Perovskite surfaces attract attention in the catalysis community due to these materials’ promising chemical properties, good ability to separate electron-hole pairs in light harvesting, and the presence of ferroelectricity in many perovskites. While perovskites possess a unique set of interesting bulk properties, their surfaces are much less understood; the main open questions are their structural stability and associated chemical reactivity and catalytic selectivity. [more]

The opto-electronic properties of the transition metal dichalcogenides (TMDs) are sensitive to their environment. For example, the presence of graphene in the vicinity of the TMDs modifies their exciton binding energy and the magnitude of the bandgap via external dielectric screening. [more]

Observing molecular dynamics experimentally with both, highest spatial and temporal resolution is one of the biggest challenges in chemistry and biochemistry. Understanding and resolving structure-dynamics relationships will help to further understand molecular function. Few experimental methods allow to resolve multi-scale dynamics and structural information in the same experiment. [more]

When working in the infrared (IR) or terahertz (THz) spectral ranges, traditional optical materials like gold and silver have extremely large and negative permittivities. This means it is difficult to use these materials for plasmonics or hyperbolic metamaterials, both of which require materials with relatively small and negative permittivities. We must therefore explore alternative materials. In this talk, I will focus on two classes of materials: heavily-doped III-V semiconductors for the IR and topological insulators for the THz. [more]

Advances in mid and far-infrared THz sources have created a new paradigm in condensed matter physics: ultrafast structural and functional control through direct lattice excitation. Striking changes in magnetism, metallicity, ferroelectricity, and superconductivity, observed experimentally on ultrafast timescales, have been tied to the anharmonic coupling between pumped infrared-active (IR) phonons and Raman-active phonons via the nonlinear phononics effect. [more]

The first topic of this talk is focused on the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst (SAAC) Pd/Cu(111) [1]. The hydrogen spillover on the Cu(111) surface from the Pd site was successfully observed in real-time using infrared reflection absorption spectroscopy (IRAS) at 80 K. The observed chemical shifts of Pd 3d5/2 in X-ray photoelectron spectra (XPS) indicate that H2 is dissociated and adsorbed at the Pd site initially. [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]

In this talk, I discuss our recent efforts in the context of nano-optics and photonics, with a special emphasis on strong light-matter interactions enabled by excitonic, phononic, electronic and magnonic material responses coupled to engineered metasurfaces. I will discuss our recent theoretical and experimental results in the context of polariton manipulation in these systems, the role of symmetries in their control, and their opportunities for technological advances. The combination of these features with photonic engineering enables giant optical nonlinearities, efficient nanoscale light manipulation and topological wave phenomena. During the talk, I will discuss the exotic light-matter interactions arising in these systems, and their opportunities for wave physics and photonics technologies. [more]

Exploration of essential photophysics at the level of individual molecules and atoms requires highly specialized optical spectroscopies that work at the very limit of instrument sensitivity or have to use plasmonic nanostructures - in order to overcome the fundamental resolution limits achievable with visible and infrared light. [more]

Symmetry and its breaking sit at the core of condensed matter physics research and determine the ordering and unique functionalities of solid-state materials. Ultrashort light pulses from visible to THz wavelength ranges offer the opportunity to probe and control the electronic, phononic, magnetic, and even time ordering in those materials. [more]

We investigate second-harmonic generation (SHG) light from a Pt surface in air under terahertz (THz) pulse irradiation. THz pulse-modulated SHG intensity shows a clear time profile of the THz field. [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]

Higgs spectroscopy is a new and emergent field that allows to classify and determine the superconducting order parameter by means of ultra-fast optical spectroscopy. There are two established ways to activate the Higgs mode in superconductors, namely a single-cycle ‘quench’ or an adiabatic, multicycle ‘drive’ pulse. [more]

Molecular motors, an important class of molecular machines, harness various energy sources to move unidirectionally [1]. The operational principles of molecular motors are distinct from those of man-made macroscopic motors, because they have nanoscale dimensions and generally work in a solution environment where viscosity is dominant. Under these low Reynolds number, overdamped conditions, they cannot rely on inertia to sustain motion. Furthermore, they are continually agitated by random Brownian motion, which provides both challenges and opportunities for the unidirectional motion. [more]

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Under the Microscope a science communication project dedicated to materials and nano science. Despite the widespread relevance of materials science to everyday life, we feel that dedicated science communication in this area is much rarer than in other fields. [more]

The study of the suppression of an order parameter by an external perturbation and the following recovery of the broken-symmetry phase is a problem relevant to systems even beyond condensed matter physics. In the context of pump-probe experiments, it has been tackled considering the suppression of the charge density wave order parameter in several compounds, and it was found a relevant role played by the fluctuations [1,2]. [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]

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Ultrashort laser pulses can induce coherent phonons, where all atoms in the crystal oscillate in phase. Using ultrafast electron diffraction, we can directly image this joint atomic motion in the time domain. [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]

Using low-temperature scanning tunneling microscopy and occasionally synchrotron radiation methods we investigate molecules at surfaces. The experiments along with model calculations reveal molecular spin states and electron transport properties as well as intermolecular interactions. [more]

In dye-sensitized solar cells (DSSCs) and in photo-catalytic devices designed for hydrogen production of CO₂ reduction, light triggers ultrafast molecular processes, such as electron, energy transfer or singlet fission. Since these processes are at the heart of the function of the devices and of their efficiencies, the design of new molecular photo-sensitizers or catalysts can be optimized rationally if these processes are monitored by ultrafast spectroscopy. [more]

Our group specializes in ultrafast spectroscopic methods, enabling in-depth studies of material chemistry in intricate environments and the control of quantum phenomena on femtosecond timescales. In the first part of this seminar, I will discuss the role of lithium in various systems from its contribution to symmetry breaking (LiNbO₃), to an exotic quantum material (polar metal LiOsO₃), to unravel the reasons behind the low hopping rate of lithium ions at the surface of a solid-state electrolyte (Li_xLa_(2-x)/3TiO₃). All these systems share the common feature that Li occupies a symmetry-broken state which we can selectively probe using extreme-ultraviolet second-harmonic generation spectroscopy (XUV-SHG), a novel spectroscopy pioneered in my group. In the second part I will discuss recent results on 1T-TiSe₂, a prototypical charge-density-wave (CDW) compound that also exhibits strong excitonic correlations in its low-temperature phase. [more]

Microscopic understanding of exciton physics in molecular materials for optoelectronics is a great challenge because of their complexity resulting from strong electron-phonon coupling and perhaps interaction to spin degree of freedom, electron spin-flip of intersystem crossing in molecular optoelectronic materials are strongly connected to molecular geometries in the excited states and vibronic coupling, and singlet fission, ultrafast generation of a correlated triplet pair state from a singlet excited state, is viewed as an extreme example of a concerted process of electron-phonon-spin degrees of freedom. [more]

Science faces the challenge of ever-increasing and more complex data that push existing evaluation methods to their limits. Additional requirements such as reproducibility, data security, consistency, transferability, and reusability cause significant effort and are hardly met by traditional manual workflows. [more]

Different processes are seen as relevant for the reaction rate enhancement of chemical reactions on optically excited metal nanoparticles. Recent studies suggest that all processes that follow a plasmon excitation, excited electrons, strong local fields and heat, can be of relevance for specific reactions. [more]

Proteins are the machinery of life ­and understanding their structure provides important clues about their mode of action. Since proteins are so important for biology and medicine, more than 100.000 protein structures have been determined experimentally and are available in databases. [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]

Liquid-Solid interfaces are the primary region where molecules interact through molecular motion and chemical reactions. However, it is challenging to gain insight into how interactions between the surface and solution modify molecular behavior. A powerful tool for evaluating these phenomena and molecular properties is in-situ surface-sensitive vibrational spectroscopy, such as surface-enhanced Raman scattering (SERS) induced by plasmonic materials.[1] [more]

Transition metal dichalcogenides (TMDC) are excellent models for the exploration of semiconductor physics at the 2D limit, with potential applications in electronics, optoelectronics, and quantum devices. The strong Coulomb interactions and distinct structural symmetries in these materials give rise to a rich variety of photoexcited states, including excitonic complexes that are tightly bound electron-hole pairs, and valley-spin polarized. However, directly accessing the momentum direct and indirect excitons and their dynamics are out of optical experimental reach. Here, I will talk about the generation of high repetition rate higher order harmonics (HRR-HHG) and its coupling with momentum microscope, to establish HRR time- and angle -resolve photoemission spectroscopy (TR-µARPES), demonstrated on a micron-scale monolayer WSe2 flake (1). This measures the momentum direct and forbidden excitonic states across entire Brillouin Zone (BZ) and measures their dynamics under different excitation conditions (2). The direct access of excitonic energy-momentum distribution leads to the measurement of excitonic wave function revealing the exciton size in real and k-space, whose electron follows the downward curvature of its partner hole (3). Beyond excitonic states, I will also visualize the evolution of the conduction band electrons in a 1LWS2-gated device captured through nano-ARPES (4). [more]

Combining lattice and electronic dynamics with functional material properties is a holy grail for condensed matter science. For example, combing semiconducting and magnetic states in a material would enable the unlocking of spin-based electronics such as non-volatile transistors, which are key for low-energy computing [1]. In this talk I will detail our efforts towards lattice-based control of material properties in two important areas. [more]

Sum frequency generation spectroscopy (SFG) is a valuable technique to study the molecular properties of surfaces. As a second-order technique, it is uniquely sensitive to the average organization of molecules at the surface. However, as most surfaces are spatially heterogeneous, it isn't easy to interpret the spectrum as a single domain. The development of SFG into microscopy has allowed a more detailed and accurate analysis of the spatio-spectro-temporal evolution of surface chemistry. The SFG microscope development will be presented, and compressive sensing and the application toward thin films will be used. [more]

We aim to control the symmetry of molecular spin structures on solid surfaces and design a two-dimensional (2D) organic quantum bit network with exceptional quantum spin properties. To achieve this, we have employed transition metal atoms and organic molecules as materials. Over the past two decades, we have investigated surface spin structures using scanning tunneling microscopy and spectroscopy (STM/STS), as well as spin-polarized STM/STS, all conducted in ultrahigh vacuum (UHV) at cryogenic temperatures, in combination with density functional theory (DFT) calculations [1-4]. [more]