Polarization tagging of two-photon double ionization by elliptically polarized XUV pulses

We explore the influence of elliptical polarization on the (non)sequential two-photon double ionization of atomic helium with ultrashort extreme ultraviolet (XUV) light fields using time-dependent full ab initio simulations. The energy and angular distributions of photoelectrons are found to be strongly dependent on the ellipticity. The correlation minimum in the joint angular distribution becomes more prominently visible with increasing ellipticity. In a pump-probe sequence of two subsequent XUV pulses with varying ellipticities, polarization tagging allows us to discriminate between sequential and nonsequential photoionization. This clear separation demonstrates the potential of elliptically polarized XUV fields for improved control of electronic emission processes.This work was supported by the WWTF through Project No. MA14-002, and the FWF through Projects No. FWF-SFB041-VICOM, No. FWF-SFB049-NEXTlite, and No. FWF-W1243-Solids4Fun, as well as the IMPRS-APS. J.F. acknowledges support by the Spanish MINECO through a Ramón y Cajal grant and the “María de Maeztu” program for Units of Excellence in R&D (MDM-2014-0377

Tuning canonical typicality by quantum chaos

One key issue of the foundation of statistical mechanics is the emergence of equilibrium ensembles in isolated and closed quantum systems. Recently, it was predicted that in the thermodynamic ($N \rightarrow \infty$) limit the canonical density matrix emerges for small subsystems from almost all pure states of large quantum many-body systems. This notion of "canonical typicality" is assumed to originate from the entanglement between subsystem and environment and the resulting intrinsic quantum complexity of the many-body state. We show that quantum chaos plays a crucial role in the emergence of canonical typicality for large but finite quantum systems. We demonstrate that the degree of canonical typicality can be quantitatively controlled and tuned by the degree of quantum chaoticity present in the many-body system.Comment: 10 pages, 10 figure

Theory of Subcycle Linear Momentum Transfer in Strong-Field Tunneling Ionization

Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. We present numerical simulations as well as an analytical description of the subcycle phase (or time) resolved momentum transfer to an atom accessible by an attoclock protocol. We show that the light-field-induced momentum transfer is remarkably sensitive to properties of the ultrashort laser pulse such as its carrier-envelope phase and ellipticity. Moreover, we show that the subcycle-resolved linear momentum transfer can provide novel insights into the interplay between nonadiabatic and nondipole effects in strong-field ionization. This work paves the way towards the investigation of the so-far unexplored time-resolved nondipole nonadiabatic tunneling dynamics. © 2020 authors

Ionisierungsphasen und Elektronenwinkelverteilungen von Vielelektronenatomen getrieben durch Attosekundenpulse

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersDie Entdeckung und Erklärung des photoelektrischen Effekts war einer der ersten Beweise der Lichtquantenhypothese. Darüber hinaus stellt die Absorption eines hochenergetischen Photons, und die darauf folgende Emission eines Elektrons, bis heute einen fundamentalen Baustein der Wechselwirkung von Licht mit Materie dar. Obwohl Photoionisation schon sehr genau untersucht worden ist, konnte seit der Jahrtausendwende mit Hilfe der Attosekundenphysik ein noch tiefergehendes Verständnis des photoelektrischen Effekts erzielt werden. So gelang es, unter anderem, die Zeit zu messen, die ein Elektron benötigt, um das Atom zu verlassen, nachdem es ein Photon absorbiert hat. Diese charakteristische Zeit ist in der komplexen Phase der quantenmechanische Wellenfunktion des Elektrons eingeprägt. Eine präzise Messung der Elektronenwellenfunktion nach der Ionisation, wie durch die Attosekundenphysik ermöglicht, erlaubt es direkte Rückschlüsse auf die zugrundeliegenden physikalischen Prozesse zu ziehen. Die quantenmechanische Phase der Wellenfunktion ist allerdings nicht direkt experimentell bestimmbar, sondern kann nur mit Hilfe von aufwendigen Messprotokollen gemessen werden. Zusammen mit theoretischen Rechnungen und numerischen Simulationen, ist es jedoch oft möglich die korrelierte Elektronendynamik zu rekonstruieren. Die Untersuchung von einfachen atomaren Systemen, für die numerisch präzise Simulationen möglich sind, bilden die Grundlage unseres Verständnisses der Wechselwirkung von ultrakurzen und starken Laserpulsen mit Materie. Wir untersuchen, teilweise in Kooperation mit experimentellen Arbeitsgruppen, unterschiedliche Photoionisationsmessprotokolle zur Charakterisierung von Photoionisation mit Hilfe von hochpräzisen ab initio Simulationen. Der Großteil der untersuchten Protokolle verwendet die Winkelverteilung der emittierten Elektronen als Observable zur Charakterisierung prototypischer Ionisierungsprozesse. Im einfachsten Fall führt die Absorption eines hochenergetischen Photons zur Emission eines einzelnen Elektrons. Um die quantenmechanische Phase der ionsierten Elektronen zu bestimmen, wurden Messprotokolle entwickelt, bei denen das bereits ionisierte Elektron ein weiteres Photon absorbiert. Eben dieser Kontinuum-Kontinuum Übergang bewirkt eine zusätzliche Phasenverschiebung der Wellenfunktion, welche mit Hilfe des winkelaufgelösten Elektronenspektrums bestimmt werden kann. Neben der Anregung eines Einteilchenkontinuum-Zustandes kann die Absorption eines Photons auch die Anregung einer sogenannten Fano-Resonanz bewirken. Wir zeigen im Detail wie ultrakurze Laserpulse verwendet werden können, um die Streuphase in der Nähe einer solchen Resonanz zu bestimmen, und wie der zeitliche Aufbau der charakteristischen Absorptionslinie unter Einsatz von kurzen, aber starken, Laserpulsen verfolgt werden kann. Der Einfluss der Elektron-Elektron Wechselwirkung kann genau untersucht werden für den Fall von Doppelionisation mit ultrakurzen Laserpulsen. Da in diesem Fall beide Elektronen das Atom nahezu gleichzeitig verlassen, enthüllt eine Messung der Winkelverteilung beider Elektronen den Einfluß der interelektronischen Coulomb-Wechselwirkung auf diesen Prozess. Um den Einfluss der Elektron-Elektron-Wechselwirkung auf den Doppelionisationsprozess zu quantifizieren, untersuchen wir den prototypischen Fall von Helium Doppelionisation mit zirkular polarisierten Laserfeldern.Photoionization, i.e. Einstein's photoelectric effect, often is one key ingredient to a broad variety of physical phenomena. Complementary to previous methods, attosecond metrology has provided the means to measure the time associated with the photoelectric effect. Neither is photoionization instantaneous nor is there a universal time associated with this process. Rather the exact emission time depends on the energy of the liberated electron and the details of the system it is leaving. The observed photoionization time delay (or advance) is imprinted on the quantum mechanical wave function as a variation of the phase shift. Being able to retrieve this scattering phase constitutes one major opportunity provided by attosecond metrology. Since the acquired quantum mechanical phase shift depends on the ionized system the measurement of photoionization phases can be used to probe and study matter. Even though the quantum mechanical phase carries a large amount of information it is, unfortunately, not directly accessible in experiment. Often, intricate detection schemes are necessary to retrieve phase information. Furthermore, in many cases theoretical calculations and analytical models are additionally needed to fully reconstruct the electronic dynamics from the measured phase information. Thus, theoretical calculations and experiments on simple atomic systems serve as backbone of our understanding and provide the playing ground for the exploration of more complex phenomena in, e.g., solids and topological materials. Within this thesis we numerically explore, in close collaboration with experimental groups, various measurement setups to investigate photoionization of atoms by ultrashort laser pulses using highly accurate ab initio simulations. We will mainly focus on the phases which can be obtained from the measurement of angular resolved electron spectra (angular distributions) for three prototypical ionization regimes and highlight which information can be gained by the different techniques. The simplest scenario we investigate is single ionization by high energetic ultrashort laser pulses. The phase acquired within this process can be determined with measurement protocols that require at least one additional photon absorption by the quasi-free electron after absorption of the high-energy photon. Within this thesis we will accurately determine the phase acquired within this free-free transition. As a second scenario, we investigate the quantum mechanical scattering phase near atomic Fano resonances, whose existence is a direct consequence of electron-electron correlation. We will determine in detail how this phase can be measured using attosecond pulses. Furthermore, we will show how strong few-cycle light fields can be used to projectively measure ultrafast electronic processes such as the build-up of the asymmetric Fano line shape. As a further application, we will show that angular distributions serve as ideal candidates to pin down electronic correlations for double ionization of atomic helium by circularly polarized light fields.20

Dynamik von Korrelationen von Fermionen in Gittern: Anwendung der zeitabhängigen Zweiteilchendichtemartix-Methode

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersCorrelations in many-body quantum states lead to interesting phenomena while at the same time being enormously difficult to describe theoretically. Recent experiments conducted with ultracold atoms in optical lattices allow to explore quantum many-body correlations in a controlled and tunable way. The underlying theoretical model is the single-band Hubbard model. In this thesis we investigate the one-dimensional Hubbard model by applying the newly developed time-dependent two-particle reduced density matrix method. The method is based upon the assumption that three-particle correlations are negligible. We assess this approximation for equilibrium and non-equilibrium states. We show that the method is well suited for weakly interacting and weakly correlated systems.9

Towards the complete phase profiling of attosecond wave packets

Realistic attosecond wave packets have complex profiles that, in dispersive conditions, rapidly broaden or split in multiple components. Such behaviors are encoded in sharp features of the wave packet spectral phase. Here we exploit the quantum beating between one- and two-photon transitions in an attosecond photoionization experiment to measure the photoelectron spectral phase continuously across a broad energy range. Supported by numerical simulations, we demonstrate that this experimental technique is able to reconstruct sharp fine-scale features of the spectral phase, continuously as a function of energy and across the full spectral range of an attosecond pulse train. In a proof-of-principle experiment, we observe the periodic modulations of the spectral phase of an attosecond pulse train due to the individual chirp of each harmonic