Atomic Physics 2019

Extended coherently delocalized states in a frozen Rydberg gas

Abumwis, Ghassan

The long-range dipole-dipole interaction can create delocalized states due to the exchange of excitation between Rydberg atoms. We show that even in a random gas many of the single-exciton eigenstates are surprisingly delocalized, composed of roughly one quarter of the participating atoms. We identify two different types of eigenstates: one which stems from strongly-interacting clusters, resulting in localized states, and one which extends over large delocalized networks of atoms. These two types of states can be excited and distinguished by appropriately tuned microwave pulses, and their relative contributions can be modified by the Rydberg blockade. The presence of so many highly delocalized eigenstates could be relevant to puzzling results in several current experiments.

Non-Adiabatic Dynamics From Motion in 3D Bulk Rydberg Gases

Agrawal, Rajat

We consider a group of highly excited Rydberg atoms[1] in a cold gas as would be excited by shining a laser on an entire ultra-cold cloud of atoms. The atoms are then set into motion by van-der-Waals and resonant dipole-dipole interaction. More interesting features arise from resonant dipole-dipole interactions, due to the emergence of conical intersections [2] leading to a change of the overall electronic state of the Rydberg system. We calculate the electronic state of Rydberg atoms coupled to their movement by using Tully’s surface hopping [3].

Detecting noise-dominated deterministic signals from high harmonics with neural networks pretrained with synthetic data

Alonso Silva, Lázaro

We train artificial neural networks with synthetic data to extract the characteristic pattern in deterministic signals even when overshadowed by noise. Using an ideal signal, namely a periodic one, we analyze how the extraction capability for a given noise level depends on the size of the training data and the architecture and complexity of the network. We find that networks pretrained with ideal periodic signals including noise are most suitable to determine the periodicity of atomic high harmonic spectra which can be considered to be naturally noisy. This renders the combination of ideal synthetic signals with noise added a promising tool to analyze real world data.

Controlling non-adiabatic ionization in ultra-short pulses

Azizi, Sajjad

Light-spin conversion of high-order harmonics generated in single layer graphene.

Boyero-Garcia, Roberto

During the last years, there has been an increasing interest in generating high-frequency beams with controllable polarization, due to their potential applications to perform ultrafast studies of chiral and/or dichroic systems at the nanometer scales. Different complex configurations have already demonstrated polarization control at the sub-femtosecond scale through high-order harmonic generation (HHG) in atomic gases. In parallel, different works have recently shown the suitability of solid systems to emit high-order harmonics in a non-perturbative regime. In opposite to what happens in atomic systems, it has been shown experimentally that high-order harmonics are emitted in graphene when driven by elliptically polarized laser pulses. This behavior allows to control the polarization in a simpler way than using gases. In this work we have performed theoretical simulations in a single layer graphene to analyze the ellipticity, tilt angle and intensity of the harmonics when driven by a laser pulse with different polarizations and orientations. Our results demonstrate a complex photon-spin conversion, leading to a rich scenario for harmonic polarization control: optical rotation, tunable polarization and ultrafast transient polarization.

Optimizing hole coherence in attosecond photoionization

Chen, Yi-Jen

Recent progress in attosecond spectroscopy makes it possible to probe the photoion left behind by ultrafast ionization. Typically, the ion subsystem is a partially coherent statistical mixture of hole states. In order to observe the ionic quantum motion, it is thus crucial to find the best set of laser parameters that maximizes the degree of ionic coherence. In this study, we optimize the hole coherence launched by attosecond photoionization for two many-electron atoms: Kr and Xe. We apply the method of Gaussian-process optimization from the machine-learning community, which allows not only efficient search for the optimal control parameters but also the construction of the target-function landscape. The training data fed into the algorithm are obtained by ab-initio solution of N-electron TDSE using a truncated configuration-interaction approach. We show that the location of the optimum and the structure of the control landscape are highly sensitive to the underlying electronic structure. As such, there is no simple universal strategy for maximizing hole coherence in attosecond ionization. The role of electronic correlations is found to be two-fold. While the coupling between the slow electron and the ion leads to decoherence in the ion subsystem, another type of many-body effects can surprisingly enhance the degree of coherence --- even if the bandwidth of the attosecond pulse itself is insufficient to create multiple ion species.

High-harmonic generation assisted by single-excited multi-electron states

de las Heras, Alba

High-order harmonic generation (HHG) represents a reliable mechanism to produce coherent high-frequency radiation up to the extreme ultraviolet or soft x-rays. The phenomenology of the process is typically described considering a single-active electron (SAE) occupying the outermost valence orbital of the atom or molecule under study. However, multi-electron effects can influence the ionization or recombination steps in HHG. By including all electron interactions in the one-dimensional Schrödinger equation, we identify a novel HHG mechanism based on electron correlation that has an unequivocal signature in the high-harmonic spectrum. The contribution of the inner electron in single-excited states of He atom manifests itself in the appearance of a secondary plateau beyond the SAE cutoff. Our finding not only provides a new picture of HHG, but opens an alternative route to study electronic correlations through high-harmonic spectroscopy.

Structure Factors for Tunneling Ionization Rates of Molecules: a New General Computational Method

Dnestryan, Andrey

Topological effects in interacting Su-Schrieffer-Heeger chains

Drüeke, Helena

Helena Drüeke and Dieter Bauer The Su-Schrieffer-Heeger (SSH) model [1] describes a linear chain with two distinct topological phases. We investigate the behavior of two interacting particles in this one-dimensional system by simulating the equivalent system of a single particle in a two-dimensional system. Even though the two particles repel each other, doublon states, where both particles are on the same lattice site, are possible. We simulate transitions between the two topological phases of the two-dimensional SSH-like system. We focus on the doublon and surface states and the relation between them. A possible experimental realization of such two-dimensional systems are optical waveguides [2]. Realizing energetically separated surface and doublon states would allow the routing of light along specific paths within a bundle of waveguides.

Relaxation of the Electron Temperature in Ultracold Plasma with Ionic Clusters

Dumin, Yurii

It is well known already from the first experiments with ultracold plasmas that cold electrons experience a violent relaxation of their velocities, leading to the sharp increase in the electron temperature, which exceeds by an order of magnitude the value expected from the virial relation. Here, we test the hypothesis that such anomalous heating is caused by the ionic clusters, which serve as the regions of multi-electron interactions and their subsequent acceleration. As follows from our numerical simulations, the above-mentioned clusterization really result in: (1) a much greater initial jump in the electron temperature on the timescale about the inverse Langmuir frequency and (2) a considerable subsequent heating on a longer timescale, which is presumably caused by the three- (or multi-)body recombination.

Strong field driven electron dynamics from a static potential

Dutta, Soumi

An intense, low-frequency, elliptically-polarized laser field lets an electron tunnel from an atom and drives it on a quivering trajectory. The asymptotic angle, measured in the famous attoclock experiment, indicating the electron's release time, is spoiled by the so-called laser-Coulomb interaction. The trajectory from a fully equivalent description with the time-dependent Kramers-Henneberger potential, shows a remarkable similarity with a conventional Kepler hyperbola, albeit with some notable deviations. We discuss those deviations, devise a *static* potential compensating for those and give an (approximate) analytical expression for the attoclock angle.

Static Coherent State Method: High-order Harmonic Generation in atomic and molecular systems in different gauges

Eidi, Mohammad Reza

Signatures of matter Talbot effect in high-order harmonic generation from periodic systems

García-Cabrera, Ana

High harmonic generation (HHG) stands as a robust technique to generate extreme ultraviolet and soft x-ray radiation in the form of attosecond pulses. The coherent nature of this process, based on electronic wave packet ionization, acceleration and recombination, allows a detailed control of the emitted radiation. Recently, the possibility of HHG in crystalline solids has attracted the attention of the community. Following the analogy between the electromagnetic wave equation in the Fresnel approximation and the time-dependent Schrödinger equation (TDSE), we explore the appearance of Talbot images of the ionized wave packets in a periodic structure at different instants of time. Synchronizing the Talbot time with the electronic wave function rescattering, we obtein a new degree of control over the spectral and temporal properties of the high-order harmonics.

High-order harmonic generation with twisted electrons

Gemsheim, Sebastian

We present analytically and numerically the spectrum of high harmonic emission generated by twisted electrons in the presence of linearly polarized light. Ensuing transitions from electronic continuum states with orbital angular momentum to bound states give rise to circularly polarized attosecond pulses. For central collisions with twisted wave packets, continuum-bound transitions are subject to dipole selection rules. For noncentral collisions, a crossover from circularly to linearly polarized emission occurs for increasing impact parameter due to the transverse topology of twisted wave packets.

Nonadiabatic Molecular Association in Thermal Gases Driven by Radio-Frequency Pulses

Giannakeas, Panagiotis

The molecular association process in a thermal gas of ${85}^Rb$ is investigated where the effects of the envelope of the radio-frequency field are taken into account. For experimentally relevant parameters our analysis shows that with increasing pulse length the corresponding molecular conversion efficiency exhibits low-frequency interference fringes which are robust under thermal averaging over a wide range of temperatures. This dynamical interference phenomenon is attributed to Stueckelberg phase accumulation between the low-energy continuum states and the dressed molecular state which exhibits a shift proportional to the envelope of the radio-frequency pulse intensity.

Ultrafast coupled electron-nuclei dynamics of the Jahn-Teller Effect following the sudden ionization of CH$_4$

Gonçalves, Cayo

C. E. M. Goncalves, A. Valentini, S. van den Wildenberg and F. Remacle Theoretical Physical Chemistry, University of Liege, Belgium The engineering of attosecond and few fs optical pulses has made it possible to follow the dynamics of nuclear reorganization with sub-femto second resolution using various pump-probe schemes.[1] Photoionizing molecukes with ultrashort pulses typically leads to a coherent superposition of the electronic states of the cation. When the optically accessed cationic electronic states are nearly degenerate (close to a conical intersection) it is therefore possible to investigate the nuclear reorganization driven by the non adiabatic couplings.[2-5] In the CH$_4$ cation, the non stationary superposition of the optically accessed states results in an ultra fast nuclear relaxation due to the Jahn-Teller (JT) effect which can be probed by High Harmonic Spectroscopy.[2] We report on quantum dynamical simulations of the coupled electron-nuclei dynamics in the Jahn-Teller region of the CH$_4$ cation following the sudden ionization of the neutral molecule. In the Jahn-Teller region, the Born-Oppenheimer approximation breaks down, making the electronic and nuclear dynamics strongly correlated.[6] In the Td geometry, the CH$_4^+$ is triply degenerate, and quickly undergoes JT distortion to a $C_{2v}$ symmetry. For some decades, it was believed that the CH$_4^+$ minimum was of $D_{2d}$ symmetry, that is indeed close in energy but about 0.16 eV higher that the $C_{2v}$ minimum. We therefore computed a nuclear grid in two reduced dimensions that includes these two geometries. The electronic structure of CH$_4^+$ was calculated with a SA-CASSCF/6-31G++(2D,2P) electronic wave function with 3 electronic states and 8 active orbitals, using the MOLPRO package [7]. The photoionization amplitudes resulting from the sudden ionization of the neutral ground electronic with a XUV pulse are computed using the methodology of ref.[8] for each grid point. Our simulations allow following in time the wave packet localization in the region of the $C_{2v}$ and $D_{2d}$ geometries for different orientations of the CH4 at the time of ionization. 1. M. Nisoli, P. Decleva, F. Calegari, A. Palacios and F. Martín, Chem. Rev. 117 (16), 10760-10825 (2017). 2. S. Baker, J. S. Robinson, C. A. Haworth, H. Teng, R. A. Smith, C. C. Chirila, M. Lein, J. W. G. Tisch and J. P. Marangos, Science 312 (5772), 424 (2006). 3. W. Li, X. Zhou, R. Lock, S. Patchkovskii, A. Stolow, H. C. Kapteyn and M. M. Murnane, Science 322 (5905), 1207 (2008). 4. P. M. Kraus and H. J. Wörner, ChemPhysChem 14 (7), 1445-1450 (2013). 5. R. Geneaux, H. J. B. Marroux, A. Guggenmos, D. M. Neumark and S. R. Leone, Philos. Trans. Royal Soc. A 377 (2145), 20170463 (2019). 6. T. Mondal and A. J. C. Varandas, J. Chem. Phys. 143 (1), 014304 (2015). 7. H.-J. Werner, P. J. Knowles, G. Knizia, F. R. Manby, and M. Schuetz, Molpro: a general-purpose quantum chemistry program package,WIREs Comput. Mol. Sci., 2012, 2(2), 242–253. 8. S. van den Wildenberg, B. Mignolet, R. D. Levine and F. Remacle, J. Chem. Phys. 151 (13), 134310 (2019).

Grid based TDCIS for helium in chiral environments

Heger, Marec

Chiral molecules, i.e. molecules that cannot be superimposed by its mirror image by rotations and translations, show different reactions to other chiral systems depending on their handiness. A prominent example is the interaction of a chiral molecule with circular polarized light which gives rise to differences in their corresponding photoelectron angular emission spectrum. The difference of the spectra for a given handiness with respect to left and right-circularly polarized light is called photoelectron circular dichroism(PECD) which can be used as a tool to detect and distinguish chiral signatures of molecules in the gas phase. Whereas experimental setups for PECD measurements are fairly widespread ,theoretical predictions are still challenging especially for large molecules. As a consequence we explore the feasibility to simulate electron ionization for PECD measurements using TDCIS. Since the ionization of a molecule will inevitably lead to electron continuum wave function contributions, it is important to select a suitable basis in order to allow for fast convergence for the bound and unbound part of the spectrum. Consequently we refrain from using a gaussian basis and instead employ a discrete variable representation(DVR). We intent to investigate the role of electron correlations and the feasibility of grid methods for helium and larger molecules in chiral environments

Rydberg Composites

Hunter, Andrew L.

We introduce the Rydberg Composite, a new class of Rydberg matter where a single Rydberg atom is interfaced with a dense environment of neutral ground state atoms. The properties of the Composite depend on both the Rydberg excitation, which provides the gross energetic and spatial scales, and on the distribution of ground state atoms within the volume of the Rydberg wave function, which sculpt the electronic states. The latter range from the “trilobites,'' for small numbers of scatterers, to delocalized and chaotic eigenstates for disordered scatterer arrays, culminating in the dense scatterer limit in symmetry-dominated wave functions which promise good control in future experiments. We characterize these scenarios with different theoretical methods, enabling us to obtain scaling behavior for the regular spectrum and measures of chaos and delocalization in the disordered regime. Thus, we obtain a systematic description of the Composite states. The $2$D monolayer Composite possesses the richest spectrum with an intricate band structure in the limit of homogeneous scatterers.

High-harmonic generation in topological graphene nanoribbons

Jürß, Christoph

The interaction of a laser field with graphene nanoribbons in the zig-zag configuration is investigated using a tight-binding description. As in the bulk Haldane model, complex-valued hopping elements between next-nearest neighbors renders the system topologically nontrivial. High-harmonic spectra are calculated for different amplitudes of the next-nearest neighbor hopping for a laser polarized linearly along the ribbon. The polarization of the emitted harmonics has a component in the direction of the incoming field as well as in the perpendicular direction, i.e., it is elliptically polarized. We find that the helicity changes at a certain harmonic order that depends on the next-nearest neighbor hopping strength. The phases of the Bloch functions at the lattice sites within the unit cell flip at certain values of the lattice momentum. In the band structure, an avoided crossing appears at that lattice momentum. The corresponding energy gap fits to the harmonic order where the change of the helicity occurs.

Generalized spheroidal wave equation revisited: Application for Rydberg states of the hydrogen molecular ion

Khoma, Mykhaylo

The generalized spheroidal wave equation has become increasingly important in the solution of a variety of physical problems in atomic and molecular physics, optic, electromagnetic theory, astrophysics, and cosmology. The calculations for very large or complex parameters still a challenging problem [1-3]. In the present study, we provide a new highly-accurate scheme for computation of eigenvalues and eigenfunctions of the generalized spheroidal wave equation with a moderate numerical effort. One of the important applications of the spheroidal wave equation is the study of the hydrogen molecular ion. Theoretical treatment of $H_2^+$ has played and is still playing an important role in the development of quantum chemistry in the scattering theory and other areas of physics. The lack of state-resolved data for highly excited $H_2^+$ ion has been mentioned in [2,3]. Our interest in highly excited states of $H_2^+$ was stimulated by their potential use as a starting point for studying of the Rydberg states of the more complex diatomic molecules. We report the application of the proposed method for calculation of the potential energy curves (PECs) of the Rydberg ($n \sim$ 100-130) $\Sigma$-states of $H_2^+$ ion at internuclear separations $R\sim$ 0-60000 au. The results obtained are compared with the data available in the literature [3]. [1] B. E. Barrowes, K. ONeill, T. M. Grzegorczyk, and J. A. Kong, On the asymptotic expansion of the spheroidal wave function and its eigenvalues for complex size parameter. Stud. Appl. Math. 2004, v.113, p.271. [2] M. C. Zammit, {\it et al.} State-resolved photodissociation and radiative association data for the molecular hydrogen ion. Astrophys. J. 2017, v.851, p.64. [3] T. J. Price, and C. H. Greene. Semiclassical treatment of high-lying electronic states of $H_2^+$. J. Phys. Chem. A 2018, v.122, p.8565.

Asymmetry and chirality hidden behind the transverse electromagnetic waves on resonance with plasmon and phonon polaritons of lossy media

Lee, Hyoung-In

A transverse magnetic (TM) wave is established on resonance across a planar interface between a dielectric medium and a plasmonic medium (say, gold) and/or phonon polaritonic medium (say, silicon carbide). The pertinent dispersion relation is well known, and the decomposition of the Poynting vector into its orbital and spin parts is thoroughly examined for lossless media. However, the characters of the orbital and spin parts are not yet fully examined for lossy media (say, metals with losses and/or silicon carbide with complex-valued permittivity data). For such lossy media in interactions with electromagnetic waves, it is well-known that the spin is indicative of the electromagnetic polarizations, whereas the orbital part represents the overall energy flows. In this talk, I will present two cases: [1] a single electromagnetic wave, and [2] two counter-propagating electromagnetic waves. In case [1], we have identified asymmetries hidden behind the normalized parameters of the orbital part and the spin part, each being divided by the Poynting vector. We could draw an analogy of our wave configuration to the well-known SSH (Su-Schrieffer-Heeger model) in the sense that there are both band gaps and chiral behaviors hidden in our model. The role of interband transitions will be examined as well. In case [2], we discovered that optical and/or polaritonic vortices are established not for the orbital part but for the spin part. In case [2] for the counter-propagating waves, we will discuss its relevance to the typical configurations for photoemission electron microscopy (PEEM). This is intended for a contributed talk (if it were given a chance). Otherwise, it can be presented as a poster.

Effect of the Boundary Conditions on the Relaxation of Temperature in Ultracold Plasmas

Lukashenko, Anastasiia

A violent relaxation of the electron velocities, resulting in the sharp increase of their temperature, is a well-known property of the ultracold plasmas created by photoionization of a neutral gas. Surprisingly, a numerical modeling of this phenomenon substantially depends on the boundary conditions imposed on the simulated volume. In this work, we studied in detail three types of the boundary conditions: "free" (i.e., a finite-size bunch of the charged particles), periodic and reflective. It was found that the minimal increase of the electron temperature takes place in the case of free boundaries. This is not surprising because the electrons accelerated due to the multi-particle interactions escape outwards and begin to move in the "halo", thereby, not experiencing a further heating. The maximum increase in the temperature was obtained in the case of the periodic boundary conditions, but a considerable part of this increase comes from a permutation of the electrons between the opposite sides of the simulated volume (i.e., has a non-physical nature). At last, the reflective boundary conditions give the intermediate magnitude of the temperature increase. Since they are characterized by a small integration errors and do not suffer from other non-physical effects, we believe that just the reflective conditions are the most robust tool for simulation of ultracold plasmas.

The laser-driven Kitaev chain

Luther, Alexander

During the recent years, research on high-harmonic generation (HHG) in solid state systems experienced an increasing interest. Most of the often used tight-binding models to describe the interaction of laser fields with solid-state systems conserve the number of electrons. The Kitaev chain, a model for a one-dimensional p-wave superconductor, contains a pairing term that does not conserve the number of electrons and thus is usually treated in Bogolyubov-de-Gennes (BdG) representation. In our work, we study the role of electrons and holes on finite Kitaev chains in the HHG process using a time-dependent tight-binding approach in which the BdG Hamiltonian contains the coupling to the external driver in length gauge. Further, we investigate the laser-driven dynamics of the Majorana edge states, which appear at the ends of of the chain in the topological non-trivial phase of a Kitaev chain.

Geometrical rearrangement and vibrational energy redistribution of atomic and molecular adsorbates

Martinez-Mesa, Aliezer

The development of novel methods for the solution of quantum many body problems constitutes one of the most important challenges in comtemporary theoretical and computational physics. The dynamics of lightweight atoms and molecules adsorbed at surfaces, or embedded in quantum solvents, belong to these class of problems. In these systems, collective quantum behaviour emerges as a consequence of the influence the interparticle interactions, the small masses, and the confinement imposed on the constituents. The present investigation focuses in the development and implementation of novel computational methodologies that enable the numerical simulation of equilibrium properties and of dynamical processes taking place in the aforementioned type of systems. The developed simulation methods are applied to the modelling of a variety of phenomena such as the photoinduced dynamics in condensed phase, the hydrogen adsorption on nanostructures, the substrate mediated water splitting, and the vibrational energy redistribution in adsorbed molecules. The results of the present simulations are found to be in very good correspondence with previous theoretical and experimental data reported in the literature.

Understanding Correlation Effects in Photoelectron Circular Dichroism

Mondelo Martell, Manel

Manel Mondelo-Martell1, Christiane P. Koch2, Daniel M. Reich1 1) Institut für Physik, Universität Kassel, Heinrich-Plett-Str 40, 34132 Kassel, Germany 2) Theoretische Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany Chirality is a fundamental symmetry breaking, defined by the impossibility to superpose the mirror images of a given object, highly relevant in the fields of AMO and chemistry. Current experimental techniques based on chiral radiation–matter interactions, such as Photoelectron Circular Dichroism (PECD)[1], provide detailed information about such systems, but theoretical models are crucial for its interpretation. Accurate numerical simulation of the photoionization process is limited to ∼three electron systems, and studies pursuing a time–resolved solution of the process for larger systems generally need to rely on a simplified ansatz to become numerically affordable, which usually leads to a poor description of electronic correlation and thus only qualitative results. We present a time–resolved simulation of photoelectron spectra in chiral environments using the MCTDHF [2] approach. This algorithm allows for a numerically efficient representation of the wave function through the use of time–dependent basis sets, and includes electronic correlation due to its multiconfigurational character. To study the suitability of this technique for the study of chiral effects in correlated many-electron systems, the photoionization of a He atom embedded in a chiral potential will be simulated. Comparison with the TDHF approach and possible improvements will be discussed. References [1] I. Powis, in Adv. Chem. Phys. (2008), pp. 267-329. [2] D. Hochstuhl and M. Bonitz, J. Chem. Phys. 134, 084106 (2011).

Ultrafast Photoelectron Circular Dichroism Changes in 1- Iodo-2-Methylbutane

Music, Valerija

Highly intense circularly polarized XUV free-electron laser pulses were used to observe the time-resolved photoelectron circular dichroism (TR-PECD) of a prototypical chiral molecule as 1-Iodo-2-Methylbutane ($C_5H_{11}I$) during laser induced fragmentation. The presented experiment was performed at the BL1 CAMP endstation at FLASH1 in Hamburg. With a two-sided velocity map imaging spectrometer, electron-ion correlations were obtained and the time-resolved ejection of ionic and neutral atomic iodine was observed. These fragments serve as observer site to monitor the evolving chirality of the molecule. For probing the different fragmentation channels of the chiral molecule (R-enatiomer, S-enatiomer and the racematic mixture), two different photon energies, i.e. 63 eV (for neutral iodine) and 75 eV (for singly charged iodine), were used.

Locally-chiral light and its interaction with chiral matter: characterization and control in the non-linear perturbative regime

Ordonez, Andres

Chiral systems are characterized by their handedness, a geometrical feature that provides an extra handle for the control of matter with electromagnetic fields. Despite groundbreaking developments in the control of chiral matter [1], efficient operation of this geometrical handle has been intrinsically limited by the spatially non-local character of light's chirality [2]. This is due to the huge disparity between the molecular size and the optical wavelengths, the latter determining the scale of the helix that circularly polarized light (CPL) creates in space. Such disparity prevents an efficient handshake between chiral matter and CPL (or any other light whose chirality depends on the spatial dependence of the field [3]). We have found a way to bypass this inherent limitation by recognizing that chirality can be encoded in the temporally non-local character of light [4]. The time evolution of the electric field vector, at a single point in space, can follow a chiral trajectory! This fundamentally new approach to the interaction between chiral light and chiral matter can be implemented at any wavelength and yields an unprecedented enantiosensitivity. We discuss how to characterize this locally-chiral light, how it interacts with chiral matter in the non-linear perturbative regime, and how it can be used to control the response of chiral matter. [1] B. L. Feringa, The art of building small: from molecular switches to motors, Nobel Lecture 2016 [2] A. F. Ordonez and O. Smirnova, Generalized perspective on chiral measurements without magnetic interactions, Phys. Rev. A 98, 063428 (2018) [3] Y. Tang and A. E. Cohen, Optical chirality and its interaction with matter, Phys. Rev. Lett. 104, 163901 (2010) [4] D. Ayuso et al, Locally and globally chiral fields for ultimate control of chiral light matter interaction, Nat. Phot, (2019) (accepted); arXiv:1809.01632 (2018)

High-harmonic generation in bicircular Laguerre-Gaussian beams

Paufler, Willi

We report on a method to generate extreme ultraviolet vortices from high-order harmonic generation with two-color counter-rotating Laguerre-Gaussian (LG) beams that carry a well-defined orbital angular momentum (OAM). We show that the OAM of each harmonic can be directly controlled by the OAM of the incident LG modes. Furthermore, we investigate the temporal evolution of the generated harmonics and show that thereby so-called attosecond light spring are shaped. We explain possibilities to control the divergence and number of coils in these light springs by the OAM of the bicircular driving beam.

Polarization control of attosecond pulses through the orbital angular momentum of light

Rego Cabezas, Laura

Ultrashort pulses of coherent structured light—light fields with custom spatial and temporal properties, such as intensity, phase and angular momentum—are opening excellent opportunities to control the primary electronic response of matter. In particular, topological light beams carrying angular momentum are particularly interesting, as they interact with matter differently, introducing mechanical motion to nanostructures, and affecting fundamental excitation rules. Light angular momentum is two-fold: spin angular momentum (SAM), related to the polarization of light; and orbital angular momentum (OAM), associated to the spatial profile of the phase of the electromagnetic wave. While angular momentum can be routinely transferred to visible/infrared (IR) beams it becomes a lot harder in the extreme-ultraviolet (EUV) and x-ray regimes. The nonlinear frequency up-conversion of an intense IR femtosecond laser pulse through high harmonic generation (HHG) has emerged as a robust mechanism to imprint SAM and OAM onto the EUV regime, providing structured attosecond pulses with controlled angular momentum properties. In this work, we identify a new mechanism to control the polarization of the high-order harmonics and the corresponding attosecond pulses, through the OAM of the driving IR pulses. By properly selecting the OAM of a bichromatic, counter-rotating driving vortex beam we are able to harness the polarization of the attosecond pulses, all the way from linear to circularly polarized. In fact, EUV harmonic beams are produced with simultaneous control over their SAM and OAM. Our results significantly extend the degree of control over short-wavelength, ultrafast radiation via manipulation of the properties of IR driving lasers.

Quasi-resonant two-photon ionization: From controlling transitions with adiabatic passage to determining spectra with deep learning

Saalmann, Ulf

Evidence of AC-Stark-shifted resonances in intense two-color counter-rotating circular polarized laser fields

Stammer, Philipp

We report on the appearance of a structure at low energies in the photoelectron momentum distribution of the hydrogen atom exposed to two-color counter-rotating bi-circular laser fields. These structures, which arise due to AC-Stark shifted resonances, break the three-fold symmetry, typical for the $\omega-2\omega$ bi-circular fields. We discuss the physical origin of this structure in terms of partial-wave interference between direct ionization channels and a resonant pathway, that passes through the AC-Stark shifted state and show how the underlying Rydberg state population depend on the field strength and pulse duration.

QPROP 3.0: faster t-SURFF for slow electrons

Tulsky, Vasily

Vasily Tulsky, Dieter Bauer University of Rostock, Institute of Physics, 18051 Rostock, Germany Obtaining a solution to the time-dependent Schrödinger equation (TDSE) for a target in a strong laser field is a complicated problem and needs numerical methods to be involved for accurate results. We present a new version [1] to our TDSE solver QPROP [1-3] which now calculates photoelectron spectra (PES) several times or even orders of magnitude faster than before: the PES are calculated with the help of the surface-flux method [4] step-by-step in time till the end of the laser pulse only; then using the semi-analytic trick [5] the infinite time interval after the pulse is covered in a single step, significantly speeding up the calculation of the PES and allowing to investigate the low-energy domain. QPROP can be downloaded at [1] V. Tulsky, D. Bauer, QPROP with faster calculation of photoelectron spectra, (submitted to Computer Physics Communications) [2] D. Bauer, P. Koval, Computer Physics Communications 174, 396 (2006) [3] V. Mosert, D. Bauer, Computer Physics Communications 207, 452 (2016) [4] L. Tao, A. Scrinzi, New Journal of Physics 14, 013021 (2012) [5] F. Morales, T. Bredtmann, S. Patchkovskii, J. Phys. B 49, 245001 (2016)

Enhancing the non-adiabatic transfer with heavier isotopomers: Effect of mass in the ultrafast electron-nuclear dynamics in LiH induced by few-cycle IR pulses

van den Wildenberg, Stephan

Exciting molecules using few-cycle intense optical pulses leads to non-equilibrium electron density and induces ultrafast nuclear response on the femtosecond timescale. By simulating the coupled electron-nuclear dynamics of the LiH molecule and isotopomers, we show that non adiabatic coupling is favored in heavier isotopomers of LiH. When a coherent superposition of coupled electronic states is pumped by the pulse, the rate of transfer is modulated by the overlap of the nuclear wave packets and the momentum acquired on the electronic states involved in the non-adiabatic dynamics. Since the non-adiabatic coupling strength is inversely proportional to the reduced mass one would expect that the increase in mass decreases the importance of non-adiabatic transfer. However, our quantum dynamical simulations show the overlap between the nuclear wave packets of heavier isotopomers is larger and that they acquire a larger momentum on the repulsive region of the potential energy curves. This leads to a significant transient bistate isotope effect and an increase in non-adiabatic transfer for the heavier isotopomers.

Unifying real-space and k-space TDDFT simulations of high-order harmonic generation in extended systems

Yu, Chuan

High-order harmonic generation (HHG) in extended systems attracts increasing research interest in recent years. Theoretical models considered in this research area are usually treated with either real-space or k-space approaches. Among them, a real-space TDDFT model of finite chains has turned out to be useful in studying the transition from atomic to solid-state HHG. By applying periodic boundary conditions, we extend the real-space TDDFT model to be unified with a k-space treatment (Brillouin zone sampling). This extension allows us to simulate HHG in the infinite-system limit, with many-electron effects taken into account within the TDDFT framework. We demonstrate that HHG from a finite model with hundreds of atoms can indeed agree with the result in the corresponding infinitely-extended system.

Creating, detecting and controlling dynamically chiral molecules

Zak, Emil

E. J. Zak1, A. Yachmenev1,2, J. Küpper1,2,3 1 Center for Free-Electron Laser Science, DESY, 22607 Hamburg 2 Center for Ultrafast Imaging, Universität Hamburg, 22761 Hamburg 3 Department of Physics, Universität Hamburg, 22761 Hamburg Chirality is a property of systems being in either of its two enantiomeric (mirror-image) forms, non-superimposable by rotations and translations in space. It is fundamental to biological activity of molecules in living organisms. Traditionally the term ‘chirality' refers to molecules that are ‘born’ to be so, owing to their quasi-rigid spatially enantiomorphic geometrical structures with high potential energy barriers between them. It has been recently theoretically shown [1] that upon excitation of certain non-chiral molecules to rotational states with high value of the total angular momentum, the molecules become chiral. Generation of such rotationally induced chirality is possible through an optical centrifuge performed on the gas phase molecular beam. 

We study computationally an optical centrifuge on prototypical PH$_3$ molecule with the aim of generating dynamic chirality. As a means of detecting dynamically chiral quantum states we suggest photo-electron circular dichroism (PECD) technique. Simulations of PECD on statically chiral molecules and dynamically chiral molecules show that it is indeed possible to observe and quantify chiral characteristics of rotationally induced quantum states of statically non-chiral molecules. The computational procedure uses high-accuracy variational simulations of molecular ro-vibrational dynamics in the presence of electric fields [2-3]. Literature: [1] A. Owens, A. Yachmenev, S. Yurchenko, J. Küpper, Phys. Rev. Lett. 121 (2018) 193201. [2] A. Owens, A. Yachmenev, J. Chem. Phys. 148 (2018) 124102. [3] A. Yachmenev, J. Onvlee, E. Zak, A. Owens, J. Küpper, Phys. Rev. Lett. (accepted).

Excitonic Wave Function Reconstruction from Near-Field Spectra Using Machine Learning Techniques

Zheng, Fulu

A general problem in quantum mechanics is the reconstruction of eigenstate wave functions from measured data. In the case of molecular aggregates, information about excitonic eigenstates is vitally important to understand their optical and transport properties. Here we show that from spatially resolved near field spectra it is possible to reconstruct the underlying delocalized aggregate eigenfunctions. Although this high-dimensional nonlinear problem defies standard numerical or analytical approaches, we have found that it can be solved using a convolutional neural network. For both one-dimensional and two-dimensional aggregates we find that the reconstruction is robust to various types of disorder and noise. Phys. Rev. Lett. 123, 163202 (2019)

Control of the geometric phase with time-dependent fields

Zhu, Xiaosong

We evaluate the geometric phase acquired during the cyclic evolution of quantum systems exposed to time-periodic external fields, using Floquet theory and comparing the phase definition by Aharonov and Anandan to that by Berry. Ions in a light field and spin-1/2 particles in a magnetic field are considered. At high frequency, the Aharonov-Anandan phase converges to zero. In the limit of low frequency, it does not always converge to the adiabatic Berry phase, even though time evolution becomes adiabatic. Using a photon-channel perspective, we explain the oscillatory frequency dependence which facilitates control of the geometric phase. Control is also possible by steering the system through a degeneracy of the adiabatic energies, which can cause an additional phase of $\pi$. We demonstrate a robust example where both phase definitions agree and not only the dynamical phase but also the nonadiabatic corrections vanish at suitable frequencies.

Attosecond pulse induced ionization dynamics in a molecular charge-transfer system with correlated electrons

Ziems, Karl Michael

The attosecond ultrafast ionization dynamics of correlated two- or many-electron systems have, so far, been mainly addressed investigating atomic systems. In the case of single ionization, it is well known that electron- electron correlation modifies the ionization dynamics and observables beyond the single active electron picture, resulting in effects such as the Auger effect or shake-up and knock up processes. Here, we extend these works by investigating the attosecond ionization of a molecular system involving the correlated two-electron dynamics, as well as the non-adiabatic nuclear dynamics. We demonstrate, employing a charge-transfer molecular model system, how elastic and inelastic correlation-driven processes can be observed. As the model system investigated here involves two differently bound electrons, a stronger and a weaker bound electron, we can distinguish different pathways leading to ionization, be it direct ionization or ionization involving elastic and inelastic electron scattering processes. We find that different pathways result in a difference in the electronic population of the parent molecular ion, which, in turn, involves different subsequent (non-adiabatic) vibrational dynamics.