DFG Priority Programme 1840 QUTIF Quantum Dynamics in Tailored Intense Fields

QUTIF International Conference 22-25 February 2021

The QUTIF International Conference 2021 brings together experts from countries worldwide working on
  • atomic, molecular and optical science with strong ultrashort pulses
  • chemical control with tailored fields
  • strong-field dynamics in new media
  • attosecond science.
Registration is open for all interested researchers in these fields. The conference serves also as a platform for presenting the work of the DFG QUTIF Programme to an international audience and for international scientific exchange.
Due to the pandemic, the conference will be held online. Talks and posters will be presented electronically using video conference software. To this end, it is planned to use the systems BigBlueButton and Jitsi, possibly WebEx.

Invited speakers

  • Thomas Brabec (University of Ottawa)
  • Oren Cohen (Technion, Haifa)
  • Paul Corkum (University of Ottawa)
  • Baptiste Fabre (Université de Bordeaux)
  • Carla Faria (UCL)
  • Shambhu Ghimire (Stanford University)
  • Hugo van der Hart (Queen's University Belfast)
  • Igor Litvinyuk (Griffith University, Brisbane)
  • Hongcheng Ni (East China Normal University, Shanghai)
  • Jean-Michel Raimond (Physical Review X and Laboratoire Kastler Brossel, Paris)
  • Paris Tzallas (FORTH, Crete)
  • Thomas Weinacht (Stony Brook University)

Schedule (click on name to see abstract)

Time (UTC+1)


Feb 22nd


Feb 23rd


Feb 24th


Feb 25th


Paul Corkum

Paul Corkum (University of Ottawa)

Exploiting quantum interference to generate THz B-fields

Coherent control relies on two-path (or more) quantum interference to control the outcome of a quantum process – a chemical reaction or a current in a semiconductor. It has a classical analogue. We use the interference of currents created by a single photon transition in GaAs with frequency 2ω and a two-photon transition of frequency ω, to produce interfering pathways that directs the current in one direction or the other. Using azimuthally polarized beams (or a spatial-light-modulator-controlled linearly polarized light together with a circularly polarized fundamental) coherent control can generate ring currents and an associated solenoidal magnetic field which should radiate as a “flying torus”. We also study the classical limit where we predict THz magnetic fields reaching 8 Tesla.

Jan Michael Rost

Jan Michael Rost (MPIPKS Dresden)

High harmonics from backscattering of delocalized electrons in finite solid-state systems

In atomic systems it is well known that through backscattering from the ion a laser driven photo electron can gain kinetic energy Up to 10 Up, where the ponderomotive energy Up characterizes the dynamics of a free electron in an intense laser field. This is much higher than the cutoff energy of 3.2 Up for high harmonic generation (HHG). We show that in a finite solid-state system, specifically in a finite chain of atoms, backscattering occurs as well. However, in contrast to atomic systems, it leads to an extension of the cutoff for HHG. Moreover, such chains exhibit equivalent strong field electron dynamics for the same ratio of their length and the wavelength of the laser light highlighting the importance of spatial scales for electrons in finite, solid-state-like systems.

Philip Dienstbier

Philip Dienstbier (University of Erlangen), Timo Paschen, Lennart Seiffert, Thomas Fennel, Peter Hommelhoff

Sub-cycle ionization and propagation dynamics in the two-color near field of a metal tip

Two-color laser fields with well-defined relative phase have enabled deep insights into strong-field physics at atoms [1]. Here, we show that few-cycle two-color laser fields can be employed to probe and disentangle field-driven from ionization-related [2] electron dynamics at the surface of a nanometer sharp tungsten needle tip. By measuring electron energy spectra as a function of the relative phase, we observe two distinct signatures. First, a modulation of the high-energy cut-off position and second, a characteristic phase dependency of the maximum yield as a function of electron energy. The comparison with 1D-TDSE simulations [3] shows excellent agreement. By combining instantaneous ionization rates derived from the quantum simulations with a classical trajectory analysis one can explain both signatures by the energy gain in the two-color near field and the temporal width of the emission window. The analysis allows us to quantify the second harmonic near field admixture of 19 ± 3 % and an emission width of 670±30 as.

[1] Shafir, D. et al. Nature 485, 343 (2012).
[2] Förster, M. et al. PRL 117, 217601 (2016).
[3] Seiffert, L. et al. J. Phys. B. 51, 134001 (2018).

Germann Hergert

Germann Hergert (University of Oldenburg)

Electron near-field dynamics between the sub-cycle and quiver regime

The dynamics of photoemitted electrons in the near field of nanostructures has been intensively studied during the past decade. Two limiting cases are already well explored, i.e. the sub-cycle and the quiver regime. In the subcycle regime, the electron is unidirectionally accelerated through the short-ranged near field while, in the quiver regime, it undergoes an oscillatory motion with the period of the driving field. The transition between both regimes is controlled by varying the near-field frequency, strength and decay length. Recently, we finalized a tunable high-repetition rate NOPA-DFG system that provides short and CEP-stable laser pulses around 2µm. We use phase-locked pairs of such pulses to record the energy spectra of electrons that are photoemitted from a sharp metal nanotaper. By finely tuning the pulse delay we see that the final spectra are not trivially depending on the field strength during the transition from the subcycle to quiver regime, revealing a new, intermediate near-field few-cycle acceleration regime. We give further insights into these results by simulating the emission process and the subsequent electron dynamics with TDSE and classical simulations.

H. Lourenço-Martins

Hugo Lourenço-Martins (University of Göttingen)

Towards the coherent control of plasmonic near-fields in an ultrafast transmission electron microscopy

Plasmonic nano-resonators enable the generation of intense and localized electromagnetic fields with applications in e.g. quantum information. Hence, an important goal is to determine how the plasmonic fields can be shaped by tailoring the optical excitation. In this talk, we will demonstrate that plasmonic near-fields can be mapped and quantitatively analyzed in an ultrafast transmission electron microscope. In a first part, we will show that a boundary element method based data analysis of optical near-field maps enables to reconstruct the population and relative phase of each plasmonic mode excited by a femtosecond pump laser. We will exemplify this technique on gold nano-triangles and demonstrate that the total plasmonic near-field results from the contribution of a large number of plasmonic modes which can be precisely controlled by tuning the laser properties. In a second part, we will theoretically and experimentally demonstrate that the optical near-field can be coherently manipulated by pumping the system with two phase-locked optical pulses of different wavelength, which enables to create a controlled beating pattern between two different modes of a single resonator.

Walter Pfeiffer
After-dinner talk

Walter Pfeiffer (Bielefeld University)

The complexity trap — skepticism, denialism and the role of non-expert scientists in the climate debate

Confronted with climate denialist arguments that sounded scientifically sound on a first glimpse I started researching the scientific case of climate change in a bit more detail. It turned out as a rather mixed experience. Climate science is an immensely broad, interdisciplinary, complex, and politically loaded field. It is not a simple task to find your way through it being not an expert. Discriminating between science, fake science, and bad science in such a complex matter is challenging, and the skepticism that works well in your own field of expertise failed big many times. Still a lot of non-expert scientists - quite a few of them either skeptics or denialists - participate in the public discussion. The presentation summarizes my experiences, puts them in the larger context of denialism to identify its basic mechanisms, and draws conclusions for a responsible role of scientists in a public debate.

Disclaimer: This talk is not about climate science or climate change, the reported issues are much broader and climate science acts merely as an example.

Hongcheng Ni

Hongcheng Ni (ECNU, Shanghai)

Strong-field dynamics time-resolved by backpropagation

Strong-field ionization is the common cornerstone of various interesting strong-field and attosecond phenomena and thus has its fundamental importance in attosecond physics. The backpropagation method is a hybrid quantum-classical method specifically tailored to investigate strong-field ionization, which retains the quantum nature of the ultrafast dynamics while providing clear local information regarding the time evolution. In this talk, we briefly overview our recent works employing the backpropagation method, including the studies of tunneling time delay, orbital deformation, subcycle linear momentum transfer, and over-barrier ionization. We show that the backpropagation method offers the unique perspective of the quantum dynamical evolution of strong-field processes inaccessible otherwise.

Hugo van der Hart

Hugo van der Hart (Queen's University Belfast)

Atomic dynamics in short arbitrarily polarised light fields

The description of multi-electron dynamics in intense short light fields remains a significant challenge to computational atomic physics. When the interaction between electrons is strong, the coupled motion of electron pairs can significantly affect the overall dynamics of the system. Over the past 15 years, we have developed R-matrix with time-dependence (RMT) theory to enable the investigation of such dynamics in multi-electron atoms and molecules.

Recently [1], we have made the RMT codes available to the community. Over the last few years, we have extended the codes to allow systems to be investigated in arbitrarily polarised light fields. We have also adapted the codes to use input data generated by the Breit-Pauli R-matrix approach to enable heavier atomic systems to be described with inclusion of the effects of the spin-orbit interaction. In this presentation, we will highlight some of our recent studies using the RMT approach, which highlight these new capabilities for arbitrary polarization and heavy atoms.

[1] A.C. Brown et al, Computer Physics Communications 250, 107062 (2020).

Paraskevas Tzallas

Paraskevas Tzallas (FORTH, Heraklion)

Linking quantum optics with strong-laser-field physics

Strong-laser-field physics (SLFP) and quantum optics (QO), are two disjoined research areas founded on the classical and quantum description of the electromagnetic radiation, respectively. SLFP [1] led to groundbreaking discoveries ranging from relativistic electron acceleration to attosecond science, while QO opened the way for fascinating achievements in quantum technology [2] advancing studies ranging from fundamental test of quantum theory to quantum information processing. Despite the progress achieved in both research areas, they remained disconnected over the years. Here, after a brief introduction of SLFP and QO, I will present how we have managed to connect these domains [3-6] and build the foundations for studies of quantum electrodynamics in strong-field physics and the development of a new class of non-classical light sources for applications in quantum technology.

[1] G. Mourou, Rev. Mod. Phys. 91, 030501 (2019).
[2] A. AcÍn, et al. New J. Phys. 20, 080201 (2018).
[3] N. Tsatrafyllis, et al., Nat. Commun. 8, 15170 (2017).
[4] N. Tsatrafyllis, et al., Phys. Rev. Lett. 122, 193602 (2019).
[5] M. Lewenstein et al., arXiv:2008.10221 (2020).

Jean-Michel Raimond

Jean-Michel Raimond (Physical Review X / LKB Paris)

The Physical Review

I will present the Physical Review journals family and particularly Physical Review X, the Open Access journal with the highest impact in physics.

Poster preview

Poster presenters

Brief poster intros

Poster session 1

Poster presenters


Shambhu Ghimire

Shambhu Ghimire (Stanford PULSE Institute)

High-order harmonic generation from topological insulators

High-order harmonic generation (HHG) is a fascinating strong-field phenomenon originally identified in gaseous media and more recently in condensed-matter systems. Recent studies in solid materials suggest that the underlying microscopic mechanism depends strongly on the type of solid [1]. In this context, three-dimensional topological insulators (TI) form a whole new category of material as they have insulating bulk bands and time-reversal symmetry protected conductive Dirac-like surface bands. In this talk, I will give a brief overview of the field and present our latest results on HHG from Bi2Se3 [2]. Our study reveals that the generation from the surface is significantly more efficient when the laser pulse is circularly polarized. The enhancement is linked to the band topology and strong spin-orbit couplings.

[1] “Review: High-harmonic generation from solids”, S. Ghimire and D. Reis, Nature Physics 15, 10-16 (2019).
[2] “Strong-field physics in three-dimensional topological insulators”, D. Baykusheva et al., Phys. Rev. A 103, 023101 (2021).

Zahra Nourbakhsh

Zahra Nourbakhsh (MPSD Hamburg), Ofer Neufeld, Nicolas Tancogne-Dejean, Angel Rubio

Theoretical investigation of high-harmonic generation from liquid water

Liquids are promising systems in ultrafast technology since they gather the advantages of both solids and gases in one material. Liquids are condensed systems, that, like gases, can tolerate highly intense driven pulses. We study the non-perturbative interaction of intense infrared laser pulses with liquid water to investigate its high-harmonic emission. Our method is on the basis of real-time density functional theory. The liquid-water structure is reproduced using Car-Parrinello molecular dynamics simulation. Our calculations predict the generation of high harmonics up to extreme ultraviolet energies. Similar to solids, the harmonic cutoff scales linearly with the peak field strength; and it is independent of pulse wavelength. In addition, we discuss the impact of nuclear dynamics during the pulse radiation on harmonic emission. By considering two structures of ice, cubic and hexagonal, we also show how harmonic emission is reduced in liquid water due to the loss of long-range order. Our results provide deep fundamental insights into the electron dynamics in liquids, opening the door to the development of ultrafast technologies based on strong-field driven liquids.

Ofer Neufeld

Ofer Neufeld (MPSD Hamburg)

Ab-initio cluster approach for high-harmonic generation in three-dimensional liquids: characteristic features

High harmonic generation (HHG) takes place in all phases of matter. In gases it has been extensively studied and is well-understood. In solids research is ongoing, but a consensus is forming about the dominant HHG mechanisms. In liquids however, no theoretical model exists yet, and approaches developed for gases and solids are generally inapplicable. Here there are many open questions such as cutoff scaling laws, the dominant HHG mechanisms, and more. Advancement on this front, which may lead to novel light sources and ultrafast spectroscopies, is hindered by the lack of theoretical frameworks for liquids interacting with strong fields. We present an ab-initio cluster approach for the interaction of liquids and intense light. We employ it to study liquid-HHG in water, ammonia, and methane, and compare the response of polar and non-polar liquids. We analyze the temporal and spectral structure of liquid-HHG and find that the spectrum routinely separates into two plateaus that are generated by distinct mechanisms. A semi-classical model is proposed to explain our findings. Our work paves the way to feasible calculations of liquid-HHG, and illustrates the unique nonlinear nature of liquids.

Vladimir Nazarov

Vladimir Nazarov (Hebrew University of Jerusalem)

High-frequency limit of spectroscopy

The high-frequency limit of spectroscopy is derived leading to a new analytic technique for molecules and materials. A system comprised of interacting electrons and exhibited to the external potential V(r,t)=R(r) C(t) cos(ω0 t) is considered. We find that, at asymptotically large ω0, UPON THE END OF THE PULSE, the leading term in the transition amplitude to a state |α〉 is proportional to ω0-n C2(Eα-E0), where C2(ω) is the Fourier transform of the envelope function squared C2(t). Furthermore, n=2 for all forms of the potential R(r), except for n=4 for the dipole case R(r)=-E·r. The linear response (LR) is suppressed while the quadratic response governs the excitations. Application to jellium slab and sphere reveals a richer excitation spectra than accessible in the LR regime and high surface sensitivity of the method. Despite the nonlinearity, observables can be conveniently expressed in terms of the linear density response function χ(r,r',ω), allowing for the use of the efficient machinery of LR TDDFT. The new technique, based on our findings, is envisaged to evolve into a powerful characterization method in nanoscience and nanotechnology.


Oren Cohen

Oren Cohen (Technion, Haifa)

New (chiral) light on selection rules in high-harmonic generation

Selection rules in nonlinear optics determine which radiation modes are allowed/forbidden in nonlinear processes. Traditionally, the theory accounts for symmetries of the medium only. I will present a general theory in which the selection rules are derived from the space-time symmetries of the entire system: medium and light, including microscopic and macroscopic degrees of freedom. I will also present selection rules for breaking selection rules – these are laws that describe the allowed and forbidden behavior for deviations from selection rules due to symmetry breaking perturbations. I will then present applications of the new theory for ultrafast spectroscopy, focusing on diagnostics of chirality.

Emil Zak

Emil Zak (CFEL Hamburg)

Dynamic chirality of deuterium sulfide

We are going to discuss a computational study of dynamic chirality in molecules. A specially designed sequence of corkscrew laser pulses excites an asymmetric top D2S molecule (deuterium sulfide) along a selected rotational energy path to populate highly-stable chiral states. We demonstrate that such an emergent (dynamic) chirality in statically non-chiral systems can be controlled with the use of weak static dc electric fields and detected with Coulomb explosion imaging or with photo-electron circular dichroism.

Andres Ordonez

Andres Ordonez (MBI Berlin)

Structuring light's chirality: the chiral double-slit experiment

Structured light, exhibiting nontrivial intensity, phase, and polarization patterns in space, has key applications ranging from imaging and 3D micro-manipulation to classical and quantum communication [1]. However, to date, its application to molecular chirality has been limited by the weakness of magnetic interactions [2]. Here we structure light's local handedness in space to introduce and realize an enantio-sensitive interferometer for efficient chiral recognition without magnetic interactions, which can be seen as an enantiosensitive version of Young’s double slit experiment. Upon interaction with isotropic chiral media, such chirality-structured light leads to unidirectional bending of the emitted light, in opposite directions in media of opposite handedness. Our work [3] introduces the concepts of polarization of chirality and chirality-polarized light, exposes the immense potential of sculpting light’s local chirality, and offers novel opportunities for efficient chiral discrimination, enantiosensitive optical molecular fingerprinting and imaging on ultrafast time scales.

[1] J. Opt. 19, 013001 (2016).
[2] J. Phys. Chem. A118, 3472 (2014).
[3] arXiv:2004.05191 (2020).

Felix Ritzkowsky

Felix Ritzkowsky (CFEL Hamburg)

Integrated attosecond time-domain spectroscopy

Time-domain sampling of arbitrary electric fields with sub-cycle resolution enables a complete reconstruction of the microscopic polarization response of matter to electromagnetic radiation. Despite the many scientific motivations, time-domain, optical-field sampling systems operating in the visible to near-infrared spectrum are seldom accessible, requiring sophisticated laboratory setups. Here, we demonstrate an all-on-chip, optoelectronic device capable of sampling arbitrary, low-energy, near-infrared waveforms under ambient conditions. Our solid-state integrated detector uses optical-field-driven electron emission from resonant nanoantennas to achieve petahertz-level switching speeds by generating on-chip attosecond electron bursts. We demonstrate our method by sampling the electric field of a ~5 fJ, broadband near-infrared ultrafast laser pulse by using only 50 pJ near-infrared gate pulses to drive our devices. Our sampling measurements recovered the weak optical transient as well as localized plasmonic dynamics of the emitting nanoantennas in situ. This field-sampling device offers opportunities for detailed study of nonlinear light-matter interaction.

Fabian Scheiba

Fabian Scheiba (CFEL Hamburg)

Shaping sub-cycle optical waveforms to generate tunable isolated attosecond light pulses

To reveal the electron dynamics occurring on ever shorter time scales, attosecond pulses generated via laser high-harmonic generation in inert gases is employed. Exquisitely controlled sub-cycle waveforms seem ideal for efficient isolated attosecond pulse generation. An approach via laser parametric waveform synthesis allows to create and shape intense electric field transients on the sub-cycle scale well suited for producing tunable isolated attosecond pulses. We demonstrate pulses down to 0.6 optical cycles of highly non-sinusoidal shape (1.5 octaves centered at 1.4 µm). The subsequent high-harmonic emission leads to broadband, isolated attosecond pulses with up to two octaves of bandwidth as well as narrow-band pulse continua with tunable central energy.

Ihar Babushkin

Ihar Babushkin (Leibniz University Hannover)

Imaging electron dynamics using Brunel harmonics

Brunel harmonics are radiated in process of electron ionization and subsequent acceleration in the continuum, induced by laser-induced-ionization. This radiation is, in a certain sense, an instantaneous process and because of this contains fingerprints of the attosecond dynamics of the electron wavepacket on its way to the continuum. We show how this can be used as a tool for electron wavepacket imaging, alternative to measuring the electrons directly or to measure recollision-based harmonics.

Marcel Mudrich

Marcel Mudrich (Aarhus University)

Collective enhancement of above-threshold ionization in resonantly excited helium nanodroplets

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we study energetic electrons generated by above-threshold ionization (ATI) of resonantly excited helium nanodroplets. The latter are excited by ultrashort XUV free-electron laser pulses and subsequently ionized by near infrared or visible pulses. We observe that energetic electrons generated by high-order ATI are enhanced by several orders of magnitude in excited helium droplets compared to helium atoms. A nonlinear scaling of the high-order ATI intensity as a function of the number of excitations per droplet suggests a local collective enhancement.

Poster session 2

Poster presenters


Thomas Weinacht

Thomas Weinacht (Stony Brook University)

Coherent control of non-adiabatic dynamics in molecules

Electronic coherence decays much more rapidly in molecules than atoms as a consequence of averaging over the different rates of phase advance for different internuclear separations associated with nonlocal nuclear wavefunctions. This is particularly dramatic for polyatomic systems that have many degrees of vibrational freedom over which this averaging occurs. Furthermore, if there is non-adiabatic coupling between electronic states, then it is not clear how the phase between states is maintained. I will discuss experiments that show via interference that the electronic coherence of a wave function in a molecule that undergoes internal conversion via non-adiabatic (non-Born Oppenheimer) coupling between electronic states is maintained [1]. The measurements demonstrate that this coherence can be used to control the population of different electronic states, and measure its decay time.

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.053202

Francoise Remacle

Francoise Remacle (University of Liege)

Steering selective bond formation with short optical pulses: quantum dynamics of a four-center ring closure

Few-cycle short strong pulses allow excitation of coherently coupled electronic states towards steering nuclear motions in neutral molecules and cations. We report on bond formation induced by an ultrashort UV pulse [1]. We investigate quantum mechanically the coherent electronic and nuclear motions during the ring closure of norbornadiene to quadricyclane induced by a short UV pulse. Norbornadiene consists of two ethylene moieties connected by a rigid C3H4 bridge. The short fs UV pulse yields a non-equilibrium electronic density in the open norbornadiene that evolves towards the closed four-atom ring quadricyclane. As the coherent vibronic dynamics unfold, the Rydberg excited states of norbornadiene change character through non-adiabatic interactions and become valence states for the two new C-C bonds of quadricyclane. We show that short UV pulses of different polarization allow tailoring markedly different initial non-equilibrium electronic densities that follow different dynamical paths, thereby opening the way to controlling bond-making by selective pumping of electronic states with attopulses.

[1] A. Valentini, S. van den Wildenberg, and F. Remacle, PCCP 22, 22302-22313 (2020).

Victor Despré

Victor Despré (University of Heidelberg)

Decoherence and revival of attosecond charge migration in silane: A joint experimental and theoretical study

The advent of attosecond physics allowed the observation and manipulation of dynamic processes occurring within the intrinsic time scale of the charge motion in atoms and molecules. This has opened the door to the realization of the dream of attochemistry, namely to control chemical reactions through the manipulation of the pure electron dynamics taking place in the first instants after the excitation of the system. Thereby, the existence of long-lasting electronic coherences in molecular systems is a key prerequisite to its realization. Furthermore, understanding the mechanism leading to or preventing the loss of coherence is necessary for its development.

The first measurement of a decoherence and revival in attosecond charge migration will be presented. This dynamics occurs after excitation of silane (SiH4) by an IR pulse. Simulations treating quantum mechanically both the electronic and nuclear degrees of freedom permitting the interpretation of the experimental results will be discussed. Using these simulations, the behavior of the coherence and the possibility to conserve coherence trough conical intersection will be rationalized.

Igor Litvinyuk

Igor Litvinyuk (Griffith University, Brisbane)

Frustrated tunneling ionization: experimental observation of AC-Stark resonances in strong-field excitation

We investigate strong-field excitation in the cross-over regime between multiphoton and tunnelling with intense near-infrared pulses. We observe periodic enhancements in the excitation yield in the region of the 14th and 16th channel closings that are attributed to AC Stark resonances. These enhancements have not been previously observed and critically demonstrate the existence of channel closing in the tunneling regime. Further, by reducing the pulse duration such that it contains only a few optical cycles, we demonstrate that these resonances become ambiguous due to the large bandwidth of the pulse. We show that at the location of the enhancements there exists an interplay of the AC Stark shift of both the continuum threshold and individual excited states which is explained by considering the relative population of (n, l) states as a function of the laser intensity. Finally, by following the motion of the wavepacket in the laser field and its overlap with the distribution of excited states, we show that in this regime tunneling dynamics must be included to fully explain the excitation.

Hendrike Braun

Hendrike Braun (University of Kassel)

Excited-state Rabi-cycling near the ionization threshold after multiphoton excitation - a general concept?

Control schemes using ultrashort laser pulses in the weak-field regime such as the basic In behaviour and spectral interference as well as in the strong-field regime such as Rabi-cycling, rapid adiabatic passage, photon locking or selective population of dressed states are well established. The excitation of molecules and atoms with near-IR fs laser pulses can merge these regimes: In many cases the first excitation step is predominantly non-resonant (weak-field interaction) while the increasing density of states near the ionization threshold results in the possibility of subsequent one-photon absorptions which can lead to excited state Rabi-cycling (strong-field interaction). To investigate coherent dynamics of atoms and molecules in this combination region we employ 2D strong-field spectroscopy using sequences of phase modulated fs laser pulses. Varying the relative optical phase and the temporal separation between adjacent pulses allows to look at the coherent response of the atomic level as well as molecular systems in the form of electronic coherences. Comparison of our experimental results with simulations confirms the subsequent weak- and strong-field behaviour.

Nicola Mayer

Nicola Mayer (MBI Berlin)

Probing Rydberg states in non-collinear bicircular high-harmonic generation via the spin-orbit coupling

The role of Rydberg states in high-harmonic generation (HHG) has been long investigated but remained quite elusive to experimental detection until recently [1-3]. By employing a bichromatic bicircular (ω,2ω) electric field as the driver for the HHG process, we have shown experimentally and theoretically in a previous publication [2] that excitation of Rydberg states can lead to symmetry-forbidden harmonic orders by breaking the field-imprinted dynamical symmetry of the attosecond bursts of radiation. Here, we extend on our previous results by using a non-collinear (ω,2ω) setup in Argon, which allows us to spatially separate in the far-field the helical components of the harmonic spectrum and obtain the relative strength of the forbidden harmonic orders to the allowed one as a function of the time-delay between the ω and 2ω fields. Moreover, we observe a splitting of the harmonic spectrum when the 2ω field precedes the ω one, whose value is consistent with the spin-orbit splitting in Argon. By comparing the experimental results with theoretical simulations using the strong-field approximation on one side and the numerical solution of the time-dependent Schrödinger equation on the other, in this contribution we show that the experiment can be fully comprehended only by including the Rydberg states in the HHG process. Most strikingly, we conclude that the splitting of the harmonic spectrum is due to the trapping of the electron in the excited states, which provides enough time for the spin of the ion core to precess and resolve the spin-induced slow modulations of the time-dependent dipole moment. Our results thus show that it is possible to probe the Rydberg states in HHG by using the inherent clock of the spin-orbit coupling.

[1] S. Beaulieu et al., Role of excited states in High-Harmonic Generation, Phys. Rev. Lett., 117, 203001 (2016).
[2] A. Jiménez-Galàn et al., Time-resolved high harmonic spectroscopy of dynamical symmetry breaking in bi-circular laser fields: the role of Rydberg states, Opt. Exp., 25, 19 (2017).
[3] E. Bloch et al., Hyper-Raman lines emission concomitant with high-order harmonic generation, New. J. Phys., 21, 073006 (2019).

Baptiste Fabre

Baptiste Fabre (CELIA - Université de Bordeaux)

What dynamical information can time-resolved PECD reveal?

After a short introduction on photoelectron circular dichroism (PECD) measurements in chiral molecules, a time-resolved study in camphor and fenchone, supported by numerical simulations, will be presented. By comparing temporal evolution of the PECD and its angular distribution in these two species, we will examine the influence of rovibrationnal degrees of freedom and isomerism effects on the dynamics.

E. Karamatskos

Evangelos Karamatskos (CFEL Hamburg)

Toward ultrafast molecular imaging in the molecular frame

Imaging the ultrafast dynamics of molecules requires experimental methods that offer atomic spatial and (sub-)femtosecond temporal resolution. The possibility to prepare cold, controlled molecular samples in the gas phase, combined with elaborate methods to fix the molecules in space, are important prerequisites to image molecular dynamics directly in the molecule-fixed frame. Here, I will present our work toward the precise spatiotemporal imaging of the prototypical UV-induced photodissociation dynamics of carbonylsulfide (OCS). This includes the time-resolved ion-imaging of the UV-initiated photodissociation dynamics of OCS, strong field-free alignment of OCS, allowing to fix the molecules in space and to access the molecular frame, as well as measurements of angularly-resolved photoelectron momentum distributions of aligned and isotropic OCS. An overview over the achieved results will be presented and an outlook given of how to proceed toward recording a molecular movie by combining the discussed imaging methods.

Patrick Rupprecht

Patrick Rupprecht (MPIK Heidelberg)

Controlling multi-electron interaction in molecules with ultrashort laser pulses

While electron-electron interactions play a fundamental role in any atom beyond hydrogen, they also govern molecular structure and reactivity. We introduce and experimentally demonstrate a general concept to control multi-electron interaction by intense, ultrashort laser fields. In particular, strong coupling to excited states allows to modify the effective exchange energy by infrared(IR)-induced valence-orbital mixing. For a proof-of-principle, we focus on the sulfur hexafluoride molecule SF6, considering the coupling of a sulfur 2p core hole with a valence-excited electron on the few-femtosecond timescale, using a combination of soft x-ray and IR laser pulses. The IR laser intensity represents a control knob to tune the effective exchange interaction energy, resulting in a characteristic change in the spin-orbit-split oscillator-strength ratio that is directly quantified in the experiment. Such direct control of effective electronic interactions and correlation is a significant step towards laser-directed chemistry on the fundamental electronic level with single-atomic site selectivity.

Carla Faria

Carla Faria (University College London)

Dissecting quantum interference in ultrafast photoelectron hologaraphy

Similarly to traditional holography, ultrafast photoelectron holography explores quantum phase differences between qualitatively different electron pathways to extract information about a target. Unfortunately, however, the residual binding potential is neglected in most theoretical models of the interference patterns encountered. This is problematic as its presence radically alters the electron dynamics. In this presentation, I will analyze a myriad holographic patterns, known and overlooked, in terms of electron trajectories using an orbit-based method that includes the binding potential and the laser field on equal footing: the Coulomb Quantum-orbit Strong-Field Approximation (CQSFA) [1]. Subsequently, I will provide applications and illustrate how photoelectron holography may be used to detect the parity of atomic and molecular orbitals [2], and introduce a novel spiral-like holographic structure highly sensitive to phase differences [3].

[1] A. S. Maxwell et al, Phys. Rev. A 96, 023420 (2017); J. Phys. B 51, 124001 (2018).
[2] H. Kang et al, Phys. Rev. A 102, 013109 (2020).
[3] A. S. Maxwell et al, Phys. Rev. A 102, 033111 (2020).

Diego Arbó

Diego Arbó (CONICET - University of Buenos Aires)

Quantum holography in atomic photoelectron spectra

When an intense short laser pulse interacts with an atom ionizing it, the photoelectron spectra present several structures that can be understood as double-slit interferences in the time domain, namely intra- and inter-cycle interferences [Phys. Rev. A 82, 043426]. Another type of structures requires interference of direct electron trajectories with others that interact with the parent core. In a classical picture, the rescattering trajectories return to the parent ion driven by the laser field and rescatter off to the detector. Some structures can be interpreted as holograms, i.e. the interference pattern between the direct (reference) and the rescattered (signal beam) electrons. In this way, the information of the interaction is encoded in the interference pattern between the reference and the signal. In this talk, we present a theoretical analysis of interferences using a numerical solution of the time dependent Schrödinger equation (TDSE) and the semiclassical two-step model (SCTS). We analyze the ionization of atomic hydrogen to characterize the role that the long-range Coulomb interaction plays in the holographic structures [Phys. Rev. A 100, 023419 (2019)].

S. Eckart & D. Trabert

Sebastian Eckart/Daniel Trabert (Goethe University Frankfurt)

Angular dependence of the Wigner time delay upon tunnel ionization of H2

We report on our results on studying single ionization of molecular hydrogen in the strong-field regime. We use a corotating circular two-color (CoRTC) laser field (780 nm & 390 nm) that is dominated by the second harmonic. The three-dimensional momenta of the fragments are recorded using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) as experimental technique. We investigate sub-cycle interferences in the measured electron momentum spectra as a function of the molecular orientation by using the technique of holographic angular streaking of electrons (HASE). To understand the microscopic origins of these sub-cycle interferences, we present a simple model which shows good agreement with our experimental findings. This allows for the measurement of the changes of the Wigner time delays on an attosecond timescale as a function of the molecular orientation (similar to RABBITT).

🏅 Poster award talk

Angelina Geyer

Strong Field Ionization of H2 in Circularly Polarized Two-Color Laser Fields

The strong field ionization of molecular hydrogen is experimentally investigated using counter-rotating circularly polarized two-color (CRTC) fields and co-rotating circularly polarized two-color (CoRTC) fields (390 nm and 780 nm). The three-dimensional momentum distributions of the charged fragments are measured in coincidence using cold target recoil ion momentum spectroscopy (COLTRIMS). For the dissociation we observe low energy electrons in coincidence with high kinetic energy releases of the ions. We were not able to find a plausible explanation for the experimental observations, but we speculate that the low energy electrons are a fingerprint of inelastically recolliding electrons, which excite the molecule and lead to the high kinetic energy release.

Thomas Brabec

Thomas Brabec (University of Ottawa)

Strong field solid-state physics - the Wannier quasi-classical approach

Quasi-classical models are the workhorse of strong field physics in atoms, molecules and in solids. Here we discuss and compare the Bloch (BQC) and the Wannier quasi-classical (WQC) method applied to high-harmonic generation (HHG) and ionization in solids. The WQC approach completes the single-body picture for HHG in semiconductors, as it is shown to be in quantitative agreement with quantum calculations. The importance of WQC theory extends beyond HHG; it enables modeling of dynamic processes in solids with classical trajectories, such as for coherent control and transport processes, potentially providing better scalability and a more intuitive understanding.

Anne Harth

Anne Harth (MPIK Heidelberg)

Phase information of multiple continuum-continuum transitions

Attosecond electron motion can be observed in a variety of systems by methods often based on time-delayed two-color fields, where an XUV attosecond pulse train ionizes the system and a probe field (e.g. IR) drives continuum-continuum (cc) transitions. The analysis of such photoelectron spectra requires particularly careful consideration of the cc-transitions. Experimental measurements of this contribution are challenging. In this talk, we present how a measured phase shift is related to the phase of cc-couplings even for multiple cc-steps and present a method with the potential to gain experimental information.

Simon Brennecke

Simon Brennecke (Leibniz University Hannover)

Photon momentum transfer in strong-field ionization — it is not always Ekin/c

In ionization of atoms by strong laser pulses, not only energy but also linear momentum of the photons is transferred to the photoelectrons and their parent ions. Recent experimental advances have made it possible to study the influence of this transferred momentum on the resulting photoelectron momentum distributions. Often it has been argued that the photoelectrons gain the linear momentum Ekin/c corresponding to the photons absorbed above the field-free ionization threshold. We show theoretically that this simple assumption is not quite correct, not even for recollision-free ionization, which is realized, e.g., in circularly-polarized laser pulses [1,2]. Furthermore, we suggest ways to use tailored laser fields to study specific properties of the momentum transfer, e.g., its sensitivity to nonadiabatic electron dynamics [2,3]. Finally, we present that also the AC Stark shift of the continuum energies - known to be equal to the ponderomotive potential in the dipole approximation - is modified by nondipole effects.

[1] A. Hartung et al, Nat. Phys. 15, 1222 (2019).
[2] A. Hartung et al, Phys. Rev. Lett. 126, 053202 (2021).
[3] H. Ni et al, Phys. Rev. Lett. 125, 073202 (2020).




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