What we do


Scientific scope

The ground state properties of complex materials as well as the reaction pathways of photo-excited states are governed by many-body phenomena and the mutual dependence of electron, spin and lattice subsystems. Ultrafast techniques provide access to these fundamental correlations as femtosecond light pulses allow for subsystem-specific excitations and time-resolved observation of the same or another subsystem’s response. In the limit of weak optical excitation, this concept may be seen as an experimental perturbative approach where correlations governing ground state properties are revealed by the system’s response to modest, but specific excitations. Employing intense ultrashort laser pulses, on the other hand, allows for the preparation of transient states of matter exhibiting strong non-equilibrium, in particular between electrons and lattice. The coupling of the subsystems of such states can be significantly different compared to the ground state and the induced photo-physical or photo-chemical reaction may follow a non-thermal reaction pathway not available in thermal equilibrium.

Illustration of electron, lattice and spin degrees of freedom in a solid. Some fundamental energy excitations of the subsystem are indicated in the respective box. Ultrashort laser pulses allow for sub-system excitation of the material, for instance excitation of electrons by a visible or near-infrared laser pulse. This results in a transient state with non-equilibrium between electrons, lattice and spins, i.e. hot electrons in a cold lattice. Subsequently, the subsystems equilibrate by transfer of energy, momentum and angular momentum. Employing sub-system sensitive probes like femtosecond electron diffraction for studying the response of the lattice or time- and angle-resolved photoelectron spectroscopy (trARPES) for watching the evolution of the electronic structure, we reveal the timescale of the equilibration and learn about the coupling mechanisms between electrons, lattice and spins.

Illustration of electron, lattice and spin degrees of freedom in a solid. Some fundamental energy excitations of the subsystem are indicated in the respective box. Ultrashort laser pulses allow for sub-system excitation of the material, for instance excitation of electrons by a visible or near-infrared laser pulse. This results in a transient state with non-equilibrium between electrons, lattice and spins, i.e. hot electrons in a cold lattice. Subsequently, the subsystems equilibrate by transfer of energy, momentum and angular momentum. Employing sub-system sensitive probes like femtosecond electron diffraction for studying the response of the lattice or time- and angle-resolved photoelectron spectroscopy (trARPES) for watching the evolution of the electronic structure, we reveal the timescale of the equilibration and learn about the coupling mechanisms between electrons, lattice and spins.

These concepts require the application of different, complementary probes revealing the temporal evolution of the different subsystems: femtosecond electron diffraction (FED) unfolds the ultrafast dynamics of the atomic structure and phonon populations, femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) reveals the evolution of the electronic structure, and femtosecond optical spectroscopy provides the optical response. We investigate correlations and photo-induced reactions by employing, and partially developing, these ultrafast techniques. As these phenomena inherently depend on the symmetry of the system, specifically on the dimensionality of the translational symmetry, we put an emphasis on the development of ultrafast probes highly sensitive to structural and electronic dynamics in two- and one-dimensional systems. In particular, we develop point-sources of femtosecond low-energy electron wave packets and use them for diffraction and point-projection microscopy.

Deutschlandfahne_2 Eine Übersicht unserer Forschungsaktivitäten in deutscher Sprache findet sich im MPG Jahrbuch 2015: Im Fokus ultrakurzer Laser: das Wechselspiel von Elektronen und Kristallstruktur.


Research projects

Non-equilibrium structural dynamics of crystals

  • Structural dynamics of semiconducting transition metal dichalcogenides

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  • Electron-phonon interaction in quantum-confined systems

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We study the microscopic energy flow within size-selected gold clusters and their coupling to carbon-based substrates.

  • Phase change materials

GSTWe investigated the relation of structural and optical properties of the phase change material GST (Ge2Sb2Te5).


Visualization of photocurrents in nanostructures

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Ultrafast electron dynamics in nanoscale materials

  • Excited state dynamics and conduction band mapping of layered semiconductors

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  • Observation and control of electron motion in solids on the attosecond time scale

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Attosecond photoelectron streaking: a time-resolved view on the photoelectric effect.


Techniques and method development

Time- and angle-resolved photoelectron spectroscopy (trARPES) with a high-repetition rate extreme ultraviolet (XUV) laser

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Femtosecond low-energy electron diffraction and imaging

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Femtosecond transmission electron diffraction

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