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 (Metteet al2025Adv. Mater. Interfaces2400573). 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.

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 NbOCl2 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. Flat bands are interesting as their high density of states and suppressed kinetic energy result in strongly enhanced electronic correlations, leading to emergent and unconventional ordered states. Here, the authors use photoelectron momentum microscopy to map an isolated flat band in NbOCl2 across the entire Brillouin zone, revealing its tunable quasiparticle band gap and offering a platform for flat-band physics exploration.

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.

Ultrafast carrier and phonon dynamics at the lattice-matched interface of GaP/Si(001) are investigated upon below-bandgap excitation of the GaP layer at different growth stages. Transient reflectivity (TR) signals exhibit an abrupt change upon photoexcitation, revealing ultrafast creation of carriers at the heterointerface and/or in the GaP layer. Temporal evolution and resonance behavior of the interfacial carrier dynamics reveals the dominance of a discrete electronic state for thin low-temperature nucleation layers and its extinction for thicker high-temperature overgrown layers. In addition, a coherent 2-THz oscillation, which was reported previously for the low-temperature nucleation layer, is observed also for the high-temperature overgrown layers. The resonance behavior of the oscillation amplitude is similar to that of the interface carrier dynamics of the respective layers, supporting its assignment as a phonon mode localized at the heterointerface and coupled strongly with the interface carriers. On the other hand, the phonon amplitude exhibits a non-monotonic dependence on the GaP layer thickness, and its optical polarization-dependence is transformed qualitatively by the high-temperature overgrowth, both of which can be explained only qualitatively by the coupling with the interface carrier dynamics. Our observations imply that the 2-THz phonon mode itself is robust against the high-temperature overgrowth, but its amplitude is dominated by the coupling with the interface electronic transition as well as by the atomic reorganization at the interface by the overgrowth.

Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the issue on "The first thirty years"of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron-magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. Following the introduction, the interested reader will find a list of contributions and a brief summary in Section 1.3.

We demonstrate, for the first time, subcycle band-structure videography, directly accessing IR-driven carrier dynamics over the entire momentum space with few-femtosecond temporal resolution. By combining photoemission momentum microscopy with strong, phase-stable, few-cycle mid-infrared and sub-10-fs extreme-ultraviolet pulses, we study lightwave-driven currents in graphene and their timing with the pump field. Twist angle-dependent scattering times are determined for monolayers with different rotation angles relative to the SiC substrate. The 2D electron distribution reveals the role of carriers and phonons in scattering dynamics, highlighting limits to fully coherent light-matter interaction.

Photoemission momentum microscopy with sub-optical-cycle resolution enables band-structure videography throughout the entire Brillouin zone and resolves how atomic-strength midinfrared lightfields act on quantum materials. Subcycle investigation of lightwave-driven currents in graphene reveals femtosecond scattering times.

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 quantitative 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 ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further show a ∼ 20% contraction of the exciton radius within 400 fs, providing direct evidence of 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.

AbstractUltrafast 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.

The crystal orientation uniformity of MoS2 monolayers grown by atomic layer deposition (ALD) on sapphire substrates is investigated, and their integration with spin-coated WS2 in a MoS2/WS2 heterostructure. Polarization-resolved second harmonic generation (SHG) microscopy reveals that the heterostructure exhibits an exceptionally consistent orientation across 180 x 180 mu m2 areas, with deviations of less than two degrees, despite the polycrystallinity of the WS2 layer. The SHG response of the heterostructure seems to be dominated by the orientation of the underlying MoS2 layer; only locally aligned WS2 domains (if present) contribute marginally. These findings demonstrate the robustness of the MoS2 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.

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.

Scanning tunneling microscopy gives access to well-controlledmanipulationof surface adsorbates by means of activating reaction pathways beyondthermal excitation schemes. For the conversion of diethyl ether onSi(001) from a datively bound intermediate into the covalently boundfinal state, we show that tip-induced electronic excitation leadsto new products when compared to thermal excitation. We emphasizethe molecular structure as a key parameter in creating such specificreaction products via electronic excitation by comparing the resultsfor diethyl ether and tetrahydrofuran, the cyclic analogue of diethylether. The final configurations of the two systems differ in termsof the reacted surface atoms and their electronic structure; moreover,a new reaction product which indicates the desorption of one of thetwo molecular fragments induced by tip-induced ether cleavage is dominantin the case of diethyl ether.

Ultrafast 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 pump-photon energies of 1.4 eV, which can be assigned to an optical transition between electronic interface states. 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 phonon mode in combination with a characteristic wavelength-dependence of, both, its frequency and initial phase, strongly indicate that the 2-THz mode is a difference-combination mode of a GaP-like and a Si-like phonon at the heterointerface and that this second-order scattering process can be enhanced by a double resonance involving the interfacial electronic states.

Strong light fields have created opportunities to tailor novel functionalities of solids1-5. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases6-15. On subcycle timescales, lightwaves can simultaneously drive intraband currents16-29 and interband transitions18,19,30,31, which enable high-harmonic generation16,18,19,21,22,25,28-30 and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet-Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator32,33 with mid-infrared fields-strong enough for high-harmonic generation-and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

AbstractWe study ultrafast population dynamics in the topological surface state of Sb$$_2$$ 2 Te$$_3$$ 3 in two-dimensional momentum space with time- and angle-resolved two-photon photoemission spectroscopy. Linearly polarized mid-infrared pump pulses are used to permit a direct optical excitation across the Dirac point. We show that this resonant excitation is strongly enhanced within the Dirac cone along three of the six $${\bar{\Gamma }}$$ Γ ¯ –$${\bar{M}}$$ M ¯ directions and results in a macroscopic photocurrent when the plane of incidence is aligned along a $${\bar{\Gamma }}$$ Γ ¯ –$${\bar{K}}$$ K ¯ direction. Our experimental approach makes it possible to disentangle the decay of transiently excited population and photocurent by elastic and inelastic electron scattering within the full Dirac cone in unprecedented detail. This is utilized to show that doping of Sb$$_2$$ 2 Te$$_3$$ 3 by vanadium atoms strongly enhances inelastic electron scattering to lower energies, but only scarcely affects elastic scattering around the Dirac cone.

Time-resolved photoemission orbital tomography (tr-POT) offers unique possibilities for tracing molecular electron dynamics. The recorded pump-induced changes of the angle-resolved photoemission intensities allow to characterize unoccupied molecular states in momentum space and to deduce the incoherent temporal evolution of their population. Here, we show for the example of CuPc/Cu(001)-2O that the method also gives access to the coherent regime and that different excitation pathways can be disentangled by a careful analysis of the time-dependent change of the photoemission momentum pattern. In particular, we demonstrate by varying photon energy and polarization of the pump light, how the incoherent temporal evolution of the LUMO distribution can be distinguished from coherent contributions of the projected HOMO. Moreover, we report the selective excitation of molecules with a specific orientation at normal incidence by aligning the electric field of the pump light along the molecular axis.

We report on new THz electromagnetic emission mechanism from deformational coupling of acoustic (AC) phonons with electrons in the propagation medium of non-polar Si. The epicenters of the AC phonon pulses are the surface and interface of a GaP transducer layer whose thickness (d) is varied in nanoscale from 16 to 45 nm. The propagating AC pulses locally modulate the bandgap, which in turn generates a train of electric field pulses, inducing an abrupt drift motion at the depletion edge of Si. The fairly time-delayed THz bursts, centered at different times (t1THz, t2THz, and t3THz), are concurrently emitted only when a series of AC pulses reach the point of the depletion edge of Si, even without any piezoelectricity. The analysis on the observed peak emission amplitudes is consistent with calculations based on the combined effects of mobile charge carrier density and AC-phonon-induced local deformation, which recapitulates the role of deformational potential coupling in THz wave emission in a formulatively distinct manner from piezoelectric counterpart.

The functionality of organic electronic devices is governed by the dynamics of charge carriers and excited states in organic semiconductors. In particular, the relaxation of excitons and the transfer of charge carriers at metal electrodes crucially determine the performance of organic optoelectronic devices. In a combined experimental study we apply time-resolved photoluminescence and two-photon photoemission to reveal the ultrafast exciton dynamics and charge transfer at prototype organic/metal contacts comprising thin molecular films on single-crystalline noble-metal surfaces. On the basis of experiments with systematically varied film thicknesses, we relate the strong quenching of Frenkel excitons and charge transfer excitons to the wave function overlap with the metal, indicating charge transfer as the dominant relaxation pathway. Moreover, the presence of an electronic interface state is found to facilitate the transfer of excited carriers across the organic/metal interface.

Transient reflectivity spectroscopy is widely used to study ultrafast carrier- and phonon-dynamics in semiconductors. In their heterostructures, it is often not straightforward to distinguish contributions to the signal from the various layers. In this work, we perform transient reflectivity measurements on lattice-matched GaP/Si(001) using a near infrared pulse, to which GaP is transparent. The pump laser pulse can generate coherent longitudinal optical (LO) phonons both in the GaP overlayer as well as in the Si substrate which have distinct frequencies. This enables us to track the amplitude of the respective signal contributions as a function of GaP layer thickness $d$. The Si phonon amplitude in the signal exhibits an oscillatory behavior with increasing $d$. This can be quantitatively explained by the interference of the probe light reflected at the air/GaP/Si heterointerface. Based on this knowledge, we can then separate the interface- and the substrate-contributions in the carrier-induced non-oscillatory transient reflectivity signal. The obtained interface signal provides evidence for ultrafast carrier injection from the Si substrate into the GaP overlayer. This is also corroborated by examining the deviation of the polarization-dependence of the GaP coherent optical phonon signal from that of the bulk semiconductor.

Lightwave electronics has pushed the control of condensed matter to unprecedented time scales. By harnessing the carrier wave of intense light pulses as an alternating voltage [1] – [12] , electrons can be driven faster than a cycle of light, opening up a fascinating quantum world full of promise for future quantum technologies.