Controlling electron transfer channels (ETCs) activated by n-photon absorption is essential for photo-assisted scanning tunneling microscopy. Here, we demonstrate systematic control of ETCs among n = 0, 1, and 2 by tuning the bias voltage and current and using field emission resonance (FER) and plasmon-assisted FER peaks as reliable indicators for photon order n. We highlight that the transition of electron tunneling from the n = 0 to n = 1 process together with the transition of FER to plasmon-assisted FER peaks can occur via decreasing the current under constant excitation laser power. In addition, we show that higher order photoemission channels could be activated by further reducing the current after the intensities of plasmon-assisted FER peaks reach the maximum. The emergence of a sloped background accompanied by the suppression of plasmon-assisted FER peaks is a characteristic signature of the photoemission process. These findings provide an efficient and practical approach for identifying and controlling ETCs in photoexcited scanning tunneling microscope junctions.

The nonlinear Hall effect (NHE) is a recently discovered member of the Hall effect family in which the Hall voltage shows a nonlinear behavior when a transverse electric field is applied. While the NHE does not require broken time-reversal symmetry, such as that induced by a magnetic field, it requires broken inversion symmetry, which limits the range of suitable systems and potential applications. Here, we demonstrate an ultrafast NHE in centrosymmetric black phosphorus through dynamical symmetry breaking using femtosecond light pulses. We provide a detailed microscopic picture of excited carrier dynamics and induced fields using momentum-resolved photoemission spectroscopy combined with ab-initio calculations. The ultrafast NHE is observed exclusively for the light polarization aligned with the armchair high-symmetry direction and persists over 300 fs, which opens new possibilities for selective and ultrafast light-to-current conversions.

The unique structural properties of interfacial water are at the heart of many important processes in electrochemistry, climate science, and biophysics. At interfaces, water molecules exhibit preferential orientations and an altered intermolecular H-bond connectivity. Characterizing this layer-dependent anisotropic structure for such a thin molecular boundary, however, is a veritable challenge, with many important details remaining unknown. Here, we combine a novel depth-resolved second-order spectroscopy with molecular dynamics simulations to study the anisotropic structure at the air-water interface through the H─O─H bending vibration. We first uncover the elusive anisotropic interfacial response by removing the bulk-like (quadrupolar) term that is found to dominate the spectrum and has hampered previous experimental investigations of the interfacial structure. Thereafter, we reveal that the molecular structure at the interface shows a pronounced layering of alternating tilt-twist motifs. This highlights the often-disregarded anisotropy in the molecular twist angle and offers a revised picture of aqueous interfaces.

Revealing the structural properties of water at charged interfaces is key for a better understanding of interfacial processes in various fields such as atmospheric chemistry, biology, and electrochemistry. One important aspect of the interfacial water structure in the presence of surface charges is its evolution with depth and how it varies with electrolyte concentration, which is still largely unknown despite various experimental efforts. In this work we investigate the anisotropic water structure in contact with insoluble charged surfactants using our recently developed depth-resolved vibrational spectroscopy which is based on a combination of phase resolved sum- and difference-frequency generation spectroscopy. By probing the line shape of the O-H stretch vibration of water in the first solvation layers of the surface charges and in interfacial regions further away from the phase boundary we obtain detailed insight into the field-induced orientational anisotropy and hydrogen-bonding properties of water molecules inside the electrical double layer. We find that the properties of the hydrogen-bond network in terms of hydrogen-bond strength and connectivity are nearly unaffected by the anisotropic molecular orientation. This is shown to hold throughout the double layer and for all measured electrolyte concentrations. The data, however, reveals significant changes of more than 40% in the amount of orientational anisotropy close to the interface as function of ionic strength, directly opposing a common and crucial assumption made in such investigations.

The molecular water structure at charged aqueous interfaces is shaped by interfacial electric fields, which can induce significant anisotropy in the molecular orientations extending over nanometer-scale distances. Despite its great relevance, very little is known about the details of this depth-dependent anisotropic water structure, mainly due to the lack of appropriate experimental techniques. Here, we present a depth-resolved study of the water anisotropy at the interface with insoluble charged surfactants using a newly developed technique, which allows for directly correlating nonlinear vibrational spectra with depth information on the nanometer scale. We demonstrate that the obtained data allows for a reconstruction of the nonlinear vibrational responses as a function of depth. The results for the case of low-salinity solutions show the presence of two pronounced regions within the interfacial anisotropy with largely deviating degrees of preferential molecular orientations. A spectral analysis of the depth-dependent vibrational responses furthermore reveals that the natural local hydrogen-bond structure of bulk water remains largely unperturbed throughout the interfacial region, including water in direct proximity to the surface charges. These findings significantly refine our understanding of the anisotropic water structure at the interface with hydrophilic charged surfactants and showcase the large potential of our depth-resolved spectroscopic technique.

Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3d transition metal elements, combined with chalcogenides or halogens. Despite the great technological promise in the field of spintronics, the connection between the d-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI3, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI3 in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I p and Cr eg states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.

Transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is an important concept in solid-state physics. However, it remained difficult to directly observe how angular momentum is exchanged and conserved among lattice modes, even though these processes are thought to play an important role in achieving magnetization equilibrium and in spin relaxation effects like the Einstein-de Haas effect. Here we demonstrate and coherently control angular momentum transfer between two lattice modes using the inverse process of anharmonic decay. The observed rotational phonon-phonon Umklapp scattering enforces the conservation of quantized crystal angular momentum, as dictated by the discrete rotational symmetry of the crystal. We thereby experimentally confirm the fundamental analogy between linear and angular momentum conservation in solids. Moreover, we establish axial nonlinear phononics as a promising handle for the ultrafast control of material properties.

We report on the design and performance of a two-color dual-oscillator infrared free-electron laser (FEL). The mid-infrared (MIR) FEL at the Fritz Haber Institute (FHI FEL) has been upgraded to include a second oscillator FEL beamline that permits lasing in the far-infrared (FIR) regime from 4.5 μm to 175 μm. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It allows to deflect electron bunches of up to 50 MeV energy alternately left and right by an angle of ±2°. It can, thus, split the high-repetition-rate (1 GHz) electron bunch train from the accelerator into two bunch trains of 500 MHz repetition rate each; one is steered to the MIR FEL and the other one to the new FIR FEL. In this two-color mode of simultaneous, synchronized operation the wavelengths in both FELs can be tuned independently over wide ranges of up to a factor of four each by undulator-gap variation. In addition, two-color operation is also available at reduced repetition rates (e.g. 55.6 MHz of both MIR and FIR pulses), as needed for some applications. This unique two-color mode opens up a wealth of novel user applications such as, MIR-FIR pump-probe experiments.

Self-assembled molecular films are omnipresent in nature, where their highly ordered anisotropic packing structures play a crucial role in governing the macroscopic properties and functional behavior of interfaces. Beyond their local anisotropy, these molecular structures can also often display pronounced heterogeneity and long-range in-plane packing order - from the molecular-to-microscopic scale. Accessing this complex structural information experimentally, however, is a veritable challenge. Phase-resolved sum-frequency generation (SFG) microscopy has recently emerged as a powerful technique for elucidating these structural aspects of thin films, but many properties have so far remained inaccessible such as details about the width and shape of the microscopic orientational distribution. In this work, we show how implementing an azimuthal-scanning approach in SFG microscopy can be used to overcome this limitation by yielding the full in-plane orientational distribution, going far beyond extracting the average molecular orientation. Specifically, by analyzing the complete set of rotational frequencies that arise from the azimuthal dependency, we show through simulated data how they are differently affected by any in-plane orientational disorder or deviation from perfect crystallinity, as well as more complex packing such as bimodal arrangements. This hence offers a route to elucidate the details of the orientational distribution. We then apply this concept to a model membrane comprised of a phase-separated mixed phospholipid monolayer, demonstrating that the molecules within the condensed domains possess micron-scale orientational correlations but nevertheless display substantial diversity in their in-plane orientation, showing both a non-negligible spread in the molecular orientational distribution, as well as profoundly different orientations for their two tail-groups. Overall, this showcased example highlights the potential for this method in future investigations on the role packing structure plays in the functional behavior in lipid membranes. Beyond this, the theoretical concepts presented in this work can be extended to a wide range of systems, from molecular samples to phononic materials, and thus has potential to open-up new directions in the structural elucidation at interfaces.

In the present work, we examine the relevance and proper interpretation of broadband-dielectric and THz-spectroscopy data for the investigation of various types of biological matter. We provide an overview of the rich variety of different dynamic processes that can be detected by these experimental methods. Several experimental examples are discussed in detail, helping to understand the information that can be drawn from such studies. This includes dielectric spectra, extending well into the GHz region, for pure water, which can be considered as a simple but highly important biological molecule. We also discuss results for a prototypical aqueous solution of a protein, belonging to one of the most important classes of biological macromolecules. Moreover, we examine broadband dielectric spectra on blood as an example of functional biological matter in organisms. To demonstrate the relevance of THz spectroscopy for the investigation of biological molecules, we finally treat such experiments applied to different amino acids.

Hyperbolic phonon polaritons - hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals - enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic β $\beta$ -Ga2O3 (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near-field optical microscopy to image HShPs in 18O bGO films homoepitaxially grown on a 16O bGO substrate, we demonstrate a spectral redshift of ∼ $\sim$ 40 cm-1 for the 18O bGO, compared to 16O bGO. 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 demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

Floquet control of band topology is a central theme in ultrafast quantum materials science. Established experimental probes of light-induced topological states include ultrafast transport and time- and angle-resolved photoemission spectroscopy, each with important strengths but also well-known limitations. Here we propose ultrafast terahertz scanning tunneling microscopy (THz-STM) as a real space energy-resolved probe of Floquet physics. We show that THz-STM enables direct local detection of bulk Floquet gaps and distinct Floquet edge state signatures. We derive a nonequilibrium Green's-function formalism for time-dependent tunneling that directly extends standard STM theory and provides an intuitive interpretation of rectified ultrafast tunneling currents. We apply the approach to bulk graphene and graphene nanoribbons of variable width. For the bulk, we show that THz-STM provides direct spectroscopic access to Floquet-induced gap openings, and we contrast pulsed pump-probe protocols with the continuous-wave Floquet steady-state limit. For finite ribbons, we demonstrate time- and space-resolved imaging of Floquet-induced topological edge states and identify the ribbon-width scale below which edge state protection breaks down. We further show how band structures of graphene nanoribbons and Floquet chiral edge modes can be reconstructed via Floquet quasiparticle interference. Finally we demonstrate that chiral impurities that break time-reversal symmetry induce characteristic spatial THz-STM signatures that can be used as a direct probe of Floquet edge state chirality.

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here we search for distinct time-domain signatures of the two SCI phenomena in an F/TI model stack of a ferromagnetic-metal layer F(Co and Fe) and a topological-insulator (TI) layer (Bi2Te3, SnBi2Te4, and Bi1-xSbx with x = 0.15 and x = 0.3), where the focus is on Bi2Te3. A femtosecond laser pulse serves to induce a transient spin voltage & micro; F s in Fand, thus, drive an ultrafast spin current out of F. SCI results in a transverse charge current with a sheet density Ic that is detected by sampling of the emitted terahertz electric field. Analysis of the dynamics of Ic(t) vs time t relative to & micro;F s (t) reveals two components with distinct timescales: (1) a quasi-instantaneous response and (2) a longer-lived response with a relaxation time of 270 fs, which is independent of the Fmaterial chosen. Component (1) is consistently ascribed to the ISHE. In contrast, we interpret component (2) as a signature of interfacial spin accumulation and the IEE at the F/Bi2Te3 interface, with a fraction of less than 10-2 of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents allow us to differentiate the ISHE and the IEE and, more generally, provide important insights into the mechanisms of spin transport and SCI in F/TI stacks.

Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large exciton binding energy ( 800 meV) and a highly anisotropic momentum space distribution of the exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material, α-MoO3, an orthorhombic crystal boasting three distinct phonon dispersions, provides three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO3 (98MoO3, Mo18O3, and 98Mo18O3) is explored. A uniform ∼5% spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM, and we report an HPhP Q-factor maxima increase in 98Mo18O3 of ∼50% along the [100] in the RB2 and ∼100% along the [001] in the RB3 with respect to 98MoO3. Observations in both real and Fourier space of higher-order HPhP modes propagating in slabs of isotopically enriched α-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual-element isotope enrichment of α-MoO3 as an intrinsic strategy to design optical, thermal, and polaritonic properties.

Abstract Liquid interfaces play central roles in biological and physicochemical processes. Sum-frequency-generation (SFG) spectroscopy is intrinsically interface-specific, but the insight gained from SFG spectra into molecular interfacial structure has been limited since spectral analysis is usually done within the electric-dipole approximation, neglecting higher-order multipole contributions. Here we introduce a general framework that includes electric and magnetic multipoles for calculating SFG spectra from molecular simulations, achieving quantitative agreement between predicted and experimental SFG spectra of the air-water interface. We show that the electric-dipole approximation breaks completely down in the water bending region and remains only qualitatively valid in the OH-stretch region. When accounting for multipole contributions, the analysis of the SFG bending band reveals pronounced biaxial water ordering in a triple-layer structure with a width of only about 0.8 nanometers. By resolving a fundamental limitation of the interpretation of SFG spectroscopy, our framework allows for the detailed extraction of interfacial molecular ordering from SFG spectra.

The substantial diversity in phospholipids within a plasma membrane, varying in tail length, degree of saturation, and head-group functionality, generates widespread structural heterogeneity. This exists both laterally across the membrane through the spontaneous formation of condensed domains that differ from their surrounding expanded phase in density, composition, and molecular packing order, as well as between its two leaflets, which normally maintain significant compositional asymmetry. Of particular importance is the exposure of phosphatidylserine (PS) lipids which is a marker for important physiological processes e.g. apoptosis. Despite this, the molecular-level alterations to the phase-structure of the membrane that result from PS exposure remain generally unknown. In this work, we utilise recently developed phase-resolved azimuthal-scanned sum-frequency generation (SFG) microscopy to investigate structural changes that occur heterogeneously across model membranes as a result of PS-lipid exposure. Specifically, by probing mixed monolayers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and deuterated 1-palmitoyl-2-oleoylphosphatidylcholine (dPOPC) in both the C-H and C-D stretching regions as well as equivalent films with DPPC exchanged with DPPS, we analyse the variations in the apparent phase distributions and domain morphologies, and quantitatively extract the density, composition, and relative out-of-plane packing order for both mixtures. We find that, in these mixtures, DPPS shows vast differences in the domain growth and coalescence behaviour compared to DPPC, as well as in the relative compositions and molecular ordering within each phase. This demonstrates the critical role the head-group plays in the heterogeneous phase structure of the membrane and may give insights into their impact on important physiological processes.

We report on tip-enhanced Raman spectroscopy of H2 and D2 molecules physisorbed within a plasmonic picocavity at 10 K. The intense Raman peaks resulting from the rotational and vibrational transitions are observed at subnanometer gap distances of the junction formed by an Ag tip and an Ag(111) surface, where a picocavity-enhanced field plays a crucial role. A significant redshift of the H-H stretch frequency is observed as the gap distance decreases, while the D-D stretch frequency is unaffected. Density functional theory, path-integral molecular dynamics, and quantum anharmonic vibrational energy calculations suggest that this unexpected isotope effect is explained by a different molecular density between H2 and D2 on the surface.

The advent of high-field terahertz (THz) sources opened the field of nonlinear THz physics and unlocked access to fundamental low energy excitations for ultrafast control of quantum materials, correlated systems and other novel functional materials. Nonlinear driving schemes extend these methods of contemporary IR-spectroscopy even to non-IR-active modes. Recent concepts employing the angular momentum of THz light for driving helical excitations, such as chiral or axial phonons, provide a novel venue to study the fundamental conservation of angular momentum in solids. Here, we employ helicity-tailored fields to prepare and directly measure coherent states of phonon angular momentum. We demonstrate how to prepare Raman-active phonons with circular or elliptic lattice trajectories on demand by employing nonlinear THz excitation pathways. By this, we witness coherent nonlinear phonon-phonon angular momentum transfer, opening the field of helical and chiral nonlinear phononics. Therefore, we generally unlock phonon angular momentum as one of the last previously missing tuning knobs for ultrafast control of matter.

Controlling quantum materials with ultrafast light pulses enables access to transient and metastable states that are inaccessible under equilibrium conditions. Yet their local dynamics remain poorly understood due to the challenge of resolving ultrafast processes with angstrom-scale spatial resolution. Here, we use terahertz scanning tunnelling microscopy (THz-STM) to probe coherent collective dynamics within a THz-induced metastable state in the layered charge density wave (CDW) material 1T-TaS2. Following ultrafast photoexcitation, we locally resolve coherent oscillations of the CDW amplitude mode at 2.5 THz together with two previously unreported modes at 1.3 THz and 0.7 THz. Comparison with phonon calculations identifies these as interlayer breathing and shear vibrations that are sensitive to the stacking configuration. These coherent dynamics are observed within a THz-induced metastable state that exhibits long-lived and spatially inhomogeneous modifications of the local density of states within the insulating gap, while higher THz fields drive a local redistribution and disordering of Star-of-David clusters near defects and domain boundaries. Our results suggest that the THz-induced metastable state involves a modification of the local interlayer stacking configuration, and demonstrate the role of interlayer degrees of freedom in the ultrafast dynamics of light-induced phases in layered quantum materials.