RTG seminar: Dr. David N Purschke
2025-03-26 13:30 - 14:30 (Europe/Berlin)
Strong‐fields and photons in solids, from classical to quantum light

Joint Attosecond Science Laboratory, University of Ottawa and NationalResearch Council of Canada

At the surface, strong‐field physics and quantum optics seem to operate in disparate regimes of light‐matter interaction. Nevertheless, the growing field of attosecond quantum opticsis beginning to bridge this gap, with the solid state providing an ideal platform of study [1]. I will discuss two experiments at this intersection: in the first, we see how a photon picture emerges from a laser‐dressed crystal using classical light [2]. In the second, we begin to study field‐driven processes with emerging non‐classical and intense sources known as brightsqueezed vacuum [3].

[1] L. Cruze‐Rodriguez et al. “Quantum phenomena in attosecond science,” Nat. Rev. Phys. 19 (2023) [2] D. N Purschke et al. “Microscopic mechanisms of highorderwavemixing in solids,” Phys. Rev. A 108 (2023)[3] S. Lemieux et al. “Photon bunching in high‐harmonic emission controlled by quantum light” arXiv:2404.05474

Venue: RUN 0.35

RTG seminar:Dr. Jan Gerrit Horstmann
2025-03-27 10:00 - 11:00
Optical control of nonequilibrium phases: from hammers to tweezers:

Department of Materials,ETH Zurich

Ultrafast pump-probe spectroscopy has not only provided remarkable insights into electronic and structural dynamics on atomic length and femtosecond time scales but has also enabled the optical control of nonequilibrium states. Over the past decade, a dominant trend in the field has been the shift from strong, unselective optical stimuli towards the selective excitation of few key degrees of freedom—in other words, from hammers to tweezers. This ap-proach promises more energy-efficient switching and access to entirely new states of matter.In this talk, I will present three examples of optical control over nonequilibrium states with increasing selectivity. The spectrum ranges from laser-induced thermal quenching of a multiferroic domain pattern to the all-optical amplification of a key phonon mode in a van der Waals ferroelectric, and finally to the ultrafast manipulation of metal-insulator nanotex-tures. Furthermore, I will provide an outlook on a novel approach for the investigation and control of structural dynamics based on non-resonant excit-ation of phonons via transient Raman forces.

Venue: RUN auditorium

RTG offsite workshop in Rötz
2025-04-07 08:00 - 2025-04-09 17:00
Venue: Hotel Wutzschleife

Guests:

Dr. Eva A. A. PognaIstituto di Fotonica e Nanotecnologie CNR-IFN

Henry Chapman FRSCenter for Free-Electron Laser ScienceDESY / University of Hamburg

Harald Gießen4th Physics InstitutePfaffenwaldring 57, 70569 Stuttgart, Germany.

RTG seminar:Prof. Dr. Götz Uhrig
2025-05-08 15:15 - 16:00
Concepts and applications in non-equilibrium physics

TU Dortmund

Non-equilibrium physics has become one of the thriving focal points in condensed matter physics. Pump-probe experiments are by now fairly common and further driving protocols attract more and more attention. We will discuss how periodic driving allows to cool a small quantum system by exploiting the commensurability of various oscillatory processes [1]. Interestingly, the Lindblad damping of a highly excited state is crucial here. Turning to ordered magnetic systems, it is common to describe their damping by Landau-Lifshitz-Gilbert equations. The question arises whether this phenomenological formalism is related to Lindblad damping. We will present such a conceptual link [2]. Finally, we explicity consider how one can switch magnetic order efficiently by electromagnetic pulses. How strong do the pulses have to be [3]? What is the influence of relaxation? These question are relevant for processing data in magnetic storage devices.

[1] Quantum coherence from commensurate driving with laser pulses and decay G.S. Uhrig; SciPost Phys. 8, 040 (2020)[2] Landau-Lifshitz damping from Lindbladian dissipation in quantum magnets G.S. Uhrig; arXiv:2406.10613[3] Exchange enhanced switching by alternating fields in quantum antiferromagnets A. Khudoyberdiev, G.S. Uhrig; Phys. Rev. B 111, 064408 (2025)

Venue: PHY 5.0.21

Special lecture: Prof. Milena Grifoni
2025-05-09 14:00 - 15:00
Introduction to Floquet theory

University of RegensburgHow to characterize properties of a quan-tum system subject to an intense time-periodic drive? We shall address this question within the framework of Floquet theory. According to the Floquet theorem, the solutions of the time-dependent Schrö-dinger equation display a quasi-stationary evolution, governed by quasi-energies, and a periodic part. Working in the Sambe space, it is possible to evaluate both the so-called quasi-energy spectrum and the Floquet functions without resorting to perturbation theory in the strength of the time-periodic drive or other commonly used approxim-ations. Noticeably, the quasi-energy spect-rum can be qualitatively different from the one of the undriven Hamiltonian, opening pathways to manipulate properties of quantum systems by a time-periodic drive. These concepts will be illustrated on the example of a strongly driven two-level system.

Venue: RUN auditorium

Lecture Notes: here.

RTG seminar: Artur Slobodeniuk
2025-05-15 15:15 - 15:45
Intraband motion corrections to the spectrum of excitons in transition metal dichalcogenide monolayers

Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

Transition metal dichalcogenide (TMD) monolayers are among the most intriguing two-dimensional materials for optoelectronic applications due to their unique valley-dependent electronic properties. In particular, the superposition of intravalley exciton states at the nonequivalent ±K points can be considered a qubit, which has potential applications in various quantum technologies. The manipulation of such qubits for dark intravalley excitons using intense magnetic fields was demonstrated in 2019 [1]. Subsequently, similar control over bright excitonic states using non-resonant circularly polarized light was both experimentally observed and theoretically explained [2,3]. In the latter, the theoretical analysis relied on the Semiconductor Bloch equations (SBE), where the dominant interband transition terms were considered. However, as it turned out, this approximation is valid for small intensities of applied optical pulses and needs to be reconsidered for more intense pulses. In this work, we analyze the solutions of the SBE, incorporating the previously neglected intraband term, which governs electron dynamics within the separate bands. We develop a new perturbation technique and calculate corrections to the shifts of intravalley exciton energies in TMD monolayers [4]. The possibility of experimentally measuring these results is also discussed.

References

[1] M. R. Molas et al, Phys. Rev. Lett. 123, 096803 (2019)

[2] A. O. Slobodeniuk et al, npj 2D Mater. Appl. 7, 17 (2023)

[3] A. O. Slobodeniuk et al, Phys. Rev. B 106, 235304 (2022)

[4] A. O. Slobodeniuk, T. Novotny, https://arxiv.org/abs/2501.06885

Venue: PHY 5.0.20

Joint RTG+IRTG onsite workshop: effective visual communication of science
2025-05-21 09:00 - 17:00
Venue: RUN 0.35

You will learn the Seyens Method™—a comprehensive design philosophy, process, and set of strategies tailored specifically for scientists to visually communicate your own complex ideas so your messages are effortlessly understood by scientists or non-scientists.It is a structured, no fluff, memorable, easy to follow, useful and fun training that will empower and enable you to create effective images, slides, posters, and grants. You will imediatelly apply the new skills to communicate your own science, draw a graphical abstract and discuss it with your peers and receive actionable feedback on your images and slides.

Find more information here and here.

RTG Colloquium:Franz Giessibl & Sabine Maier
2025-06-06 14:00 - 16:00
Venue: Run auditorium2 pmAtomic Force Microscopy (AFM)This tutorial presentation builds on Jascha Repp’s introduction to scanning tunneling microscopy (STM) six months ago. STM relies on the monotonic exponential decay of the tunneling current with distance. This decay at a factor of ten per Angstrom distance reduction ensures a high spatial resolution and a relatively simple implementation of STM. AFM is harder, as the forces between a sharp tip and a sample often have strong long-range van-der-Waals compo-nents, the forces are non-monotonic and the experimental measurement of small forces measurements (nN and below) is more difficult than the measurement of tunnling currents (nA and below). Frequency modulation AFM with stiff self-sensing cantilevers (qPlus sensors) provides a powerful solution that even obtains higher spatial resolution than STM.

Prof. Dr. Franz J. GießiblUniversity of Regensburg

3 pmBottom-up fabrication and electronic structure of atomically precise low-dimensional materialsThe ability to engineer atomically precise low-dimensional structures with tailored electronic properties is essential for the development of future electronic and quantum materials. This presentation focuses on molecular architectures constructed through bottom-up surface reactions. By combining scanning probe microscopy with density-functional theory, we investigate their atomic-scale structure and electronic characteristics. Recent advances in the synthesis and characterization of surface-supported 2D polymers and nanographenes illustrate how structural precision enables electronic property control. These studies offer fundamental insights into the design and functionality of low-dimensional materials.

Prof. Dr. Sabine MaierFAU Erlangen-Nürnberg

RTG Colloquium:Jan Vogelsang & Gerti Beliu
2025-07-04 14:00 - 16:00
2 pmFrom single‐molecule fluorescence spectroscopy to (fast) super‐resolution microscopySuper‐resolution (SR) microscopy has revolutionized far‐field fluorescence imaging by overcoming the diffraction limit of light. The technique relies fundamentally on the ability to detect single fluorescent molecules, and the first part will focus on achieving this, which requires carefully optimized technical and photophysical needs, including the manipulation of fluorophore behavior through photoinduced redox reactions. The second part will focus on a novel approach using DNA origami nanoantennas to dramatically enhance fluorescencesignals in single‐molecule experiments. Such nanoantennas can increase the fluorescence of single molecules by an order of magnitude, which increases the time resolution and facilitates the observation of ultrafast processes such as thediffusive barrier crossing events between two potential energy minima in disordered proteins.

Dr. Jan VogelsangUniversity of RegensburgInstitute of Experimental and Applied Physics

3 pmClick. Tag. Image –A Hitchhiker’s Guide to Molecular BioimagingSuper‐resolution microscopy holds the promise of visualizing molecular processes in cells with nanometer precision. However, realizing this potential requires labeling strategies that are not only bright and site‐specific, but also biologically orthogonal. Many commonly used labeling approaches interfere with protein function or fail to access conformationally restricted or sterically masked epitopes in live‐cell environments. In our work, we develop and apply biochemical labeling strategies that combine genetic code expansion, bioorthogonal click chemistry, and fluorogenic dyes to overcome these limitations. By minimizing linkage errors and enabling stoichiometric labeling under native conditions, we aim to push the limits of resolution, specificity, and functional integrity in molecular imaging. This talk will provide an overview of our toolbox, recent applications, and future directions for nanoscale imaging of proteins in living systems.Venue: RUN auditorium

Prof. Dr. Gerti BeliuUniversity of RegensburgInstitute of Pharmacy

RTG Colloquium:Ulrich Höfer
2025-07-11 14:00 - 16:00
Formation of Floquet-Bloch bands in solidsPeriodic driving of electrons in solids by strong light fields can lead to the formation of Floquet- Bloch bands, i.e., sidebands separated by multiples of the driving frequency. The induced change of the electronic structure is one route to materials design by light. In 2009, Oka proposed that circularly polarized light can turn the semimetal graphene into a Chern insulator in this way. While transport experiments could so far not provide compelling evidence for the predicted anomalous Hall effect, time-resolved photoemission has clearly demonstrated the formation of Floquet-Bloch bands for several materials, including graphene. Moreover, with subcycle resolution, it has become possible to reveal their ultrafast build-up. These experiments show that Floquet sidebands emerge after a single optical cycle, concurrently with intraband acceleration of the electrons.Venue: RUN auditorium

RTG Seminar: Ryo Shimano
2025-07-18 10:00 - 12:00
Higgs mode in superconductors studied by nonlinear terahertz spectroscopy Cryogenic Research Center & Department of Physics

The University of Tokio

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Recent developments of experimental techniques based on ultrafast spectroscopy have enabled the access to low energy collective modes in quantum materials in a time-resolved manner. In particular, the amplitude mode of the order parameter in superconductors, called the Higgs amplitude mode, has attracted a growing interest from the viewpoint of non-equilibrium dynamics in symmetry-broken ordered phases. In this presentation, I will introduce the study of the Higgs mode and other collective modes in conventional and unconventional superconductors using various nonlinear terahertz spectroscopic schemes.

Venue: RUN Auditorium

RTG seminar: Patrick Rupprecht
2025-07-23 14:00 - 16:00
Shedding light on ultrafast quantum dynamics with attosecond spectroscopyFeodor Lynen Postdoctoral FellowLeone & Neumark GroupsLawrence Berkeley National LaboratoryUSA

The physics and chemistry of matter is determined by the interaction of electrons and nuclei on ultrafast timescales down to the attosecond (10-18 s) regime. In this seminar, I will highlight how attosecond photon spectroscopy grants us a detailed look into the intricate quantum dynamics within atoms and molecules. Beyond precision metrology, shortest laser pulses allow us to precisely manipulate the quantum interactions of electrons and nuclei.

Venue: RUN auditorium

Tag der Naturwissenschaften
2025-07-25 08:00 - 17:00

RTG seminar:Martin Švec
2025-07-29 14:00 - 17:00
Photon-induced processes at single molecules in SPM nanocavitiesInstitute of PhysicsCzech Academy of SciencesPraha 6,Czech Republic

Exploration of isolated chromophores – optically active molecules – often reveals stunningly complex internal dynamics of excitations and transitions between electronic configurations, influenced by coupling to vibrational levels and reorganization of local environments. One of the promising tools for studying the photophysics of individual molecules with nanometer resolution is optical spectroscopy that employs plasmonic, atomically sharp tips in Scanning Probe Microscopy (SPM), which overcomes the fundamental spatial resolution limits of visible and infrared light. This methodology enables measurements that correlate electronic structure and atomic arrangement with various photon spectroscopies – e.g., fluorescence, absorption, and Raman scattering – as a function of applied electric field and measurement geometry. In this lecture, I will present our experiments with photon spectroscopies using SPM, focusing in particular on photon absorption-induced phenomena such as photoluminescence, and photon-induced current generation, through which we achieved a direct visual discrimination among the components of a multiconfigurational state of an open-shell chromophore.

Venue: RUN 0,35

RTG Seminar:Dr. Carolin Gold
2025-09-22 10:00 - 11:00
From microscopic to macroscopic phenomena:Bridging length scales in two- dimensional quantum materials

Columbia University

Two-dimensional quantum materials have driven a revolution in condensed matter physics through the realization of a large variety of exotic quantum phenomena, ranging from strong correlations and quantum magnetism to topological states. At the core of this revolution is the unique tunability of the quantum phases emergent at the macroscopic scale through variations in the microscopic roperties of the material. Despite decades of intensive theoretical and experimental endeavors, bridging these two length scales has remained a major challenge, leaving numerous questions nanswered.In this talk, we demonstrate how atomic force microscopy (AFM) can be used not only to (i) probe quantum electronic states across multiple length scales, but also to (ii) locally control and engineer the interplay between microscopic and acroscopic properties in van der Waals materials. Using the AFM tip as a mechanical actuator we bend narrow van der Waals ribbons, realizing (moiré) heterostructures with engineered twist angle and strain gradients. Combining these structures with low-temperature conductive AFM imaging not only reveals their unique electronic properties, but also opens exciting avenues for studying the interplay between microscopic and macroscopic length scales through versatile and novel AFM techniques.

Venue: RUN Auditorium

RTG Colloquium:Prof. Dr. Miriam Vitiello
2025-11-21 14:00 - 15:00
Detector-less near-field quantum nanoscopy in the far-infraredNEST, CNR – Istituto Nanoscienze and Scuola Normale Superiore, Pisa, Italy

Near-field nanoscopy at terahertz (THz) frequencies (wavelength range, 3 mm – 3 µm) enables studies of objects over a large span of scales – from 300 µm to the nanometer scale – and disciplines, from physics to biological science. The talk will show the potential of a series of near-field nanoimaging approaches that, while exploiting the same core building block (a detector-less QCL-based nanoscope), rely on different physical mechanism for imaging reconstruction. I will also highlight their potential for tracing the realspace propagation of THz-frequency polaritons in isotropic and anisotropic nanomaterials and meta-elements.

Venue: RUN Auditorium

Special lecture: Prof. Dr. Sebastian Loth
2025-11-28 14:00 - 16:00
Stochastic and coherent dynamics of spins and electrons at the atomic scaleInstitute for Functional Matter and Quantum Technologies, University of Stuttgart

Many fascinating phenomena at the atomic scale, such as energy transfer between quantumsystems or collective behavior of electrons in correlated materials, remain hidden to staticmeasurements. The combination of ultrafast spectroscopy with scanning probe microscopypresents a unique opportunity to study the dynamics of matter locally at its intrinsic length andtime scales. This lecture introduces recent advances using all-electronic microwave spectroscopy to capture quantum dynamics of magnetic atoms and small quantum magnets, as well as lightcoupled THz pump-probe techniques with femtosecond resolution that reveal phonon propagation and collective excitations in charge-density wave materials.

Venue: RUN Auditorium

RTG Colloquium:Prof. Dr. Jim Freericks
2025-12-02 14:15 - 15:15
How to describe pump-probe experiments in quantum materialsGeorgetown University, Washington, USA

I will describe both the challenges, and how to address them, when theoretically modelling ultrafast pump-probe experiments in strongly correlated materials and in materials with strong electron-phonon coupling. After providing an introduction, I will focus on three case studies: (i) examining the response at picosecond times of charge-density waves with ultrashort pulses that drive the electrons; (ii) examining the response on femtosecond times for nonresonant electronic Raman scattering; and (iii) examining how one can measure correlated charge dynamics in f-electronsystems using x-ray photoemission spectroscopy and x-ray absorption spectroscopy.

Venue: H34

RTG Seminar:Assistant Professor Ipsita Mandal
2025-12-04 15:15 - 16:15
Detectable signatures of topology of the bandstructures in 3d semimetalsShiv Nadar Institution of Eminence (SNIoE), India

We will elucidate the nature of the linear-response tensors in planar-Hall and planar-thermal-Hall set-ups, where we subject a 3d semimetal (harbouring nodal points of topological nature) to the combine influence of an electric field (and/or temperature gradient) and a weak (i.e. non-quantising) magnetic field. We will explain why it is essential to include the effects of the intrinsic orbital magnetic moment of the wavepackets in conjunction with the Berry curvature, in order to obtain a holistic picture of the effects of topology of the Brillouin zone in the linear-response tensors. Going beyond the well-studied example of Weyl semimetals, we will discuss the cases of their multifold cousins, harbouring higher Chern numbers. We will highlight how we can compute the exact solutions from the Boltzmann equations, which gets rid of the often-inaccurate results of the naive relaxation-time approximation. Finally, we will outline how to derive surface states, in the form of Fermi arcs, characterising generic nodal points.

Venue: PHY 5.0.21

RTG seminar:Dr. Kirill Voronin
2026-01-12 14:00 - 15:00
Theory of near-field microscopy andquantum optics of anisotropic materials

Donostia International Physics Center, (DIPC) San Sebastian, Spain

The talk presents our recent theoretical advances in near-field microscopy and quantum optics of anisotropic media. First, I discuss an analytic theory of near-field microscopy, its extension to layered structures, and open theoretical challenges elevant for experiments. I then address quantum electrodynamics of strongly anisotropic materials, including quantization of hyperbolic phonon polaritons and emitter–polariton coupling, highlighting open questions and potential directions for future studies.

Venue: RUN Auditorium

RTG Colloquium: Prof. Dr. Rudolf Bratschitsch
2026-01-16 14:00 - 15:00
Title: Magnonic waveguide networks

Ultrafast solid-state quantum optics and nanophotonics, Physical Institute, University of Münster, Münster, Germany

Spin waves (magnons) have emerged as a promising platform for next-generation physical computing. However, current experimental realizations of magnonic waveguides suffer from limited spin-wave propagation lengths and inefficient dispersion tuning capabilities. In my talk, I will present low-loss magnonic waveguides in a thin magnetic insulator film, fabricated with a new maskless silicon ion implantation technique. We demonstrate a large-scale magnonic network, paving the way for wafer-scale magnonic integrated circuits.

Venue: RUN Auditorium

RTG Colloquium:Dr. Anna Rosławska
2026-01-23 14:00 - 15:00
Single-molecule optics with atomic precision

Atomic Scale Optics, Max Planck Institute for Solid State Research, Stuttgart, Germany

Luminescence, photosynthesis, and energy harvesting rely on processes originating at the spatial scale of individual molecules. Investigating the details of these mechanisms requires reaching subnm precision in optics, which becomes possible by combining optical spectroscopy with scanningprobe microscopy. In my talk, I will discuss how atomic-scale optics can be used to study the optical properties of single molecules and to drive photochemistry with sub-nm precision.

Venue: RUN Auditorium

RTG Colloquium:Prof. Dr. Renske van der Veen
2026-01-30 14:00 - 15:00
Fast electrons and hard X-rays for unraveling atomic-scale dynamics in light-energy conversion

Department Atomic-Scale Dynamics in Light-Energy Conversion, Helmholtz Zentrum Berlin

The increasing demand for renewable and low-cost energy motivates intensive research aimed at characterizing and optimizing materials that can efficiently convert (sun) light into usable energy in the form of electricity or chemical fuels. Conventional characterization techniques either lack the spatial resolution necessary to resolve individual atoms, or they lack the temporal resolution required to capture structural rearrangements as they evolve. Our group develops complementary X-ray and electron-based tools to visualize light-induced processes in materials on atomic length and time scales. In this talk I will introduce you to the techniques of ultrafast/time-resolved X-ray spectroscopy and transmission electron microscopy and provide several examples of how these techniques can be used in the fields of solar energy and catalysis.

Venue: RUN Auditorium

RTG Seminar:Dr. Zijian Lyu
2026-02-06 10:00 - 11:00
Attosecond quantum spectroscopy with entangled photon pairs

Peking University, China

Entangled photon pairs provide nonclassical correlations that can be up-converted in strong-field nonlinearities. I will show how these correlations map onto high-order harmonics in solids, enabling correlation-resolved spectroscopy of carrier dynamics. By introducing a two-color driving field, we further achieve ultrafast, in-situ control of the quantum state of the emitted light.

Venue: RUN.0.35

Softskill workshop: Unconscious Bias
2026-02-25 09:00 - 16:00
Venue: RUN 0.35

“Our life is marked by prejudices and unconscious biases that influence our thoughts and actions. In this workshop, we will embark on a journey to become aware of these unconscious biases and develop strategies for better handling them in our daily lives. We will create space for open discussions and respectful exchanges to collaboratively devise solutions and have a positive impact on our personal relationships and society as a whole. This workshop provides an opportunity to gain a deeper understanding of the topic of bias and to take concrete steps towards change.” [J. Handschuh]

Find more information here.

RTG offsite workshop
2026-03-23 08:00 - 2026-03-25 17:00
Hotel Wutzschleife, Rötz

RTG Colloquium jointly held with the Physics Colloquium: Oliver Graydon
2026-04-20 16:00 - 17:00
The role of AI in scientific publishing

Chief Editor Nature PhotonicsSpringer Nature, UK

Artificial intelligence (AI) is set to bring profound transformations in how science is conducted and communicated. This talk with discuss the trends, opportunities, risks and challenges associated with AI from the viewpoint of scientific publishing with a focus on clarifying the present policies and principles at Nature journals and Springer-Nature.

Venue: H34

RTG Colloquium: Zhiyang Zeng
2026-04-24 14:00 - 15:00
Dynamical control of structural order

Condensed Matter Dynamics DepartmentMPI for the Structure and Dynamics of Matter, Hamburg

Coherent excitation of lattice vibrations has emerged as a powerful approach to dynamicallycontrol functional material properties. In this talk, I will present our recent work on using nonlinearphononics—a mechanism that enables the selective displacement of specific lattice vibrations—tomanipulate structural functionalities with light [1,2]. In particular, I will discuss how this approachenables the ultrafast control of chirality [3] and ferroaxial order [4], offering new pathways to tailorsymmetry and drive phase transitions in solids on demand.

[1] Först, et al. Nature Physics 7 (2011): 854-856.[2] Disa, et al. Nature Physics 17 (2021): 611-618.[3] Zeng, et al. Science 387.6732 (2025): 431-436.[4] Zeng, et al. Science 390.6769 (2025): 195-198.

Venue: RUN Auditorium

RTG Seminar: Yiqi Fang
2026-05-21 13:00 - 14:00
Shaping electron dynamics with structured light

FAU Erlangen, Institute of Laser Physics

Laser-excited electron motions allow the investigation of fundamental physical phenomena with unprecedented resolution in time, space and energy. Structured light offers new opportunities to tailor and control the optical response of materials, far beyond the ability of conventional linearly or circularly polarized light. In this talk, I will show the physics and application of structured light when it interacts with bound electrons in atoms as well as when it interacts with electron pulses in free space.

Venue: RUN.0.35

RTG Colloquium: Thomas Taubner
2026-05-22 14:00 - 15:30
Infrared near-field spectroscopy of few-layer graphene

RWTH Aachen, Germany

Infrared scattering-type scanning-near-field optical microscopy (s-SNOM) enables the imaging of nanostructures with sub-diffraction resolution by elastic light scattering at a sharp tip. s-SNOM also boosted the field of 2D materials by enabling real-space imaging of plasmon- or phonon polaritons, e.g., in graphene and hexagonal Boron Nitride (hBN), respectively. Polariton imaging with s-SNOM has allowed for indirectly mapping (grain) boundaries and (stacking) defects in fewlayer Graphene (FLG) via polariton reflection.Here, I will present how spectroscopic IR s-SNOM is able to determine the local stacking order at nanoscale resolution not only of bare few-layer graphene, but also when it is encapsulated below hBN layers. I will explain the contrast mechanisms, mainly based on the characteristic and stacking-dependent interband transitions of FLG, for bilayer graphene, trilayer (TLG) graphene and tetralayer graphene (4LG). We retrieve and reconstruct these characteristic complex optical conductivity resonances from the amplitude and phase of the scattered light, which e.g. allow for the unambiguous assignment of previously undetected ABCB domains in 4LG.Coupling the (stacking-dependent) FLG with phonon polaritons in a hBN TLG heterostructure also leads to coupled polaritons (hyperbolic phonon plasmon polaritons). We explore how they allow for super-resolution imaging of subdiffraction-sized defects in graphene through the hBN cover layer via the so-called hyperlensing effect. Furthermore, we show how resonators for polaritons in 2D materials like hBN can be easily fabricated and optically reconfigured using Phase-Change Materials.

Venue: RUN Auditorium
