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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.

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The European Research Council has awarded Melanie Müller an ERC Starting Grant for her project FASTOMIC. With a total amount of €1.5 million for a period of five years, this grant will allow her and her team to push the limits of ultrafast scanning tunneling microscopy to study the emergence and dynamics of nonequilibrium quantum states in real time and space. The project entitled “Ultrafast atomic-scale imaging and control of nonequilibrium phenomena in quantum materials” is one out of 494 funded projects in the 2024 ERC-StG call.

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The manipulation of optical and electrical properties at the nanoscale has been critical for the development of miniaturized devices such as single-molecular sensors and photoswitches. Localized surface plasmons (LSPs), which are confined light waves generated at surfaces of nanomaterials, are powerful tools for building optoelectronic devices, as they enable reaction pathways that are inaccessible with far-field light. Traditionally, metallic structures have been targeted for LSP-induced photochemistry, which has not expanded the application of this technology to more promising platforms for nano-optoelectronics, such as semiconductor substrates.

In their recent work published in Nature Communications, the Atomic-Scale Microscopy & Spectroscopy group demonstrated the atomic-precision control of plasmon-induced single-molecule switching on a silicon surface using laser-coupled scanning tunneling microscopy. A single perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule works as a photoswitch by toggling between the silicon surface and a silver tip, triggered by LSPs at the tip excited by visible laser irradiation. This process involves the reversible breaking and formation of O–Si bonds between the molecule and the surface, as characterized with the simultaneous conductance measurement and tip-enhanced Raman spectroscopy (TERS). They achieved precise control over the reaction dynamics by adjusting the gap distance of the silver tip from the molecule in the order of 10 picometers. Their findings suggest that metal–single-molecule–semiconductor nanojunctions could serve as a versatile platform for nano-optoelectronics, or even beyond, reaching the realm of picoscale control.

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On June 1^st, 2024, physicist Sebastian F. Maehrlein will commence his professorship in high-field terahertz physics at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). In a joint appointment with the Technical University of Dresden, he will head the High-Field THz-driven Phenomena group at the Institute of Radiation Physics at the HZDR.

https://pc.fhi-berlin.mpg.de/tsd

https://www.hzdr.de/db/Cms?pOid=72114&pNid=0

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Biological membranes consist of ordered molecular assemblies of various chiral phospholipids, most of which are spatially inhomogeneous. This heterogeneity is expressed by the formation of specialized condensed regions that take on specific functions in biological processes.  The physicochemical properties of these lipid islands are substantially governed by the details of in-plane molecular packing structure, which is still largely unknown despite extensive research over several decades. The reason for this shortcoming is related to the lack of appropriate experimental techniques.

In their latest experimental work recently published in Nature Communications, the Nonlinear Interfacial Spectroscopy group present a breakthrough in molecular imaging that allows for unraveling the structural details in such molecular systems. Using a newly developed Sum-Frequency-Generation (SFG) microscope, the authors fully uncover the hierarchical molecular packing structure in condensed domains of mixed phospholipid monolayers. These domains are shown to consist of lipid molecules with spiraling distributions of their in-plane orientations. This spiraling packing structure is found to be dictated by the chirality of the lipid molecule and changes direction upon enantiomeric exchange. Clear breaking in mirror symmetry between the packing structures formed in domains with different enantiomeric mixtures meanwhile reports on enantioselective effects in the formation process. These results underline the importance of enantioselectivity in phospholipid assemblies and give important insights into the long-standing question of why evolution drove towards homochiral membranes.

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Active control over macroscopic properties of solids is highly desirable for a broad range of applications. A promising pathway to on-demand material properties are metastable hidden states, which are nonequilibrium phases that can only be reached after a quench by ultrashort optical or electrical pulses. As hidden states often host new emergent properties and can be switched on ultrafast timescales through non-thermal reaction pathways, they offer exciting novel functionalities for solid-state quantum devices. Yet, the fundamental processes that govern the dynamical pathway to hidden phases remain a largely open subject. Thus, switching is mostly based on empirical protocols, resulting in low efficiencies and limited control over stability.

In a new experimental work recently published in Science Advances, the Dynamics of Correlated Materials group studies the dynamical pathway to the metastable hidden quantum state of 1T-TaS₂ after optical excitation and reveal the critical role of a structural coherence governing the transition. Using time- and angle-resolved photoemission spectroscopy (trARPES), we investigate the electronic states during the transition from the insulating ground state of TaS₂ to the metallic hidden phase. Employing a multi-pulse optical excitation scheme, we demonstrate a high degree of coherent control over the switching process to the hidden phase, providing strong evidence for the key role of the collective amplitude mode as the driver of the transition.

The full scientific study is now open access published under Link. A comprehensive press release in German and English can be found here at FHI News.

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Lead halide perovskite (LHP) semiconductors emerged as an excellent material platform for next-generation solar cells and optoelectronic devices, but the microscopic origin of their surprising optoelectronic properties, such as long charge-carrier lifetimes and long diffusion lengths, continues to be debated.  These LHP properties are typically tailored by fine-tuning the lattice structure through modifications in chemical composition or morphology. Phonon-driven ultrafast material control, representing the dynamic counterpart to chemical engineering, has nevertheless been mostly elusive.

In a new experimental work recently published in Science Advances, the THz Structural Dynamics group employs intense single-cycle THz electric fields to investigate the ultrafast lattice responses in hybrid CH₃NH₃PbBr₃ and all-inorganic CsPbBr₃ perovskites. By probing the THz-induced Kerr effect, they witness a strong THz polarizability and achieve direct lattice control via nonlinear excitation of coherent octahedral twist modes in these materials. The observed Raman-active phonons between 0.9 and 1.3 THz govern the ultrafast nonlinear lattice response in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability. The results indicate that these phonons might contribute to dynamic charge carrier screening with implications for LHPs’ optoelectronic properties beyond the conventional Fröhlich polaron picture. Moreover, this work paves the way for selective control of LHPs’ vibrational degrees of freedom governing phase transitions and dynamic disorder, and puts ultrafast material design of these semiconductors into reach. 

The full scientific study is now open access published under https://doi.org/10.1126/sciadv.adg3856. A comprehensive press release in German and English can be found here at FHI News.

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Absorption of light in a molecular system typically leads to formation of a correlated electron-hole pair with zero spin, called singlet exciton. If energetically allowed, this singlet exciton may eventually transform into two triplet excitons (with spin 1 each), thereby doubling the number of excited charge carriers. Singlet fission may therefore boost photovoltaic efficiency. The primary step of singlet fission is the ultrafast creation of a correlated triplet pair. Whereas several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states. In this work, time- and angle-resolved photoemission spectroscopy is employed to observe the primary step of singlet fission in crystalline pentacene. Our results indicate a charge-transfer mediated mechanism with a hybridization of Frenkel and charge transfer states in the lowest bright singlet exciton. Detailed knowledge is obtained about the localization and the orbital character of the exciton wave functions recorded in momentum maps. This allows to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states on the basis of their orbital character. A more general perspective of this work is that orbital- and localization-resolved dynamics promise deep insights into the mechanics governing molecular systems and solids, in particular, topological materials.

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The Max Planck Fellow Programme promotes cooperation between outstanding university professors and Max Planck Society researchers. The appointment of university professors as Max Planck Fellows is limited to a five-year period and also entails the supervision of a small working group at a Max Planck institute. Institutes have been able to apply for an extension to the funding period for Max Planck Fellows on a one-off basis since 2009. The option of a one-off extension also exists here.

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The functionality of a material and its macroscopic response to external stimuli are dictated by the microscopic interactions of its elementary degrees of freedom – electrons, lattice, and spin. Understanding the fundamental mechanisms and complex many-body interactions that determine relevant material properties is a main goal of modern solid state research. In spatially inhomogeneous crystals, atomic-scale variations of the crystal lattice and/or its electronic structure will alter the microscopic response – may it be due to imperfection in the presence of defects, or by design due to the growth of nanoscale heterostructures. Whereas femtosecond pump-probe techniques provide insight into microscopic couplings by following the non-equilibrium response and dynamics of a system in real-time, very few experimental approaches exist to date which are capable to resolve such dynamics locally on the relevant length scales down to a single atom.

Coherent phonon spectroscopy is a powerful tool to monitor ultrafast lattice dynamics under nonequilibrium conditions. Such collective, in-phase vibrations of a crystal lattice can be excited by ultrafast laser pulses and can be used, for example, to monitor a light-induced phase transition in strongly correlated materials in real time and even to control its outcome. Despite the important role of the local crystal structure for such dynamics, it has not been possible so far to measure coherent lattice vibrations with high, few nanometer, spatial resolution. In this work we demonstrate nanoscale coherent phonon spectroscopy by means of ultrafast laser-induced scanning tunneling microscopy (STM) in a plasmonic junction. Comparison of the CP spectra with tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In the all-optical pump-probe STM approach, the laser-excited coherent phonons periodically modulate the photoinduced tunneling current generated by the probe pulse, for which it is crucially that the laser excitation is resonant with an optical transition in the electronic structure of the hybridized ZnO/Ag(111) system. Moreover, in contrast to the Raman spectra, the relative CP intensities exhibit strong nanoscale spatial variations, which correlate with changes in the local density of states recorded via scanning tunneling spectroscopy. A new aspect of this work is that the photoinduced tunneling current is not modulated by a periodic change of the tip-sample distance, but likely by coupling of the phonons to the local electronic structure, which in turn leads to a modulation of the resonantly excited photocurrent. Our work introduces a new approach to probe the ultrafast structural response of a photoexcited solid using STM, and opens up new possibilities to study and control of ultrafast dynamics at solid surfaces and in low-dimensional quantum materials.

Original publication: S. Liu, A. Hammud, I. Hamada, M. Wolf, M. Müller, T. Kumagai, Nanoscale coherent phonon spectroscopy, Science Advances.

Science Advances 8, eabq5682 (2022) • DOI: 10.1126/sciadv.abq5682