Akitoshi Shiotari, who currently leads Atomic-Scale Microscopy & Spectroscopy Group in our department, will take up the position of Associate Professor at the Institute for Solid State Physics (ISSP), the University of Tokyo, in February 2027. He will establish a laboratory focusing on precisely evaluating the chemical and physical properties of molecules on solid material surfaces, beyond the resolution of conventional nanoscale analyses. He expects that comprehensive characterization of these properties from the picoscale to macroscale will contribute to a deeper understanding of diverse surface phenomena.
This is from index.phpThe interaction between light and materials is fundamental to many cutting-edge technologies, from high-speed communications to ultra-sensitive sensors. When light couples to collective lattice vibrations at a crystal surface, it creates unique quasi-particles called surface phonon polaritons. These hybrid light-matter excitations enable precise control over light at the nanoscale, paving the way for next-generation photonic devices. Until now, studying these evanescent modes has relied on tip-based scanning techniques, which, while powerful, are often time-consuming and limited in their ability to capture the full vector field of polariton modes.
In their latest work published in Optical Materials Express, the Lattice Dynamics group and their collaborators employ wide-field sum-frequency generation spectro-microscopy. In this approach a visible laser is overlapped with the in-house infrared free electron laser (FHI-FEL) to image the polaritons excited in the infrared with visible resolution. This method allows for selective probing of in-plane polariton field components, offering a faster and more comprehensive way to study complex nanophotonic structures. By combining high spatial resolution with polarization-sensitive imaging, the team demonstrates a powerful tool for uncovering the intricate dynamics of polaritons, opening new avenues for high-throughput nanophotonic research.
In September 2026 Youngwook Park will commence an assistant professorship at the Department of Chemistry, at Pohang University of Science and Technology (POSTEC), South Korea in the Surface Chemistry Lab. For his research he will develop a scanning tunneling microscopy (STM) experiment to study single-molecule manipulation and chemistry on surfaces, in particular, molecular motors, photo-chemistry and photo-induced synthesis on surfaces.
This is from index.phpSymmetry principles constitute the foundation of physics, linking conservation laws to translational and rotational invariance. In crystalline solids, the transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is a well-established concept governing thermal conductivity and equilibrium properties of materials. However, it has remained an open experimental challenge to directly observe how angular momentum is exchanged and conserved among lattice modes, even though angular momentum transfer within a crystal lattice is thought to play an important role in achieving magnetization equilibrium and in spin relaxation phenomena such as the Einstein-de Haas effect and ultrafast demagnetization.
In their Nature Physics paper, the THz Structural Dynamics group demonstrates the transfer of angular momentum between two lattice modes by employing the inverse process of anharmonic phonon decay. Umklapp scattering of rotational phonon-phonon is observed, which enforces the conservation of quantized crystal angular momentum as dictated by the discrete rotational symmetry of the crystal lattice. These findings provide direct experimental confirmation of the fundamental analogy between linear and angular momentum conservation in solids. Moreover, the work establishes axial nonlinear phononics as a promising new handle for the ultrafast control of material properties.
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This is from index.phpWater is arguably the most important molecule on Earth – playing a critical role in processes within physiology, at the ocean surface, and in the atmosphere. As these processes involve interfaces between water and other media, however, it is primarily the incredibly thin layer of water directly at the boundary that governs their behaviour. Crucially, the sheer presence of the interface perturbs the molecular structure of water, generating preferential orientations and an altered H-bond network, which give rise to profoundly different interfacial properties. While these unique structures are at the heart of many interfacial phenomena, characterising them is monumentally difficult.
In their latest work published in Science Advances, the Nonlinear Interfacial Spectroscopy group in collaboration with the group of Roland Netz from FU Berlin combine their newly developed depth-resolved interfacial spectroscopy technique with high-level simulations to investigate the fine details of the layer-dependent water structure at the interface to air. Using this experimental method, which combines phase-resolved sum- and difference-frequency generation spectroscopy, the authors could isolate the purely interfacial water spectrum. The analysis of the depth-dependent sum-frequency signals then reveals the pronounced layered structure of interfacial water with alternating tilt-twist orientations. These findings show that the common structural analysis in terms of “pointing up or down” of water molecules is largely insufficient by underlining the importance of the depth-dependent molecular twist distribution at the interface to air providing a revised structural picture of interfacial water.
The ability to manipulate light by squeezing it into smaller and smaller dimensions and steering it in the desired direction has given rise to a new field of study known as nanophotonics. One of the goals of nanophotonics is to create all-optical circuits that would replace electrons with photons in telecommunications technology. Over the past decade, hyperbolic phonon polaritons have played a key role in confining light to the nanoscale with low losses and high directionality. These hybridized modes arise from the ultrastrong coupling of infrared light to anisotropic lattice vibrations in uniaxial or biaxial polar crystals. In materials with lower symmetry, such as monoclinic beta-phase gallium oxide (bGO), hyperbolic shear polaritons (HShPs) emerge and their directionality is further enhanced. However, HShPs have the disadvantage of only existing within a limited spectral range.
This paper of the Lattice Dynamics group reports spectral tuning of HShPs in bGO via isotopic substitution. Employing near-field optical microscopy to image HShPs in 18O bGO films homoepitaxially grown on a 16O bGO substrate at Paul Drude Institute, the publication demonstrates a spectral redshift of approximately 40 cm−1 for the 18O bGO, compared to 16O bGO isotope. The technique allows for direct observation and a model-free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far-field measurements and ab initio calculations – in good agreement with the near-field data – confirm the effectiveness of this estimation. This multifaceted study, resulting from a collaboration with the University of Iowa and with the Theory Department of FHI, demonstrates isotopic substitution to enable significant spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons. Reference: Carini et al., Adv. Mater. 38, e14561 (2026)
Melanie Müller, head of the “Ultrafast Scanning Probe Microscopy” research group in the PC Department, took up a new professorship for experimental condensed matter physics at the University of Bonn on April 1, 2026. The group’s research continues to focus on the experimental investigation of light-matter interactions at the atomic scale and on ultrafast time scales. By combining low-temperature scanning probe microscopy with ultrashort light pulses and high-resolution spectroscopy, her group explores the ultrafast dynamics and out-of-equilibrium behavior of solid-state surfaces, quantum materials, and nanostructures at extreme spatiotemporal scales. These innovative experimental approaches enable new insights and a detailed understanding of nonequilibrium phenomena at surfaces.
https://www.fhi.mpg.de/2220902/2026-04-10_Professorship-Melanie-Mueller
This is from index.phpThe unique molecular water structure at charged interfaces governs various important interfacial processes in electrochemistry, environmental chemistry, and biology. Despite its great relevance, very little is known about the interfacial water structures, particularly their evolution with depth, which is mainly due to the lack of appropriate experimental techniques.
In their latest work the Nonlinear Interfacial Spectroscopy group presents a novel experimental approach which combines structural sensitivity with depth resolution on the nanometer scale. Using this technique, which exploits the complementary information from phase resolved sum- and difference frequency generation signals, the authors are able to successfully isolate the spectroscopic signal from the first water layers in direct contact with the surface charges (bonded interfacial layer, BIL) and compare it to the signals from water in the layers below (diffuse layer, DL). The analysis shows a remarkable change in structural anisotropy at the transition from DL to BIL by 2 orders of magnitude while the spectral analysis reveals that the anisotropy in both regions is clearly dominated by an anisotropic orientational molecular distribution without any notable changes in the hydrogen-bond structure. These findings significantly refine our understanding of the anisotropic water structure at charged interfaces and showcase the large potential of the presented depth-resolved spectroscopic technique.
This is from index.phpLaurenz Rettig formerly headed the research group “Dynamics of Correlated Materials” in our deopartment and has now been appointed as a professor at the Rheinland-Pfälzische Technische Universität (RPTU) in Kaiserslautern on January 1st, 2026. The aim of his group is to understand complex interaction phenomena in solids, for instance between electrons and lattice or spin excitations combining various complementary ultrafast investigation methods, in particular, time- and angle-resolved photoemission spectroscopy and momentum microscopy. Another focus are experiments on dynamics of magnetic materials, including experiments on large-scale research facilities. In Kaiserslautern will focus on ultrafast optical control of quantum materials using a combination of different methods and ülans to expand on these, for example, with spin- and time-resolved photoemission.
This is from index.phpMonolayers of hexagonal boron nitride (hBN) have been – besides their high relevance in 2D materials research – traditionally very difficult to handle due their lack of optical resonances making them essentially invisible in any optical microscope. Making use of the strong infrared resonance associated with a lattice vibration in hBN, the recently developed sum-frequency generation (SFG) microscope was shown to drastically enhance imaging contrast, enabling live-imaging of hBN monolayers. Furthermore, the phase-resolved SFG signal enables absolute determination of the crystal orientation which even allowed to extract the atomistic edge termination of triangular hBN flakes.