
Parallel Sessions:
A  Location: Seminar room 1 D1 // Chair: Frederic Hummel
B  Location: Seminar room 4 // Chair: Jonathan Dubois

09:00  09:20

A: Daniel Bosworth
(University of Hamburg)
Electronic and vibrational structure of charged Rydberg trimers
We show that the recently observed class of longrange ionRydberg molecules can be divided into two families of states, which are characterised by their unique electronic structures resulting from the ioninduced admixture of quantum defect split Rydberg $nP$ states with different lowfield seeking high$l$ states. We predict that in both cases these diatomic molecular states can bind additional ground state atoms lying within the orbit of the Rydberg electron, forming charged ultralongrange Rydberg molecules (ULRM) with binding energies similar to that of conventional nonpolar ULRM. To demonstrate this, we consider a Rydberg atom interacting with a single ground state atom and an ion. The additional atom breaks the system's cylindrical symmetry, which leads to mixing between states that would otherwise be decoupled. The electronic structure is obtained using exact diagonalisation over a finite basis and the vibrational structure is calculated using the MultiConfiguration TimeDependent Hartree method. Due to the lobelike structure of the electronic density, bound trimers with both linear and nonlinear geometrical configurations of the three nuclei are possible. The predicted trimer binding energies and excitation series are distinct enough from those of the ionRydberg dimer to be observed using current experimental techniques.


B: Samuel Schöpa
(University of Rostock)
Towards highharmonic generation in larger, organic molecules: benzene in circularly polarized light
We solve the Schrödinger equation for benzene by expanding the wave function in a linear combination of groundstate KohnSham orbitals. Those have been calculated previously via groundstate density functional theory. This method is orders of magnitude faster than comparable full timedependent density functional theory simulations but neglects the update of the Hartreeexchangecorrelation potential during time evolution. Our simulation is able to describe the qualitative properties of the harmonic spectrum. The $n=6N \pm 1$ selection rules stemming from the 6fold symmetry of the benzene molecule as well as the opposite polarization of each harmonic couple are observed. We evaluate the performance of different density functional theory basis sets and examine how well the continuum is represented in the test case of hydrogen in a linearly polarized laser field.

09:25  09:45

A: Panagiotis Giannakeas
(MPIPKS Dresden)
Wiggle interferometry in cold gases: Shaken, not Stirred
Knowing the intrinsic aspects of fewbody systems, e.g. binding energies, lifetimes etc, it is always desirable since they dictate the macroscopic properties of a gas.
On a twobody level there are wellestablished dynamical protocols addressing their attributes, however, this is not the case for threebody systems.
This talk presents a novel way to dynamically probe such systems.
In particular, we consider three bosons subjected to a double sequence of magnetic field pulses.
This Ramseylike protocol, i.e. wiggle interferometry, permits the creation of trimers and by varying the dark time between the two pulses an oscillatory pattern emerges in the fraction of formed trimers.
The spectra of these interference fringes provide simultaneously the binding energy of trimers as well as their lifetimes, and they are robust against thermal effects that arise from the temperature of the gas.
This means that wiggle intereferometry protocol can potentially used to determine the intrinsic characteristics of exotic fewbody states, such as the Efimov states, regardless the sign or the magnitude of the scattering length.


B: Slawomir Skruszewicz
(DESY Hamburg)
On $(\mathcal{POP})$ controversy in $\mathrm{C_{60}}$ fullerene, and some universal scaling law of strongfield scattering
The propagation of the electronic wavepacket upon strongfield scattering with a nanosystem can be obstructed by its geometry. This effect becomes particularly pronounced when the size of the nanosystem is larger than the width of the electron wave packet. We demonstrate by means of recently introduced the phaseofphase spectroscopy $(\mathcal{POP})$, that the smallangle scattering of the electron wave packet in the prototypical nanosystem $\mathrm{C_{60}}$ molecule and propagation of any trajectory which attempt to cross the molecular plane is hampered by the molecular cage. This socalled $(\mathcal{POP})$ \textit{controversy} has been resolved in the ionization of $\mathrm{C_{60}}$ by $(\mathcal{POP})$ spectroscopy. Furthermore, we observe the transition to the atomiclike limit of the strongfield driven scattering, where the smallangle scattering is activated. This is realized by expanding the vertical size of the electron wave packet beyond the diameter of the $\mathrm{C_{60}}$ cage. Obviously this scenario is valid when the electron wave packet spread in size beyond of the molecular cage by extending the excursion of the electron wave packet. We anticipate that hampering of the smallangle scattering effect holds potential for enhancing the highharmonic generation radiation in the nanosystems and energetics of the scattering in the strong laser fields. Our results are corroborate by results of the Simple Man's Theory (SMT) modeling.
In the second contribution we report on a universal energy dependence governing the rescattering of electrons emitted from small quantum systems in intense laser fields. The empirical powerlaw, which describes the fielddriven electron rescattering probability, is derived by recording photoelectron spectra from rare gas and metal atoms as well as from $\mathrm{C_{60}}$ fullerenes. We find that the ratio between rescattered and direct photoelectrons scales with the ponderomotive energy as $U_{\text{p}}^{2}$ independently of laser intensity, laser wavelength and target system. This dependence on the cycleaveraged kinetic energy is in close analogy to the fundamental Rutheford scattering law, which also predicts a decreasing scattering probability with kinetic energy of the projectile according to $E_{\text{kin}}^{2}$ for a fixed detection angle.

09:50  10:10

A: Agata Wojciechowska
(University of Warsaw)
Exotic ultralongrange Rydberg molecules in the ultracold regime
The discovery of Rydberg matter empowers prospects in ultracold science.
Recently, scientists took another step forward by measuring the Rydberg series
of highly magnetic lanthanide – Er. In our work, we examine the complex entity
of the ultralongrange Rydberg molecule and search for intriguing energy
structure, novel transitions, and unusual magnetic properties. We use the
perturbation theory due to the lack of sufficient experimental data for a
general model. We discuss the limited capabilities of this approach in the case
of complex Er molecules and prospect precise calculations. A step towards
developing a proper model is understanding molecules with a twovalence
electron Rydberg atom. Here, the Hg*Rb molecule serves as a platform. We
present promising energy spectra with easily accessible trilobite states and
approach homonuclear Hg molecules calculations.


B: Stefanie Gräfe
(Friedrich Schiller University Jena)
Highorder harmonic generation in quantum dots
We report on results of systematic experimentaltheoretical investigation of highorder harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference thin film of bulk CdSe. We observe a strong decrease in the efficiency, or even complete suppression of harmonic generation for harmonics with energies of quanta above the bandgap for the smallest dots, whereas the intensity of below bandgap harmonics remains almost unaffected by the dot size. This becomes even more pronounced for longer laser driving wavelength. These systematic investigations allow us to develop a simple physical picture explaining the observed suppression of the highest harmonics: the discretization of electronic energy levels seems to be not the predominant contribution to the observed suppression but rather the confined dot size itself, causing fielddriven electrons to scatter off the dot’s walls. The reduction in the dot size below the classical electron oscillatory radius and the corresponding scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency the emission of highestorder harmonics. Our results demonstrate a new regime of intense lasernanoscale solid interaction, intermediate between the bulk and single molecule response.

10:15  10:35

A: Riaan Philipp Schmidt
(PTB  National Metrology Institute)
TimeEvolved Twisted Light Absorption Patterns
The time evolution of an atomic ensemble under the influence of twisted light has been investigated within the framework of the densitymatrix theory based on the Liouville–von Neumann equation. Special attention has been paid to the effect of the external homogeneous magnetic field orientation on the atomic absorption profile. While the derived expressions can be employed to study the photoabsorption by any atomic system, detailed calculations have been performed for the $5s {}^2 S_{1/2} \, (F=1) \rightarrow 5p {}^2 P_{3/2} \, (F=0)$ transition in $^{87}$Rb. Our calculations indicate how the evolution of the atomic absorption profile is governed by the initial conditions, the relaxation, the spatial structure of the light field, as well as by the applied magnetic field. This study contributes to a better understanding of the potential of twisted light and atomic clouds for magnetic sensing as has been proposed by Castellucci et al. [Phys. Rev. Lett. 127, 233202 (2021)].


B: Philipp Stammer
(ICFO  The Institute of Photonic Sciences)
Quantum electrodynamics of intense lasermatter interactions: A tool for quantum state engineering
Intense lasermatter interactions are at the center of interest in research and technology since the development of high power lasers. They have been widely used for fundamental studies in atomic, molecular, and optical physics, and they are at the core of attosecond physics and ultrafast optoelectronics. Although the majority of these studies have been successfully described using classical electromagnetic fields, recent investigations based on fully quantized approaches have shown that intense laseratom interactions can be used for the generation of controllable highphotonnumber entangled coherent states and coherent state superposition. We provide a comprehensive fully quantized description of intense laseratom interactions. We elaborate on the processes of highharmonic generation, abovethresholdionization, and we discuss new phenomena that cannot be revealed within the context of semiclassical theories. We provide the description for conditioning the light field on different electronic processes, and their consequences for quantum state engineering of light. Finally, we discuss the extension of the approach to more complex materials, and the impact to quantum technologies for a new photonic platform composed by the symbiosis of attosecond physics and quantum information science.

10:45  11:15

Coffee break
Location: Foyer seminar room 1 D1


Location: Seminar rooms 13


Chair: Manfred Lein

11:15  11:45

Franck Lepine
(CNRS Lyon)
First instants following attosecond XUV excitation, from polyatomic carbon structures to proteins
Ultrashort XUV pulses allow to investigate physical processes down to the attosecond timescale. Here we present recent progresses made in the investigation of "large" spatially extended systems using ultrashort XUV pulses. We study how the spatial distribution of atoms in a complex molecule influences processes such as attosecond ionization delays or proton dynamics. The goal of this work is to be able to apply ultrafast XUV technology to extremely complex molecular systems.

12:00  12:30

Robert Moshammer
(MPI for Nuclear Physics Heidelberg)
HHG and FEL based experiments on molecular fragmentation
Recent results of pumpprobe measurements with small molecules will be presented. Using a 50kHz laser amplifier and highharmonic generation we performed RABBITlike experiments on ionization and fragmentation of H2 molecules with a reaction microscope. Emphasis is given to signatures of possible quantum entanglement after breakup. This includes the creation, observation and modification of twoparticle entanglement in the final state. In the second part of the presentation XUV pumpprobe experiments with DiIodoMethane molecules at the XUVFEL in Hamburg will be discussed. Here, clear signatures of intermolecular dynamics and rearrangement processes after the first ionization step could be resolved.

12:45  14:00

Lunch followed by scientific discussions


Chair: Stefanie Gräfe

14:00  14:30

Manfred Lein
(Leibniz University Hannover)
Control of electron wave packets using nearsinglecycle THz waveforms
This talk reports on a joint experimentaltheoretical study of xenon ionized by combined infrared and terahertz pulses.
A femtosecond laser pulse is used to create a temporallylocalized electron wave packet through multiphoton absorption
at a well defined phase of the timesynchronized nearsinglecycle terahertz field. We focus specifically on the lowenergy
electrons close to the continuum threshold, which are particularly sensitive to the electroncore Coulomb interaction.
By recording the photoelectron momentum distributions as a function of the time delay, we observe signatures of various
regimes of dynamics, ranging from recollisionfree acceleration to coherent electronion scattering induced by the terahertz
field. The measurements are confirmed by threedimensional timedependent Schrödinger equation simulations. A
classical trajectory model allows us to identify scattering phenomena analogous to strongfield photoelectron holography
and highorder abovethreshold ionization.

14:45  15:15

Emilio Pisanty
(King's College London)
The imaginary part of the highorder harmonic cutoff
I will present a rigorous definition of the highharmonic cutoff which is applicable to arbitrary driver waveforms. This new definition provides a natural meaning for the imaginary part of the cutoff energy, which controls the strength of quantumpath interference in the plateau. This construction radically simplifies quantumorbit calculations. Using this definition, we build the HarmonicCutoff Approximation to calculate the exact location and brightness of the cutoff at a fraction of the cost of the state of the art — giving access to a much wider class of optimization tasks.
The harmonic cutoff is one of the key concepts in highharmonic generation, but it is also an elusive object which has long escaped a precise definition. In this talk I will present and demonstrate the first natural definition for the harmonic cutoff; I will show how to find it, in a simple and computationally effective way; and I will show how it can be used.
The construction is built based on a new type of quantum orbit, which recollides at the harmoniccutoff times $t_\mathsf{hc}$ defined by a secondorder saddlepoint equation for the usual action $S$,
$$
\frac{\mathrm d^2 S}{\mathrm dt^2}(t_\mathsf{hc}) = 0.
$$
The cutoff energy is then given by the real part of $\Omega_\mathsf{hc} = \frac{\mathrm dS}{dt}(t_\mathsf{hc})$. This provides a natural value for $\mathrm{Im}(\Omega_\mathsf{hc})$, which controls the strength of quantumpath interference in the plateau. The quantum orbits emerge as different branches of a unified Riemann surface, which are united at branch points — the harmoniccutoff times $t_\mathsf{hc}$.
More practically, the times $t_\mathsf{hc}$ can be used to classify the solutions of saddlepoint equations into quantum orbits in a simple and efficient way, making calculations more flexible and effective.
Last (but not least), the information at the times $t_\mathsf{hc}$ is enough to fully characterize the harmonic spectrum at the cutoff. This allows us to build a new approach — the HarmonicCutoff Approximation (HCA) — which gives a quantitatively accurate estimate of the location and brightness of the cutoff, and a qualitative estimate of the spectrum in the upper plateau. This requires solving a single saddlepoint equation — as opposed to dozens or hundreds of equations, as in previous approaches.
Reference: E Pisanty et al, J. Phys.: Photon. 2, 034013 (2020)

15:30  16:00

Nina Rohringer
(DESY Hamburg)
Probing nonlinear electronic response functions by xray optical wave mixing
Nonlinear frequency conversion processes are broadly explored and applied in the visible spectral region. Extending these effects into the xray spectral domain is challenging due to their inherently low cross section [1]. Yet, these processes are accessible at highbrilliance storagering based xray sources and xray free electron lasers. The process of xray parametric down conversion (XPDC) of xray photons in a pair of xray (signal) and visible (idler) photons is predominantly mediated by the valence electron density and provides a valence sensitive probe of electronic structure. Recently, we developed a quantitative theory [1,3] that linked the measured nonlinear scattering signal to an electronic currentdensity density correlation function. We identified a characteristic signature, that allows for an unequivocal experimental identification of this extremely weak process – the XPDC emission cone [1]. In a recent experiment, we successfully measured the XPDC cone resulting of nonlinear xray scattering of photons of 10 keV from diamond, for idler energies in the range of 100 eV. The XPDC scattering cone clearly shows the contribution of two distinct branches, manifested by a positive and negative signal with respect to the Compton scattering background. We interpret this finding as inelastic xray scattering from a XUV polaritonic excitation in diamond. A simple polariton model of two coupled electronic and photonic excited states concurrent with energy and momentum conservation of the nonlinear scattering process agrees with the data and corroborates our interpretation. Momentumresolved nonlinear xray scattering thus provides a means to determine the microscopic structure of EUV polaritons on the length scale of the xray wavelength.
[1] C. Boemer, D. Krebs, A. Benediktovitch, E. Rossi, S. Huotari & N. Rohringer, Faraday Discuss. 228, 451 (2021).
[3] D. Krebs and N. Rohringer, Phys. Rev. X (under review), arXiv:2104.05838

16:15  16:45

Coffee break

16:45  17:30

Scientific discussions

17:30  19:45

Excursion to Christmas market

20:00

Dinner at Freiberger Schankhaus
