Research

Time- and angle-resolved photoemission spectroscopy

Time- and angle-resolved photoemission spectroscopy is a pump-probe technique where a pump pulse excites the sample and a time-delayed probe pulse visualizes the pump-induced changes of the electronic structure. The pump pulse can be an infrared pulse resulting in electronic excitations, a narrow-band mid-infrared pulse exciting infrared active phonons, or a Terahertz pulse that coherently modulates the momentum of the electrons inside the solid. We use extreme ultraviolet probe pulses to eject photoelectrons from the sample and measure the resulting photocurrent as a function of emission angle and kinetic energy of the photoelectrons. This gives direct access to the time-dependent electronic structure as well as transient electron distribution in momentum space.

ERC project DANCE

The project is funded with 1.9 Million EUR from November 2020 to October 2025.

The band structure of solids is mainly determined by the orbital overlap between neighboring atoms. Therefore, electronic properties are commonly controlled via the chemical composition that determines the relevant structural parameters such as bond angles and lengths. DANCE will use a radically different approach where control of the effective orbital overlap is achieved by periodic modulation of the solid with strong mid-infrared and terahertz light fields. In this way, DANCE will control the band structure including topology, many-body-interactions, and spin. The induced band structure changes will be investigated with time- and angle-resolved photoemission spectroscopy.

We will implement two different driving schemes that either coherently modulate the atomic positions or the momentum of the Bloch electrons. Resonant excitation of infrared-active phonon modes results in a periodic modulation of the band structure at twice the driving frequency and, thus, a modified average band structure. In addition, non-linear coupling to Raman-active phonons leads to new quasi-static crystal and band structures. Coherent modulation of the Bloch electron’s momentum becomes possible if the scattering time is bigger than the inverse driving frequency and is predicted to result in various topological phase transitions as well as dynamical localization of carriers.

We will apply this approach to different low-dimensional solids with strong electron-phonon coupling and Dirac materials with long scattering times.

DANCE will address the following key questions:

• Can we switch between metallic, insulating and topological phases?
• Can we shape the potential energy surface of the solid to stabilize symmetry-broken ground states?
• Can we generate artificial magnetic fields to control the electron spin?

The success of DANCE will establish dynamical band structure engineering as a new method for electronic structure control and pave the way for novel optoelectronic and optospintronic devices.

CRC project on ultrafast charge transfer in van der Waals heterostructures

Solar energy conversion plays an important role in satisfying mankind’s ever-increasing energy usage in an environmentally friendly way. Efficient light harvesting devices need to combine strong absorption in the visible spectral range with efficient ultrafast charge separation. These features commonly occur in novel ultimately-thin van der Waals (vdW) heterostructures made by stacking different two-dimensional (2D) semiconductors such as monolayer transition metal dichalcogenides (TMDCs) and graphene in a lego-like manner. The occurrence of ultrafast charge separation can be traced back to the band alignment of the heterostructure. The relevant microscopic scattering channels, however, remain poorly understood.

In this project, we will combine time- and angle-resolved photoemission spectroscopy – a technique that allows us to investigate ultrafast charge transfer processes with unprecedented detail – with ab initio theory to seek answers for the following key questions:

• What are the relevant microsc