The interaction of light with matter is a central research topic in physics, as it provides fundamental insights into atomic and molecular structures and their dynamics. The photoelectric effect, i.e. the release of an electron from a material through the absorption of light, is an important experiment for the physical understanding of light-matter interaction. Its theoretical interpretation with the help of light quanta (photons) by Albert Einstein in 1905 contributed significantly to the foundation of quantum physics. In addition to its fundamental significance, the photoelectric effect is an essential tool in modern basic research with a wide range of applications, for example in photovoltaics for power generation and in light detection and imaging using highly sensitive sensors. Photoelectron spectroscopy, whose development was honoured in 1981 with the Nobel Prize in Physics awarded to Kai Siegbahn, enables high-resolution analysis of electronic states in atoms, molecules and solids. The development of coherent light sources, in particular lasers, has enabled the targeted control of quantum mechanical dynamics on ultrashort time scales and opened up new applications in solid-state physics, plasma physics, medicine and quantum information.

In recent decades, groundbreaking advances in ultrafast spectroscopy have been recognised by several Nobel Prizes, including for femtochemistry (Zewail, 1999), the development of the optical frequency comb (Hänsch, 2005), chirped pulse amplification (Mourou & Strickland, 2018) and attosecond physics (Krausz, Agostini & L'Huillier, 2023). The interaction of intense laser pulses with matter leads to non-linear optical multiphoton processes, which play a central role in ultrafast spectroscopy. Their theoretical description goes back to Maria Göppert Mayer, who predicted the simultaneous absorption of several photons to excite atomic states as early as 1931, thus laying the foundation for modern non-linear optics. One such non-linear optical process is multiphoton ionisation (MPI), in which electrons are released from atoms or molecules by absorbing several photons.

In our research group ULTRA (ultrafast coherent dynamics) we investigate the laser control of atomic and molecular MPI dynamics induced by temporally structured femto- and attosecond laser pulses. In the experiment, we reconstruct the released three-dimensional photoelectron wave packets using tomographic methods.

Ultrashort laser pulses not only enable the time-resolved observation of ultrafast quantum phenomena, they are also the key to the active control of quantum mechanical processes. The aim of quantum control is to selectively and efficiently steer a quantum system from its ground state to a desired target state with the help of shaped femtosecond laser pulses . According to Louis de Broglie, quantum particles such as the electron also have the character of waves and are described by matter waves. The physical mechanism of quantum control is based on the targeted manipulation of constructive or destructive interference of these matter waves using specially customised laser pulses.

A prominent implementation of this principle is the Brumer-Shapiro scheme, in which the quantum system is excited with two pulses of different colours and a specially tuned frequency ratio. The quantum system is simultaneously steered via two different paths into the target state in which the matter waves interfere. The type of interference, i.e. whether the matter waves are amplified or cancelled out, is precisely controlled by the optical phase of the pulses.

The control of MPI with polarisation-shaped femtosecond laser pulses is particularly fascinating. This generally opens up a multitude of excitation paths. The excitation dynamics and inter-distance phenomena are correspondingly rich, which makes MPI highly interesting for research into fundamental mechanisms of quantum control.

High-intensity laser fields not only offer significantly higher excitation efficiency, but also open up completely new control mechanisms as they manipulate the structure of the quantum system itself. Coupled light-matter states arise, which are referred to as "states dressed by the light field". One research focus of our group is the development of novel strong-field control mechanisms for the selective excitation of these dressed states, known as SPODS (from "Selective Population of Dressed States").