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 U_p to 10 U_p, where the ponderomotive energy U_p characterizes the dynamics of a free electron in an intense laser field. This is much higher than the cutoff energy of 3.2 U_p 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 (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 (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.

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.

Poster presenters

Brief poster intros

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 (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 (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(E_α-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.

http://arxiv.org/abs/2101.09467

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 (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 (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 (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 (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 (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.

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 (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.

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 (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).

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 (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.

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)].

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).

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.

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 (Leibniz University Hannover)

Photon momentum transfer in strong-field ionization — it is not always E_kin/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 E_kin/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).