rottke, Mark Mero, Marc J.J. Vrakking, Varun Makhija, and Jochen Mikosch

When a molecule interacts with a strong, infrared laser field, a number of phase-locked attosecond processes can be initiated. From the perspective of transient probing of molecular structure, Laser-Induced Electron Diffraction (LIED) is of particular recent interest. In LIED, the tunnel-ionized electron wavepacket is accelerated and driven back to the parent molecule, where it rescatters elastically. With midinfrared driving laser fields, where the achieved electron kinetic energies are high, bond lengths and angles of molecules can be extracted from the electron scattering images, by fitting the measured differential cross section with an independent atom model. We are particularly interested in ionization channel-resolved studies, since LIED can be performed independently with two different continuum wavepackets, on the same molecule, at the same time. Such experiments are hence very powerful in testing the way in which structural information is retrieved from the data. Current measurements on 1,3-butadiene molecules are performed in a reaction microscope, which is coupled to a 100kHz repetition rate, mid-infrared OPCPA laser system. We will report on differences we found between the rescattering associated with ground and excited state ionization continuum and on extracting three-dimensional molecular frame information.

Dissociation of HeH⁺ by long wavelength ultrashort laser pulses Philipp Wustelt

The laser-induced fragmentation of the helium hydride ion, the simplest heteronuclear molecule, is investigated using an ion beam apparatus. We show that changing the wavelength of the driving laser alters dramatically the fragmentation dynamics of HeH⁺ in strong laser fields. While at 400 nm and 800 nm laser wavelength the dominating fragmentation pathway is ionization and almost no dissociation can be observed, at longer wavelength a strong increase of the dissociation probability is observed. This remarkable behavior can be explained by the special properties of HeH⁺. The extreme asymmetric nature of HeH⁺ is manifested in a strong permanent dipole, which allows for direct vibrational excitation without electronic excitation. Therefore, depending on the photon energy substantial dissociation can be triggered using longer laser wavelengths. The contributions from different vibrational states are traced by measuring the momentum distribution of the dissociation fragments.

Ionization and dissociation of HeH⁺ in strong two- color fields Florian Oppermann

Our previous study of ionization and double ionization of HeH⁺ in strong 800 and 400nm laser pulses has shown the important role of nuclear motion before and during the electron removal [1]. Here we move our focus to laser parameters where both dissociation and ionization are of comparable probability. According to simulations, this implies wavelengths around 1 to 2μm. For fixed molecular orientation the ratio ionization/dissociation can be controlled (sometimes even reversed) via the relative phase in a collinearly polarized ω-2ω laser pulse. Numerical simulation results for HeH⁺ in two-color fields are presented using different levels of approximation in order to shine light on the interplay between nuclear motion and electronic excitation.

[1] Wustelt et al., Phys. Rev. Lett. 121, 073203 (2018)

Observation of dynamical bloch oscillations in dielectrics Adrian Pfeiffer

The counter-intuitive prediction of band theory that electrons alternate their direction when they are accelerated beyond the Brillouin zone edge cannot easily be observed. In DC fields, scattering prevents the electrons in bulk crystals from reaching the zone edge. The recent trend of studying strong-field effects in dielectric solids, especially high-order harmonic generation, has established that dynamical Bloch oscillations occur during the course of a laser cycle. Especially for drive pulses with longer wavelengths (>800 nm), intensities well below the damage threshold suffice for acceleration out of the first Brillouin zone. In this regime, dynamical Bloch oscillations are believed to be the dominant mechanism for solid-state HHG. This mechanism is consistent with the fact that the HHG cutoff scales linearly with the electric field, and not quadratic as for gas-phase HHG. Here the intensity region where the electrons leave the Brillouin zone for the first time is scrutinized. This occurs at 10-20 TW/cm2 for 800 nm pulses in SiO2. This region is below intensities where dynamical Bloch oscillations cause HHG, because this requires multiple zone-crossings. At intensities that enable only the first zone-crossing, the electrons alternate their direction only twice per laser half-cycle. This effectively shifts the current from oscillating at the fundamental frequency of the laser, as it is the case at low intensities, to oscillations at the third harmonic frequency. It is challenging to observe this effect, because the total polarization response comprises not only the intraband contribution (the current), but also the interband polarization. Therefore, noncollinear spectroscopy is employed where the interference of a double pulse in the deep-ultraviolet (DUV) reveals the dispersion due to the interband contribution. A clear interference structure is observed when the intensity is scanned in the regime of the first zone-crossing. Simulations based on semiconductor Bloch equations show that this interference structure contains information about the coherence properties of the process.

Ab-initio transient XMCD spectroscopy Peter Elliott

Studying the spin dynamics induced by intense, femtosecond, laser pulses has revealed many new phenomena, such as ultrafast demagnetization or all-optical switching. To understand the underlying physics, these processes may be probed using X-ray Magnetic Circular Dichroism (XMCD) spectroscopy. However, in recent years it has become more common to use HHG to create XUV sources and probe in this regime. In this talk, I will demonstrate an ab-initio method to calculate the transient XMCD/XUVMCD signal which combines real-time time-dependent density functional theory (TDDFT) with auxiliary linear-response TDDFT calculations. This allows for a more direct comparison between theory and experiment. As an example, the XUVMCD dynamics of CoPt at the Co M edge and the Pt N and O edges are calculated and tested.

A more detailed look into enhanced ionization in intense laser fields Alejandro Saenz

Enhanced ionization, i. e. a strongly increased ionization probability found for specific internuclear separations of a molecule exposed to an intense laser field, is one of the most paradigmatic molecular strong-field effects. This phenomenon has been investigated experimentally and theoretically for a number of molecules, especially the hydrogen molecular ion and neutral hydrogen molecules. Recently, strong experimental evidence was even found that enhanced ionization can even occur simultaneously, i.e. more than one carbon-hydrogen bond may break in corresponding organic molecules. Besides this plethora of studies, only few investigations have been devoted to heteronuclear systems. Motivated by a combined experimental and theoretical study on HeH+ in ultrashort intense laser pulses, we have performed fully correlated calculations of this molecule in intense laser pulses by solving the corresponding time-dependent Schrödinger equation in full dimensionality for fixed, but varying internuclear separation. A pronounced influence of the carrier-envelope phase of the laser is found that can lead to a variation of the products by a factor 50 or more! The detailed analysis reveals an interesting electron dynamics that will be presented and discussed in this talk after a general introduction into enhanced ionization is given.

Ultrafast non-adiabatic relaxation in XUV-excited molecules Alexander Kuleff

Exposing molecules to XUV radiation populates typically highly- excited cationic states that triggers complex ultrafast dynamics in which both the electron and the nuclear motions are strongly coupled. A fully quantum description of these dynamics in small polycyclic aromatic hydrocarbons (PAH) will be reported and compared to time-resolved experimental results [1,2]. It will be shown that the non-adiabatic relaxation dynamics gets slower the closer the initial excitation is to the double-ionization threshold and also when increasing the size of the system. Moreover, it will be demonstrated that the dynamics in these energy range is governed by the so-called correlation bands, features created by the strong electron correlation in the inner- valence, and a simple electron-phonon scattering model may be used to explain and predict the relaxation dynamics of whole classes of molecules.

[1] A. Marciniak, et al., Nature Commun. 6, 7909 (2015) [2] A. Marciniak, et al., Nature Commun. 10, 337 (2019)

Ultrafast electron dynamics and its control in the presence of non-adiabatic effects Victor Despré

The advent of attosecond physics allowed the observation and manipulation of dynamic processes occurring within the intrinsic time scale of charge motion at atomic scale. This has opened the door to the realization of the dream of attochemistry, namely to control chemical reactions through the manipulation of the pure electronic dynamics taking place in the first instants after the excitation of the system. The full simulation of such a control scheme is a multi-scale problem going from the initial ionization triggering the dynamics to the final chemical reaction of the molecule. I will present results exemplifying different aspects of attochemistry, paving the way to its full simulation. I will talk about pure electron dynamics triggered by ionization, termed charge migration, and how it is possible to control the charge migration process with tailored IR pulses. Using the propiolic acid molecule, for which we had performed a fully quantum treatment of the electronuclear dynamics, it will be shown that even though the nuclear motion can lead to a very fast decoherence of the electron dynamics, long lived electron coherences permitting the observation and control of charge migration are possible. Long lived coherence for the silane molecule, both neutral and cationic, will also be discussed.

Lightwave topology for strong-field valleytronics Álvaro Jiménez

Modern light generation technology offers extraordinary capabilities for sculpting light pulses, with full control over individual electric field oscillations within each laser cycle. These capabilities are at the core of lightwave electronics -- the dream of ultrafast lightwave control over electron dynamics in solids, on a few-cycle to sub-cycle timescale, aiming at information processing at tera-Hertz to peta-Hertz rates. At the same time, quantum materials have opened the way to dissipationless electron transport and to the possibility to harness extra electronic degrees of freedom, such as the valley pseudospin, that can be used as additional information carriers. In this talk, I will merge these two fields, and show a robust and general approach to ultrafast, valley-selective electron excitations in two-dimensional materials by controlling the sub- cycle structure of non-resonant driving fields at a few- femtosecond timescale. Bringing the frequency-domain concept of topological Floquet systems to the few-femtosecond time domain, I will demonstrate a transparent control mechanism in real space to induce and control topological properties on topologically-trivial monolayers, and an all-optical, non- element-specific method to coherently write, manipulate and read selective valley excitations using fields carried in a wide range of frequencies, on timescales orders of magnitude shorter than valley lifetime, crucial for implementation of valleytronic devices.

Theoretical investigation of HHG/SHG from hBN rotators Jin Zhang

The emergence of van der Waals (vdW) materials by stacking two-dimensional (2D) layers vertically paves the way for exploring novel physics and device applications. The generation of high-order harmonics (HHG) from different 2D materials (graphene, hBN etc) enables the production of high-energy photons and ultrashort isolated pulses. Using TDDFT calculations, we successfully reproduced the twisted-angle and layer-number dependence of second harmonic generation (SHG) intensity from hBN thin films. Our calculations confirm the external electric effects in SHG from even-number layer hBN, which explains the intensity cross-over between odd and even layers. The interfacial interactions in hBN thin films are crucial in the SHG spectra. This work paves the way to modulate SHG/HHG by tuning the interlayer interactions in 2D vdW materials.

Ionization dynamics of electrons from the lowest Brunel harmonics Ihar Babushkin

Ionization dynamics of atoms and subsequent motion of free electrons in continuum attracts significant attention over the years. Typical approaches to study this dynamics are the measurement of electrons at a distant detector, or measurement of high harmonics, typically lying in the XUV range. Here we show that the subcycle dynamics of ionization, or even some information about electrons in the continuum can be extracted from the observation of polarization state of very low harmonics of the atomic response, in particular 0th and 3rd harmonics.

Photophysics in the gas phase illuminated by ultrafast x-rays and electrons Markus Gühr

The conversion of light energy into other energy forms in molecules is the result of a concerted and ultrafast motion of electrons and nuclei, often under breakdown of the Born- Oppenheimer approximation. This talk is about ultrafast experiments aimed at resolving light induced ultrafast molecular dynamics with x-ray probe pulses using free electron lasers as well as relativistic electron pulses. First, I will present experiments on internal conversion of the nucleobase thymine as well as 2-thiouracil. Those were performed at the LCLS as well as at FLASH using femtosecond x-ray spectroscopy at the oxygen K-edge and sulfur L-edge respectively. We use information from time-resolved Auger, absorption and photoelectron spectroscopy to discern molecular processes. In addition, I will present results from femtosecond electron diffraction experiments performed at the relativistic UED source at SLAC. The experiments on electronically excited states of small molecules unravel wavepacket dynamics with Angstrom level spatial resolution and femtosecond domain temporal resolution.

Phase information of continuum-continuum couplings Anne Harth

Attosecond pulses allow the observation of attosecond dynamics of electron motion in a variety of different systems. Methods to measure such ultrafast dynamics are often based on two-color field ionization: an attosecond pulse (XUV) ionizes the system, and a probe field (e.g. IR) drives continuum-continuum dipole transitions. The analysis of such photoelectron spectra requires careful consideration of the latter contribution, the continuum-continuum transitions. Experimental measurements of this contribution are challanging. In this talk, we present a method that has the potential to gain general information about dipole transitions in the continuum.

Multiphoton ionization of chiral molecules: what can we control and where’s the button to controll it? Andres Ordonez

We have developed a new approach to analyze the photoelectron angular distribution resulting from multiphoton ionization of isotropic molecular samples. Our formalism reveals how the correlations encoded in the tensor that describes the orientation averaged photoelectron angular distribution can be cleanly factorized into a molecular tensor invariant expressed in the molecular frame and an electric field tensor invariant expressed in the laboratory frame. By explicitly determining this one-to-one connection between molecular and field tensor invariants we show which polarizations of the electric field must be used to address specific molecular tensors. Our findings provide not only a solid basis for the characterization of molecular tensor invariants (including their phase), but also allow a transparent application of coherent control of the correlations in the photoelectron angular distribution. Using our formalism we have successfully identified the molecular and electric field tensor invariants responsible for the enantiosensitive asymmetry observed in the photoionization of isotropically oriented chiral molecules via a phase-locked crossed-polarized ω-2ω in Ref. [Phys. Rev. Lett. 121, 253201 (2018)]. Our results clearly reveal the role of the chiral setup [Phys. Rev. A 98, 063428 (2018)] (essential for chiral discrimination in the absence of chiral light) in the absence of a well defined rotation direction of the electric field, expose the fundamental connection between the chiral setup and the electric field tensor invariants, and reveal the role of the molecular phase in the formation of the asymmetry. Our approach yields general and compact expressions which rely neither on a specific form of the scattering wave function nor on a specific polarization of the electric field. This approach is a vast generalization of the ideas introduced recently in Ref. [Phys. Rev. A 98, 063428 (2018)], extending them into the realm of multiphoton ionization and coherent control, both for chiral and achiral molecules.

Polarization of chirality David Ayuso

The spatial polarization of the electronic clouds of molecules governs chemistry, from stopping fatty acids to dissolve in water to giving specificity to the biological activity of enzymes. We bring together two important physical concepts that had so far remained completely unrelated: chirality and polarization. In this talk, I will present the concept of polarization of chirality and show that, like charge, handedness can be polarized. We have demonstrated this general concept using light, and found how to engineer chirality-polarized optical fields of alternating handedness in space. Despite being achiral, these racemic space-time structures interact differently with chiral media of opposite handedness. The polarization of light’s handedness is recorded in the phase of the ultrafast electronic response of the chiral medium, which controls the macroscopic optical response in the far field. Thus, control of polarization of light’s chirality gives us full control over the enantio-sensitive direction of harmonic emission: we can make a medium of randomly oriented chiral molecules emit light to the left, or to the right, depending on the molecular handedness and on the polarization of the light's handedness. Our work opens new opportunities for efficient chiral discrimination and for control of chiral and chirality-polarized fields of light and matter on ultrafast time scales.

Circularly polarized high harmonics from solids originating from intraband dynamics Nicolai Klemke

N. Klemke, N. Tancogne-Dejean, A. Rubio, F. X. Kärtner and O. D. Mücke

Recently, we demonstrated that the polarization states of high harmonics from crystalline solids can differ from those of the driving pulses [1, 2]. This is especially striking in the observation of circularly polarized high-harmonics with elliptically polarized single-color driving pulses. A time-dependent density functional theory approach is able to describe this behavior accurately [2]. However, its level of sophistication makes it costly and challenging to extract an intuitive picture of the underlying physics. Here, we therefore perform single-particle intraband- only calculations [3, 4] and find that we can reproduce some of the most striking phenomena. For instance, our calculations yield circularly polarized harmonics from elliptically polarized driving pulses that sensitively depend on the driving conditions, as well as a major-axis rotation of the harmonics' polarization ellipse. Furthermore, we perform measurements on ZnS which show qualitatively similar features to the ones observed before in silicon. Our calculations show reasonable agreement with experiment, especially for large driver pulse ellipticities. Our work suggests a method for the distinction of different generation mechanisms underlying high-harmonic generation from solids. Moreover, it paves the way to compact high- harmonic sources with controllable polarization states.

[1] N. Tancogne-Dejean, O. D. Mücke, F. X. Kärtner, and A. Rubio, Nat. Commun. 8, 745 (2017) [2] N. Klemke, N. Tancogne-Dejean, G. M. Rossi, Y. Yang, F. Scheiba, R. E. Mainz, G. Di Sciacca, A. Rubio, F. X. Kärtner, and O. D. Mücke, Nat. Commun. 10, 1319 (2019) [3] M. W. Feise, and D. S. Citrin, Appl. Phys. Lett. 75, 3536 (1999) [4] O. D. Mücke, Phys. Rev. B 84, 081202 (2011)

Rescattering effects in two-color photoemission from tungsten needle tips Timo Paschen

Timo Paschen, Philip Dienstbier, Lennart Seiffert, Thomas Fennel, and Peter Hommelhoff

Photoemission from tungsten needle tips using a synthesized two-color laser field allows for sub-femtosecond control of electron emission by changing the optical phase between the two laser pulses [1,2]. Quantum coherent control between different emission pathways results in a pristine photoemission yield modulation with a visibility of 97.5%. While in the perturbative photoemission regime only the excitation dynamics are influenced by changes of the optical phase, in the strong- field regime also trajectory modifications are expected [3]. In this talk, we show experimental evidence for the modification of both ionization dynamics and field-driven trajectories in electron energy spectra. Characteristic markers in the energy spectra, such as the energy-dependent critical phase, are identified and we utilize them to disentangle the influence of ionization from trajectory modifications. Spectra obtained by simple-man’s model simulations and calculation of the time- dependent Schrödinger equation show an excellent agreement with the experimental data. Furthermore, the phase-dependent cutoff-modulation gives direct insight into the near-field strength of the second harmonic laser field, allowing for in-situ probing of small field admixtures.

[1] Förster et al., PRL 117, 217601 (2016) [2] Paschen et al., J. Mod. Opt. 64, 10-11, 1054 (2017) [3] Seiffert et al., J. Phys. B. 51, 134001 (2018)

X Ray-induced helium nanoplasmas - ultrafast charge migration delays Coulomb explososion Markus Debatin

M. Debatin, D. Schomas, C. Medina, L. Ben Ltaief, R. B. Fink, S. Mandal, S. Rama Krishnan, R. Michiels, F. Stienkemeier, C. Ott, R. Moshammer, T. Pfeifer, A. Heidenreich and M. Mudrich

The ultrafast response of heterogeneous nanoparticle to irradiation by intense x-ray pulses crucially determines the achievable resolution of single-shot coherent diffraction images. Here, we use doped helium (He) nanodroplets irradiated by ultrashort soft x-ray pulses and probed by intense near- infrared (NIR) pulses to probe the ionization dynamics of a two- component nanosystem. Owing to the low absorption cross section of helium at high photon energies, the x-ray pulse selectively ionizes the cluster core formed of heavy rare-gas atoms. Ultrafast electron migration from the He shell to the highly-charged core atoms activates the nanodroplet and thereby facilitates the ignition of a He nanoplasma by the NIR pulse, as shown by molecular dynamics simulations. This is experimentally evidenced by the ultrafast rise of the He nanoplasma signals within 70 fs. At the same time, the expansion of the cluster core is strongly delayed, which demonstrates that He droplets may serve as tampers that reduce radiation damage of embedded nanostructures in x-ray imaging experiments.

Poster session

01. Branchi, Federico & Mikosch, Jochen Ionization Channel-Resolved Molecular Orbital Imprint in Laser-Driven Electron Rescattering 02. Dietrich, Christian Markus Brunel radiation from semiconductor nanostructures 03. Eickhoff, Kevin Dynamic quantum state holography 04. Henke, Jan-Wilke Probing structural chirality using coherent electron-light interaction 05. Hergert, Germann Observing ultrafast charge deflections of slow electrons in a nanoresonator 06. Karamatskos, Evangelos Optimization of field-free alignment of molecules for imaging experiments 07. Kerbstadt, Stefanie Control of molecular alignment using tailored picosecond laser pulses 08. Lourenco-Martins, Hugo Towards exciton mapping in an ultrafast electron microscope 09. Mhatre, Saurabh Dynamics of HeH⁺ in Long-Wavelength Intense Laser Fields 10. Nourbakhsh, Zahra Isolated attosecond pulse by solid target: Ab-initio study of two-color laser pulse 11. Shah, Ronak Petahertz field reconstruction for the investigation of electronic dynamics in nanostructures 12. Yue, Shengjun Probing Coulomb time delays in high harmonic generation 13. Zhu, Xiaosong Non-adiabatical geometric phase and high harmonic generation in solids 14. Ziems, Karl Michael Attosecond pulse induced dynamics in a molecular charge-transfer system with correlated electrons

Participants

Name Institution E-Mail Max-Born-Institut Ayuso, David [email protected] Berlin Azizi, Sajjad MPIPKS Dresden [email protected] [email protected] Babushkin, Ihar Universität Hannover hannover.de Max-Born-Institut Bengs, Ulrich [email protected] Berlin Debatin, Markus Universität Kassel [email protected] [email protected] Despré, Victor Universität Heidelberg heidelberg.de Dietrich, Christian Universität Hannover [email protected] Markus Weizmann Institute Dudovich, Nirit [email protected] of Science kevin.eickhoff@uni- Eickhoff, Kevin Universität Oldenburg oldenburg.de Max-Born-Institute Elliott, Peter [email protected] Berlin Universität Duisburg- andrea.eschenlohr@uni- Eschenlohr, Andrea Essen due.de armin.feist@uni- Feist, Armin Universität Göttingen goettingen.de thomas.fennel@uni- Fennel, Thomas Universität Rostock rostock.de [email protected] Gebauer, Andreas Universität Bielefeld bielefeld.de Gräfe, Stefanie Universität Jena [email protected]

Groß, Petra Universität Oldenburg [email protected]

Name Institution E-Mail

Gühr, Markus Universität Potsdam [email protected]

Harth, Anne MPI-K Heidelberg [email protected] [email protected] Henke, Jan-Wilke Universität Göttingen goettingen.de Universität Hergert, Germann [email protected] Oldenburg Max-Born-Institut Ivanov, Mikhail [email protected] Berlin Max-Born-Institut Jiménez, Álvaro [email protected] Berlin Karamatskos, DESY Hamburg [email protected] Evangelos Kerbstadt, Stefanie DESY Hamburg [email protected]

Kim, Doyeong Uni Jena [email protected]

Klemke, Nicolai DESY Hamburg [email protected]

Kübel, Matthias Universität Jena [email protected] Universität [email protected] Kuleff, Alexander Heidelberg heidelberg.de Lein, Manfred Universität Hannover [email protected] Lourenco-Martins, hugo.lourenco-martins@uni- Universität Göttingen Hugo goettingen.de Universidad Martin, Fernando [email protected] Autonoma de Madrid [email protected] Medina, Cristian Universität Freiburg freiburg.de Participants

Name Institution E-Mail

Mhatre, Saurabh Universität Jena [email protected] Max-Born-Institut Mikosch, Jochen [email protected] Berlin Universität Morgner, Uwe [email protected] Hannover Mücke, Oliver DESY Hamburg [email protected] Universität Müller, Niklas [email protected] Oldenburg Nourbakhsh, Zahra MPSD Hamburg [email protected] Universität [email protected] Oppermann, Florian Hannover hannover.de Max-Born-Institut Ordonez, Andres [email protected] Berlin Universität Paschen, Timo [email protected] Erlangen-Nürnberg Paulus, Gerhard G. Universität Jena [email protected]

Pfeiffer, Adrian Universität Jena [email protected]

Pfeiffer, Walter Universität Bielefeld [email protected] Universität Rao, Han [email protected] Hannover Max-Born-Institut Rouzee, Arnaud [email protected] Berlin Humboldt- [email protected] Saenz, Alejandro Universität Berlin berlin.de [email protected] Sansone, Giuseppe Universität Freiburg freiburg.de

Name Institution E-Mail lennart.seiffert@uni- Seiffert, Lennart Universität Rostock rostock.de [email protected] Shah, Ronak Narendra Universität Freiburg freiburg.de Max-Born-Institut Sharma, Sangeeta [email protected] Berlin [email protected] Shukla, Archana Freie Universität Berlin kassel.de Tancogne-Dejean, nicolas.tancogne- MPSD Hamburg Nicolas [email protected] Max-Born-Institut marc.vrakking@mbi- Vrakking, Marc Berlin berlin.de Max-Born-Institut tobias.witting@mbi- Witting, Tobias Berlin berlin.de matthias.wollenhaupt@uni- Wollenhaupt, Matthias Universität Oldenburg oldenburg.de Wustelt, Philipp Universität Jena [email protected]

Yachmenev, Andrey DESY Hamburg [email protected] [email protected] Yue, Shengjun Universität Hannover hannover.de Zak, Emil DESY Hamburg [email protected]

Zhang, Jin MPSD Hamburg [email protected] Max-Born-Institut Zhavarankau, Mikalai [email protected] Berlin [email protected] Zhu, Xiaosong Universität Hannover hannover.de karl-michael.ziems@uni- Ziems, Karl Michael Universität Jena jena.de Berlin public transport

With a valid ticket, ticket holders have access to all public transport in Berlin: S-Bahn, U-Bahn, buses, trams and ferries. The fare depends on the tariff zone and the ticket's period of validity.

Tariff Zones & Network Maps: Berlin is divided into three tariff zones: AB, BC und ABC. Tariff zone AB includes the urban area to the city boundary. Zone ABC additionally includes Berlin's surrounding area and Potsdam Hauptbahnhof. The conference location at Berlin-Adlershof is still within tariff zone B, so you need an “AB-ticket” if you want to go there from Berlin center. Tegel airport is also still within the zones AB. If you have to go from or to Berlin-Schönefeld airport, you need to include zone “C” in your ticket.

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Conference dinner on Thursday at 7.30 pm

The conference dinner will take place on Thursday, 27th February 2020, at 7.30 pm at the Restaurant „Hackescher Hof“ in Berlin city center.

For those conference participants who will attend the Reichstag tour from 5.30 pm on the same day, there will be a bus transfer from the Reichstag to the restaurant.

If you will not attend the Reichstag tour, here are directions to go to the “Hackescher Hof”:

Hackescher Hof Rosenthaler Str. 40/41 10178 Berlin – Mitte Hakescher Markt

The restaurant "Hackescher Hof" is located directly at the Hackescher Markt in Rosenthaler Straße 40/ 41 and forms the entrance to the famous Hackesche Höfe, an ensemble of old Berlin style courtyards (the largest single courtyard complex in Germany).

The easiest way to reach the restaurant is to go by S-Bahn to S-Bahn station “Hackescher Markt” (direct connection by S9 from S-Bahn station Adlershof) and then just walk 5 minutes via the Hackescher Markt square to the Hackeschen Höfe.

Schedule

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