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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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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 been 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.

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Molecular hydrogen is a key renewable energy material, and its adsorption on solid surfaces is important for its storage, transport, and catalytic reactions. Characterizing the structure and dynamics of these small and fast-diffusing molecules weakly adsorbed (physisorbed) on metallic surfaces has long been a major challenge. We investigate hydrogen molecules on a silver surface and demonstrate that low-temperature tip-enhanced Raman spectroscopy (LT-TERS) reveals the vibrational and rotational properties of the physisorbed molecule at the local tip–molecule–surface junction. For LT-TERS a silver probe tip is illuminated by a visible laser, causing plasmonic resonance to create a ultraconfined electric field on the (1 nm)³ scale in a plasmonic picocavity which significantly enhances the intrinsically small Raman scattering efficiency. We observe an enormously large isotope effect in the vibrational frequencies between hydrogen (H₂) and deuterium (D₂) in the junction. Theoretical calculations show that the nuclear quantum effects on the molecular density on the surface cause the distinctive isotope effect. The state-of-the-art LT-TERS technique will contribute to a better microscopic understanding of physical and chemical properties of hydrogen and other physisorption systems.

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The Gerhard Ertl Young Investigator Award recognizes young researchers for their outstanding talent and research in surface science. The award has been sponsored since 2010 by Elsevier and is awarded to the winner of a competition between 5 selected nominees, which present their work with an invited talk in a special session at the DPG Spring Meeting of the Surface Science Division. Niclas S. Müller wins the Gerhard Ertl Young Investigator Award 2025 with a talk entitled “Imaging Infrared Materials Excitations with Sum-Frequency Spectro-Microscopy” on investigations of hBN monolayers and SiC metasurfaces. With the nonlinear optical microscopy techniques developed by the Paarmann and Thämer groups it is possible to obtain spatial, spectral, momentum and crystallographic information of material properties and light-matter coupling with infrared sub-diffractional spatial resolution.

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Terahertz cavity electrodynamics is an emerging research area, offering exciting new opportunities for research of light-matter interactions. In a new experimental work, researchers leverage electro-optic sampling to directly measure electromagnetic fields inside of a macroscopic planar cavity for the first time.

Unlike conventional cavities, which rely on external substrates that introduce interferences and dispersive effects, the cavities in this work are produced by depositing mirrors directly onto a free-standing electro-optic crystal. This allows direct measurement of the electric fields inside the cavity as opposed to relying on transmitted light. To develop a route towards measuring the local fields of strong coupling, they designed a tunable electro-optic cavity, which allows the extraction of the cavity field in combination with full cavity frequency tunability and the ability to introduce a sample. Surprisingly, this ‘hybrid’ cavity geometry displays avoided crossings of the cavity modes, a signature of strongly-coupled oscillators, even in the absence of any material excitations, typically observed in strong-coupling. These new findings spurred theoretical developments, explaining how these avoided crossings arise from the hybridization of standing waves in each of the constituent cavity layers.

Looking forward, they expect this novel strategy to provide critical new insights into cavity field-driven phenomena, particularly at the intersection of fundamental polariton research and laser-driven nonlinear excitations.

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Metasurfaces are artificial structures that enable flat optical components and pave the way for on-chip light processing. A promising route to new functionalities is to build metasurfaces out of materials with strong optical resonances. Light then propagates across the metasurface in the form of polaritons – mixed light-matter particles. This recently enabled coherent thermal light sources and directional waveguiding. So far, it has been challenging to characterize metasurfaces because of their small sub-wavelength building blocks, their overall large spatial extent, and their wavelength-dependent properties, requiring complimentary imaging and spectroscopy techniques.

In their experimental work recently published in Advanced Materials, the Lattice Dynamics group introduces a new technique to image infrared metasurfaces with combined sub-wavelength spatial resolution and full spectral information. The authors developed a sum-frequency microscope, where the light of a tunable infrared free-electron laser is mixed with a visible upconversion laser inside of a metasurface. With this nonlinear technique it was possible to visualize how different types of phonon polaritons hybridize and propagate inside a SiC metasurface. By correlating spatial and spectral information, the authors found that strong coupling opens a route to tune light propagation in metasurfaces and even activates new polaritonic edge states, which are important characteristics for on-chip photonic devices and novel light sources.