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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.

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Symmetry 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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Water 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.

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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

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The 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.

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Laurenz 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.

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Monolayers 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.

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The confinement of electromagnetic radiation to sub-wavelength scales relies on strong light–matter interactions. In the infrared and terahertz spectral ranges, phonon polaritons are commonly employed to achieve such subdiffractional light confinement and these optical modes offer much lower losses in compared to plasmon polaritons. Hyperbolic phonon polaritons in anisotropic materials, such as hafnium-based dichalcogenides, offer a promising platform and we report here on ultraconfined phonon polaritons with confinement factors exceeding λ₀/250 in the terahertz spectral range. This extreme light compression within deeply subwavelength thin films is enabled by the large magnitude of the light–matter coupling strength in these compounds and the natural hyperbolicity of HfSe₂. These findings emphasize the role of light–matter coupling for polariton confinement, which for phonon polaritons in polar dielectrics is dictated by the transverse–longitudinal optical phonon energy splitting. Our results demonstrate transition-metal dichalcogenides as an enabling platform for terahertz nanophotonic applications.

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The National Institutes of Natural Sciences (NINS) is a corporation of inter-university research institutes in Japan including the Institute of Molecular Sciences (IMS) and the National Astronomical Observatory (NAOJ). Each year, NINS presents the Young Researcher Award to outstanding early-career researchers affiliated with or who have collaborated with its institutes. In 2025, Dr. Akitoshi Shiotari was selected as an award winner for his exceptional achievements in single-molecule photochemistry, thanks to his strong collaboration with several research groups at IMS. The award ceremony took place in Tokyo, where NINS president Prof. Maki Kawai presented the winners with the award certificate. For the award lecture especially targeting high school students, Dr. Shiotari and the other winners introduced their latest research in an easy-to-understand manner, which was broadcast via web services (in Japanese). This gave the audience an opportunity to learn about the excitement and challenges of cutting-edge natural science and inspired the next generation of scientists.

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Scattering-type scanning near-field optical microscopy (s-SNOM) is a robust method for visualizing the optical response of surfaces with a spatial resolution down to 10 nm. Near-field signal detection relies on lock-in harmonic demodulation referring to tip oscillation driven by atomic force microscopy (AFM). The improvement of the spatial resolution requires stabilizing the sub-nanometer-scale tip-sample junction and improving the duty cycle of the near-field detection using a low tapping amplitude. However, both strategies are difficult to achieve with a conventional room-temperature setup based on tapping-mode AFM. In this study, 1-nm resolution s-SNOM is demonstrated based on noncontact-mode AFM using a quartz-tuning-fork sensor at a cryogenic temperature. The stable cantilever oscillation with an ultralow tip-oscillation amplitude allows for the sensitive detection of the near-field localized at the plasmonic Ag-tip–Ag-sample junction under visible laser illumination. With a Ag(111) sample partially covered by Si monolayer islands, we obtained s-SNOM images reflecting the material contrast between Si and Ag with 1-nm spatial resolution. The effective combination of noncontact-mode AFM, plasmonic cavity, and the elastic near-field detection has high potential for optical response imaging of photoactive detects and single molecules at atomic resolution.