Ulrich Hofer

Strong light fields have unlocked entirely new possibilities to tailor coherent electron trajectories, engineer band structures and shape emergent phases of matter all-optically ^1–9 . Unravelling the underlying quantum mechanisms requires a visualization of the lightwave-driven electron motion ^10,11 directly in the band structure. Although photoelectron momentum microscopy has imaged photoexcited electrons averaged over many cycles of light ^12–19 , subcycle band-structure videography has been limited to small electron momenta ^8,20 . Yet, lightwave-driven elementary processes in quantum materials often occur throughout momentum space. Here we introduce attosecond-precision, subcycle band-structure videography covering the entire first Brillouin zone and visualize one of the most fundamental but elusive strong-field processes: non-adiabatic Landau–Zener–Majorana tunnelling. The interplay of strong-field-driven acceleration within the Dirac-like band structure of graphene and periodic Landau–Zener–Majorana interband tunnelling manifests in a coherent subcycle displacement and distortion of the carrier distribution at the Brillouin zone edge. The extremely non-thermal electron distributions also allow us to disentangle competing scattering processes and assess their impact on coherent electronic control. Our panoramic view of field-driven electron motion enables a new quality of microscopic understanding of strong-field control of quantum materials.

Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT) combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy to directly image the momentum-space distribution and ultrafast dynamics of excitons in α -sexithiophene thin films. We introduce a model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with calculations within the framework of many-body perturbation theory. Time-resolved measurements further indicate an approximately 25% contraction of the exciton radius within 400 fs, suggesting self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions—with spatial, phase, and temporal sensitivity—in a broad class of molecular and low-dimensional materials.

The broken inversion symmetry and strong spin-orbit coupling of transition metal dichalcogenides and the resulting spin-valley locking are the foundation for potential two-dimensional spin devices and valleytronics. However, ultrafast valley polarization is difficult to characterize due to the need to image both intra- and inter-valley scattering across the Brillouin zone. Here, we present an all-optical method for measuring the rate of ultrafast spin- and momentum-conserving scattering through circularly-polarized time-resolved pump-probe second-harmonic spectroscopy. With Wollaston-prism-based detection for facile and simultaneous measurement of both K and K’ valley populations, using the example of exfoliated tungsten diselenide monolayers we resolve intra-valley and inter-valley scattering rates of 460 fs and 20 fs, respectively. These ultrafast time scales indicate rapid room-temperature valley depolarization through either intervalley exchange coupling or the Elliot-Yafet mechanism, and are consistent with recent time-resolved molybdenum diselenide and tungsten disulfide results.

Lattice-matched GaP layers without extended defects can be grown on Si(001) substrate via a two-step growth procedure, consisting of low-temperature nucleation followed by high-temperature overgrowth. A transient reflectivity experiment on a thin, low-temperature nucleation layer discovered a previously unknown phonon mode at 2 THz upon below-bandgap optical excitation ( Adv. Mater. Interfaces 2025 , 2400573). Here we examine the influence of the two-step growth process on the ultrafast carrier and phonon dynamics of the GaP/Si interface. We find that the discrete electronic state, which governed the interfacial carrier dynamics of the thin nucleation layer, becomes suppressed when a thicker layer is formed by high-temperature overgrowth. The coherent 2-THz oscillation is observed also in the high-temperature overgrown structures, at the constant frequency regardless of the GaP layer thickness. Its resonance behavior closely follows that of the carrier dynamics at the respective growth stage. This supports its assignment to a phonon mode generated at the heterointerface and strongly coupled to the interfacial carriers. The phonon amplitude exhibits a non-monotonic dependence on the GaP layer thickness, and its optical polarization dependence is qualitatively altered by the high-temperature overgrowth, neither of which is accounted for by the carrier–phonon coupling alone. Our results demonstrate that the 2-THz interfacial phonon mode is robust against high-temperature overgrowth, while its amplitude is determined by both coupling to interfacial electronic transitions and atomic-scale structural reorganization at the interface.

Strong light fields have unlocked previously unthinkable possibilities to tailor coherent electron trajectories, engineer band structures and shape emergent phases of matter all-optically. Unravelling the underlying quantum mechanisms requires a visualisation of the lightwave-driven electron motion directly in the band structure. While photoelectron momentum microscopy has imaged optically excited electrons averaged over many cycles of light, actual subcycle band-structure videography has been limited to small electron momenta. Yet lightwave-driven elementary processes in quantum materials often occur throughout momentum space. Here, we introduce attosecond-precision, subcycle band-structure videography covering the entire first Brillouin zone (BZ) and visualize one of the most fundamental but notoriously elusive strong-field processes: non-adiabatic Landau-Zener-Majorana (LZM) tunnelling. The interplay of field-driven acceleration within the Dirac-like band structure of graphene and periodic LZM interband tunnelling manifest in a coherent displacement and distortion of the momentum distribution at the BZ edge. The extremely non-thermal electron distributions also allow us to disentangle competing scattering processes and assess their impact on coherent electronic control through electron redistribution and thermalization. Our panoramic view of strong-field-driven electron motion in quantum materials lays the foundation for a microscopic understanding of some of the most discussed light-driven phenomena in condensed matter physics.

Dispersionless electronic bands lead to an extremely high density of states and suppressed kinetic energy, thereby increasing electronic correlations and instabilities that can shape emergent ordered states, such as excitonic, ferromagnetic, and superconducting phases. A flat band that extends over the entire momentum space and is well isolated from other dispersive bands is, therefore, particularly interesting. Here, the band structure of the van der Waals crystal NbOCl ₂ is revealed by utilizing photoelectron momentum microscopy. We directly map out an electronic band that is flat throughout the entire Brillouin zone and features a width of only ~ 100 meV. This band is well isolated from both the conduction and remote valence bands. Moreover, the quasiparticle band gap shows a high tunability upon the deposition of cesium atoms on the surface. By combining the single-particle band structure with the optical transmission spectrum, the optical gap is identified. The fully isolated flat band in a van der Waals crystal provides a qualitatively new testbed for exploring flat-band physics.

The crystal orientation uniformity of MoS ₂ monolayers grown by atomic layer deposition (ALD) on sapphire substrates is investigated, and their integration with spin‐coated WS ₂ in a MoS ₂ /WS ₂ heterostructure. Polarization‐resolved second harmonic generation (SHG) microscopy reveals that the heterostructure exhibits an exceptionally consistent orientation across 180 × 180 µm ² areas, with deviations of less than two degrees, despite the polycrystallinity of the WS ₂ layer. The SHG response of the heterostructure seems to be dominated by the orientation of the underlying MoS ₂ layer; only locally aligned WS ₂ domains (if present) contribute marginally. These findings demonstrate the robustness of the MoS ₂ crystal orientation and provide insights into interfacial alignment mechanisms and the orientation coherence achievable by ALD, providing a foundation for layer‐selective studies of interfacial ordering in wafer‐scale transition metal dichalcogenide heterostructures.

ConspectusIn this Account, we will focus on how chemoselective strategies were employed for the controlled synthesis of organic multilayers on silicon, thus forming well-defined semiconductor-organic interfaces. We will start with the chemoselective [2 + 2] cycloaddition of functionalized cyclooctynes to Si-Si dimers which was used as the key step for the surface-orthogonal first layer attachment of bifunctional organic molecules to the Si(001) surface. It results in well-ordered monolayers of bifunctional molecules on the highly reactive Si(001) surface, which serve as a starting point for the further layer-by-layer growth.Various strategies for the covalent attachment of further molecular layers are then developed; they include, among others, a combination of UHV- and solution-based chemistry for the application of selective and orthogonal reaction schemes in solution as well as a carefully tuned enolether/tetrazine cycloaddition for click chemistry under UHV conditions.In particular, well-ordered multilayers of organic molecules were realized on prefunctionalized Si(001) via a two-step reaction cycle in solution. Alternating the application of two selective, orthogonal reaction steps based on asymmetric building blocks led to a well-controlled layer-by-layer growth of organic molecular layers on Si(001).Further exploitation of these strategies, e.g., in the framework of on-surface synthesis, will be discussed; our approach for the selective growth of covalently bound layers perpendicular to the surface might be further combined with lateral structuring of the surface for the synthesis of organic architectures with control in all three dimensions.

Ultrafast charge‐carrier and phonon dynamics at the buried heterointerface of GaP/Si(001) are investigated by means of two‐color pump‐probe reflectivity measurements. The carrier‐induced reflectivity signal exhibits a resonant enhancement at a pump‐photon energy of 1.4 eV, which is assigned to an optical transition at the interface. In addition, the transient reflectivity is modulated by a coherent oscillation at 2 THz, whose amplitude also becomes maximum at 1.4 eV. The observed resonant behavior of the oscillation, in combination with the characteristic wavelength‐dependencies of its frequency and its initial phase, strongly indicates that the 2‐THz mode is a difference‐combination mode between a GaP‐like and a Si‐like phonon at the heterointerface and that the corresponding second‐order Raman scattering process can be enhanced by a double resonance involving the interfacial electronic states.

Selective adsorption of multifunctional molecules is rarely observed when the different functional groups react via nonactivated reaction channels. Although the latter is also the case for ether cleavage and the adsorption of C=C double bonds on the highly reactive Si(001) surface, we find that allyl ethers, which combine both functional groups, react on Si(001) selectively via the cleavage of the molecules' ether group. In addition, our XPS measurements at 90, 150, and 300 K indicate an increased reactivity of the ether group when compared to monofunctional ethers. STM investigations furthermore reveal different final adsorption configurations after ether cleavage of allyl methyl ether when compared to diethyl ether as the monofunctional reference molecule. The interaction of the two functional groups in one molecule thus leads to new reaction channels with higher reactivity for ether cleavage on Si(001). As a further consequence, the reactivity of the C=C double bond is suppressed up to room temperature, leading to the observed selective adsorption.