Electrons, phonons, and their mutual interactions are crucial for the complex phenomena in strongly correlated materials. In this talk, I will show that electron and phonon dynamics can be investigated at the atomic scale by combining THz pump-probe spectroscopy and a scanning tunneling microscope (STM) [1,2]. [more]

The study of confined fields in plasmonic nanocavities and their interaction with molecules and nanostructures is an area of research with vast applications, including enhanced spectroscopy techniques as well as photoinduced/photocatalytic non-equilibrium phenomena. From the theoretical perspective, either classical electromagnetic models or atomistic/quantum descriptions are usually considered. However, in many cases these models ignore the electronic and nuclear quantum effects arising from the chemical nature and dynamics of a junction, such as tunneling, adsorption geometry, structure of the interface, vibrations, etc., or include them only approximately. Hence, a full quantum dynamical description is sometimes inescapable. [more]

Transition metal dichalcogenides (TMDs) are an exciting model system to study ultrafast energy dissipation pathways, and to create and tailor emergent quantum phases [1,2]. The versatility of TMDs results from the confinement of optical excitations in two-dimensions and the concomitant strong Coulomb interaction that leads to excitonic quasiparticles with binding energies in the range of several 100 meV. [more]

Advances in time-resolved pump-probe spectroscopies have enabled us to follow the microscopic dynamics of quantum materials on femtosecond time scales. This gives us a glimpse into the inner workings of how complex, emergent functionalities of quantum many-body systems develop on ultrafast time scales or react to external forces. [more]

I will talk about the theoretical perspective on a microscopy technique that combines the atomic-scale resolution of a scanning-tunneling microscope (STM) with optics. [more]

Terahertz Time Domain Spectroscopy (THz-TDS) has become a ubiquitous tool in many scientific fields and is also increasingly deployed in industrial settings. While these systems become more and more mature, efficient and lab-based THz generation methods combining broad bandwidth and high dynamic range (e.g., as provided by high THz average power and correspondingly high repetition rate) remain rare. [more]

Femtosecond laser pulses irradiating a solid material induce a cascade of processes starting with the excitation of so-called hot electrons and passing through various relaxation processes. Several scattering mechanisms act on different timescales. At sufficiently high energy densities, phase transitions and ultrafast structural dynamics can be induced.We simulate the dynamics of a large ensemble of excited electrons using complete Boltzmann collision integrals. We consider the excitation of conduction electrons in a metal with visible light. On a femtosecond timescale, the electrons' energy distribution deviates strongly from a Fermi distribution. We extract spectral electron densities within specificenergy windows, and find complex behavior that cannot be matched with a single relaxation time.We show that electron-electron and electron-phonon scattering mutually influence each other during thermalization. For materials with several electronic systems, e.g. itinerant ferromagnets or dielectrics, we observe that temperatures and partial densities can be independent quantitieson picosecond timescales. [more]

In the last couple of decades, non-thermal (“hot”) carriers in nanostructures have been simultaneously an inspirational concept to which a series of effects were ascribed, but also a source of confusion and hot debates. My talk will be aimed at describing the advances we obtained in the understanding of the role played by “hot” carriers in metals as well as transparent oxides via rigorous modelling of their generation process and dynamics, and extensive comparison to previous and new collaborative experimental work. [more]

Driving phase transitions in materials with light on the ultrafast enables rapid control over material properties. Ultrafast spectroscopies have advanced so that we can track these events with attosecond temporal resolution. However, in general, time-resolved measurements are spatially averaged, so we do not know what is happening spatially. In (or close to) equilibrium, we know that phase transitions are often heterogeneous, with both phases existing on different length scales, but currently we do not know what happens on ultrafast timescales as we lack probes that can measure in time and space. In this talk, I will present our work where we exploit the power of X-ray lasers (XFELs) to study light-induced phase transitions on a range of length scales. I will show how we can track and control how the distribution of atomic positions changes during the phase transition. How we can use X-rays to measure phase transitions at a surface of a crystal, and how we can use resonant soft X-rays to image dynamic heterogeneity on the nanoscale. [more]