## Ultrafast and strong-field physics

**In the last decade tremendous progress has been made in the generation of well-controlled light fields. In the ultraviolet frequency regime, ultrashort light pulses with durations of less than 50 attoseconds (1as = 10- ^{18}s) have been created, while in the infrared regime strong femtosecond (1fs = 10^{-15}s) pulses that last only few optical cycles with intensities reaching several Petawatts (1 PW = 10^{15}W) per square centimeter have become available. As a result, the direct observation and control of electronic motion in atoms, molecules, and solids on its natural time scale have become possible, giving rise to new fields of research called attoscience and strong-field physics. **

**The new experimental possibilities pose great challenges to theory as an exact time-dependent quantum mechanical treatment of (multi-)electron dynamics in strong and short external electromagnetic fields is extremely difficult, most notably for many-electron systems. We tackle these problems with state-of-the-art theoretical approaches.**

**Time-resolved photoemission: ****Watching the “birth” of electrons**

The photoelectric effect, i.e., the emission of an electron after the absorption of a photon, is one of the most fundamental processes in the interaction of light with matter. The progress in the creation of ultrashort light pulses during the last decade has enabled the time-resolved study of photoemission with attosecond precision (1as = 10-18s). Photoionization has been found to be not instantaneous as conventionally being thought, but the departure of the outgoing wavepacket is temporally shifted relative to the arrival of the ionizing light pulse, typically by a few attoseconds.

We theoretically study the formation and emission of electronic wave packets from atoms and simple molecules by employing accurate quantum mechanical simulations based on the numerical solution of the time-dependent Schrödinger equation. We address fundamental issues of time and time delay as observables in quantum theory. In particular, we investigate the influence of electronic correlation on the time-resolved photoemission processes of multi-electron atoms, how the internal geometric structure of molecules influences the formation of the outgoing wavepacket, how time-resolved photoemission from complex targets such as atoms trapped in cages of Buckminster fullerenes (endohedral C60) offers new insights into transport and screening effects on the attosecond scale, or how the time elapsed between two photoabsorption events can be experimentally accessed in multi-photon ionization.

Selected publications:

Delay in Photoemission

M. Schultze et al., Science 328, 1658 (2010)

**How long does it take for electrons to leave a solid**

In a macroscopic piece of metal or doped semi-conductor, electrons move almost like a classical particle. They can thereby conduct electric current, enabling modern-day semiconductor electronics. In the quest to put more and more circuits onto a single chip, the width of individual Silicon structures is fast approaching the atomic scale where the quasi-classical movement of electrons may be modified by quantum mechanics. We investigate how electrons move through matter on the atomic length and time scales by staging a race between electrons through a few atomic layers . The time difference between the winner and loser can be measured with extremely high precision by the "attosecond streaking" technique. Similar measurements may reveal the concerted many-electron response in nano-particles or even a single buckminsterfullerene molecule as the local electric field influences the paths of emitted electrons.

Selected publications:

Direct observation of electron propagation and dielectric screening on the atomic length scale

S. Neppl et al, Nature 517, 342 (2015)

**Correlated electronic dynamics in helium: The quantum few-body problem**

Understanding the role of the electronic interaction in atoms, molecules, and solids has been a central theme in physics and chemistry since the early days of quantum mechanics. Unfortunately, the analytical solution of the Schrödinger equation is already impossible for the simplest many-electron atom, i.e., helium. However, the two-electron Schrödinger equation can still be solved numerically to high accuracy using supercomputers.

We have implemented an efficient highly parallelized solver of the time dependent Schrödinger equation for two electrons. With its help we study electronic correlation in the interaction (collision) of helium with photons, neutrons, and charged particles. Unravelling the dynamics in this simple, few-body system is crucial to understanding more complex atoms and molecules.

The questions we address include: the intricate interaction dynamics of continuum electrons in multi-photon double ionization processes which can be observed with the intense photon beams at free electrons lasers, the ultrafast dynamics of the two-electron wave packets which can be controlled and probed by ultrashort attosecond light pulses, and accurate (differential) cross-sections for single and double ionization in various collision processes with photons, antiprotons and neutrons.

Selected publications:

Nonsequential two-photon double ionization of helium

J. Feist, S. Nagele, R. Pazourek, E. Persson, B. I. Schneider, L. A. Collins, and J. Burgdörfer, Phys. Rev. A 77, 043420 (2008)

**Strong field ionization of atoms: highly-nonlinear quantum dynamics **

The dynamics of atoms and molecules in very intense laser fields (reaching the Petawatt regime) involves the interaction with a large number of photons and cannot be understood perturbatively. When the applied electric field is strong enough, it bends the confining Coulomb potential allowing the initially bound electron to leave the atom by tunnelling through the resulting potential barrier formed by the combination of the atomic potential and the external strong field.

The unfolding complex dynamics of the liberated electron is the starting point for several fascinating non-linear phenomena, such as high harmonic generation (i.e., the emission of highly energetic light pulses when the emitted electrons recombine with the parent ion), above-threshold ionization (i.e., the creation of the highly energetic electrons), or complex multi-electron dynamics (when the emitted electron is driven back to the parent ion where it can interact with the bound electrons).

**Current in dielectrics induced by strong fields**

Intense laser pulses can steer the electrons inside a solid and may thereby change the properties of the material dramatically within a very short time. We explore how a laser pulse can turn glass into a metal on the femtosecond time scale thereby envisioning a route to petahertz signal processing. On the other hand, short laser pulses can also steer electrons near metal nanostructures, yielding ultrashort pulses of electrons mirroring the sub-femtosecond quantum dynamics taking place at the nano-structure. We describe the quantum dynamics from first principles with time-dependent density functional theory (TD-DFT) as well as supporting calculations based on Maxwell's equations and (semi-) classical mechanics. This research is done in collaboration with Prof. Yabana from University of Tsukuba.

Selected publications:*Ab Initio* Simulation of Electrical Currents Induced by Ultrafast Laser Excitation of Dielectric Materials

Georg Wachter et al, Phys. Rev. Lett. 113, 087401 (2014)