Spin-crossover (SCO) complexes are highly flexible bistable molecular switches with two distinct spin states that can be switched into each other by external stimuli such as temperature, pressure, or light. In the condensed phase, this spin switching phenomenon is determined not only by the chemical and structural properties of the SCO compound but also by intermolecular interactions within the SCO material and their coupling to surfaces. These interactions lead to cooperative effects in the spin switching behavior that are, for instance, reflected in the thermal hysteresis of the spin switching or determine the speed of the thermal and optical spin transitions within a thin film of SCO complexes. In this study, we shed new light on the cooperativity-driven spatial and temporal dynamics of the collective-light-induced spin switching of thin SCO films on a gold single crystal. We quantify the spatial and temporal dynamics of the transition between the low-spin and high-spin light-induced excited spin-state trapping (LIESST) states of the Fe(phen)(2)(SCN)(2) compound. Using real-time photoemission electron microscopy with millisecond time resolution, we uncover spectroscopic signatures of the light-induced spin-state switching to the LIESST state even 200 mu m outside the spot of direct optical excitation. This observation is identical for both continuous-wave and femtosecond laser light excitations in the visible spectral range. The time scale of the spin switching dynamics is indicative of a photothermal process in which molecules far from the laser spot relax back to the low-spin ground state faster than those within the directly illuminated region. All of these observations highlight the crucial role of cooperative interactions in spin-state switching of the SCO thin film.

The speed of optically driven magnetization dynamics is fundamentally determined by how efficiently angular momentum can be transferred between electronic, spin and lattice degrees of freedom. In rare-earth magnets, this process is typically slow because optical excitation primarily addresses itinerant electrons, whereas the magnetic moment resides in localized 4f states. Here we show that selective optical excitation of localized magnetic states can overcome this limitation. Using femtosecond pump-probe magneto-optical spectroscopy of ferrimagnetic gadolinium iron garnet, we resonantly excite an intra-4f transition of Gd3+ at 4.65 eV and resolve the ensuing dynamics of the antiferromagnetically coupled Gd and Fe sublattices. Direct excitation of the 4f manifold induces an ultrafast demagnetization of the Gd sublattice with a characteristic time of 38 fs, more than two orders of magnitude faster than in elemental gadolinium and even faster than the response of the Fe sublattice in the same material. By contrast, off-resonant excitation strongly suppresses the acceleration of the Gd dynamics while leaving the Fe response largely unchanged. These results demonstrate that the ultrafast magnetic response of rare-earth systems is governed not only by intrinsic material properties but also by the optical excitation pathway. Selective access to localized magnetic states therefore provides a powerful photonic handle for engineering angular-momentum flow and controlling magnetism far from equilibrium.

Intrinsically chiral metal surfaces, where handedness arises from the asymmetric step-kink-terrace topology of high-Miller-index planes, are model systems for enantiospecific catalysis, sensing, and spintronics. Yet, no consistent method exists to classify their handedness directly from experimental observables. We report a dual-domain machine learning framework that decodes crystallographic surface chirality from two independent image representations: atomic structure models in real space and simulated momentum-resolved photoemission maps of the Fermi surface projections in reciprocal space. ResNet18, a deep convolutional neural network, fine-tuned on a database of labeled images achieves 73

We introduce a plasmonic computing platform that unites multiplexed optical inputs, static weighting, and nonlinear activation within a single nanoscale architecture-marking a critical step toward the realization of a plasmonic neural cell. In this approach, the orbital angular momentum (OAM) of light serves as an orthogonal and inherently parallel encoding scheme, enabling distinct optical channels to be directed into individual plasmonic waveguides. The weighting of input signals is governed by precisely engineered nanoscale gaps, whereas the system's nonlinear activation is revealed through two-photon photoemission, spatially resolved using photoemission electron microscopy (PEEM). By confining electromagnetic fields far below the diffraction limit and minimizing surface plasmon polariton (SPP) propagation lengths, the design mitigates optical absorption losses while fully exploiting the unique field enhancement capabilities of plasmonic nanostructures. This physically integrated platform is compatible with scalable nanofabrication defined through a two-step electron-beam lithography (EBL) process and provides a compact, energy-efficient building block for ultrafast neuromorphic photonic circuits. Together, these results outline a tangible route toward dense plasmonic neural networks capable of performing all-optical information processing at the native speed of light.

The orbital angular momentum of light is an intriguing property for developing light driven applications. It emerged as an independent degree of freedom by which to manipulate light and, consequently, the interaction of light with matter. Several methods exist for the generation of light carrying orbital angular momentum, mostly employing transmitting or reflecting optical components, which radially modulate the phase profile of the light. As one of such components, transmissive q-plates established themselves as standard elements due to their usability over a broad wavelength range. Here, we present our approach to build a highly reflective q-plate based on a plasmonic metasurface capable of converting orbital angular momentum from the nanostructure to ultrashort laser pulses without temporal broadening. We highlight its working principle over a wide range of wavelengths for reflection under normal and gracing incidence.

Quantum confinement of surface electrons in two-dimensional metal-organic networks offers a powerful route to engineer electronic states for quantum and spintronic technologies. Here, we demonstrate the pivotal role of vertical variations in the surface potential on the quantum confinement of surface electrons. We investigate the confinement of both Shockley surface state and image-potential state electrons with their distinct vertical electron density distributions in a Cu-T4PT network on Cu(111). We find a substantial renormalization of the band mass of the image state, whereas the Shockley state remains almost unchanged. This notable divergence arises from the distinct three-dimensional potential landscape of the Cu-T4PT network, with a strong repulsive potential at the vertical position of the molecular backbone and leaky channels beneath the Cu coordination spheres. Our findings demonstrate the essential role of vertical potential engineering in designing quantum-confined states, thereby advancing control over electronic properties and quantum phenomena at the nanoscale.

We investigate the magnetization dynamics of an ultrathin Co (1.5 nm)/Py (1.5 nm) bilayer system from femtosecond (fs) to nanosecond (ns) timescales. Magnetization dynamics in the fs timescales is characterized as a highly nonequilibrium regime due to an ultrafast reduction of magnetization by laser excitation. On the other hand, the dynamics in the ns timescales is characterized as a close-to-equilibrium regime involving the excitation of coherent magnons. We demonstrate that the interfacial interaction between the Co and Py layers in these two nonequilibrium regimes across the timescales is dynamic and simultaneously influences the magnetization loss in the fs timescales and the magnon dynamics in the ns timescales. On ultrafast (fs) timescales, comparison between time-resolved magneto-optical Kerr effect (TR-MOKE) measurements and temperature-based mu T model simulations reveals that the bilayer exhibits demagnetization dynamics intermediate between those of its individual layers. When driven far from equilibrium by ultrashort laser pulse excitation, the magnetization dynamics of the individual Co and Py layers appear to remain decoupled and evolve independently in the initial stages of the ultrafast response. On the other hand, in the ns regime, the two individual layers of the bilayer precess together at the same frequency in a coupled manner as one effective single layer. These results improve our understanding of magnetization dynamics across timescales in ultrathin exchanged-coupled ferromagnetic bilayers and provide valuable insights for the design of high-frequency and energy-efficient spintronic device concepts.

Layered 2D van der Waals materials, such as transition metal dichalcogenides, are promising for nanoscale spintronic and optoelectronic applications. Harnessing their full potential requires understanding both intrinsic transport and the dynamics of optically excited spin and charge carriers, particularly the transition between excited spin polarization and the conduction band's intrinsic spin texture. Here, we investigate the spin polarization of the conduction bands of bulk WSe_{2} using static and time-resolved spin-resolved photoemission spectroscopy, complemented by photocurrent calculations. Electron doping reveals the intrinsic spin polarization, while time-resolved measurements trace the evolution of excited spin carriers. We find that intervalley scattering is spin-conserving, with spin transport initially governed by photoexcited carriers and aligning with the intrinsic conduction band spin polarization after ∼150  fs.

We investigate the magnetization dynamics of an ultrathin Co (1.5 nm) /Py (1.5 nm) bilayer system from femtosecond (fs) to nanosecond (ns) timescales. Magnetization dynamics in the fs timescales is characterized as a highly non-equilibrium regime due to an ultrafast reduction of magnetization by laser excitation. On the other hand, the dynamics in the ns timescales is characterized as a close-to-equilibrium regime involving the excitation of coherent magnons. We demonstrate that the interfacial interaction between the Co and Py layers in these two non-equilibrium regimes across the timescales is dynamic and simultaneously influences the magnetization loss in the fs timescales and the magnon dynamics in the ns timescales. On ultrafast (fs) timescales, comparison between time-resolved magneto-optical Kerr effect (TR-MOKE) measurements and temperature-based {\mu}T model simulations reveals that the bilayer exhibits demagnetization dynamics intermediate between those of its individual layers. When driven far from equilibrium by ultrashort laser pulse excitation, the magnetization dynamics of the individual Co and Py layers appear to remain decoupled and evolve independently in the initial stages of the ultrafast response. On the other hand, in the ns regime, the two individual layers of the bilayer precess together at the same frequency in a coupled manner as one effective single layer. Furthermore, by correlating the ultrafast demagnetization to precessional damping we attempt to bridge the two non-equilibrium regimes across fs to ns timescales. These results improve our understanding of magnetization dynamics across timescales in ultrathin exchanged-coupled ferromagnetic bilayers and provide valuable insights for the design of high-frequency and energy efficient spintronic device concepts.

We calculate the influence of diffusive and ballistic transport on ultrafast magnetization in thick metallic films. When only diffusive transport is present, gradients of magnetization in the material remain up to picosecond timescales. In contrast, when ballistic transport dominates, the magnetization changes homogeneously in space. We calculate the measurable magneto-optical responses for a 40 nm nickel film. Although the resulting Kerr rotation dynamics are very similar in the two limits of transport, our simulations reveal a clear signature of magnetization gradients in the Kerr ellipticity dynamics, namely a strong probe-angle dependence for the case when diffusive transport allows gradients to persist. We then compare our theoretical results with probe-angle dependent complex magneto-optical Kerr effect measurements on an optically excited 40 nm nickel film. The angle dependence of the measured Kerr signals closely matches the simulated response with diffusive transport. We conclude that the influence of ballistic transport on ultrafast magnetization dynamics in such films is negligible.

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.

For nearly 90 years, precession and relaxation processes have been thought to dominate the magnetization dynamics. Only recently has it been considered that, on short time scales, an inertia-driven magnetization dynamics should become relevant, leading to additional nutation of the magnetization vector. Here, we trigger magnetic nutation via a sudden excitation of a thin Ni80Fe20 (Permalloy) film with an ultrashort optical pulse, that leads to an abrupt tilting of the effective field acting on the magnetic moments, separating the dynamics of the magnetization from that of its angular momentum. We investigate the resulting magnetization dynamics in the inertial regime experimentally by the time-resolved magneto-optical Kerr effect. We find a characteristic oscillation in the Kerr signal in the range similar to 0.1 THz superimposed on the precessional oscillations with GHz frequencies. By comparison with atomistic spin dynamics simulations, we demonstrate that this observation cannot be explained by the well-known Landau-Lifshitz-Gilbert equation of motion but can be attributed to inertial contributions leading to nutation of the magnetization vector around its angular momentum. Hence, an optical and nonresonant excitation of inertial magnetization dynamics can trigger and control different magnetic processes, ranging from demagnetization via nutation to precession in a single device. These findings will have profound implications for the understanding of ultrafast spin dynamics and magnetization switching.

Hyperbolic polaritons have drawn great attention in nanoscale light manipulation due to their unique properties. Currently, most studies focus on natural hyperbolic phonon materials in the mid-infrared, limiting their application in the visible to near-infrared range. Here, we present a work on broadband near-infrared hyperbolic plasmon polaritons in a van der Waals material MoOCl2 by a perturbation-free direct imaging technique of photoemission electron microscopy. In particular, the hyperbolic polariton behavior has been dynamically tailored and manipulated by wavelength, polarization, interlayer twist, and artificial structure, providing a reconfigurable platform for nanophotonic applications. Notably, the full iso-frequency contours can be reconstructed via polarization-selective excitations. Our work has contributed to hyperbolic materials in the broadband near-infrared with MoOCl2, and has revealed PEEM to be an ideal method for studying hyperbolic plasmon polaritons at the space-time limit.

Optical manipulation of the magnetization of thin films opens up exciting possibilities for ever-faster magnetic storage applications. In this context, ${L1}_{0}$ chemically ordered $\mathrm{Fe}\mathrm{Pt}$ thin films are of particular interest due to their high perpendicular magnetic anisotropy and their use as a storage material for heat-assisted magnetic recording devices. However, these materials are difficult to manipulate with external fields due to their high coercivity field. Thus, we want to explore the possibility of tailoring the properties of these materials to enable switching using all-optical techniques. While stochastic all-optical switching between partially magnetized states has been reported for undoped $\mathrm{Fe}\mathrm{Pt}$ thin films, we have investigated to what extent doping with third elements can influence the switching behavior. Reducing the saturation magnetization may be one way to facilitate all-optical switching. While this is expected with the introduction of additional elements, we also want to highlight the role of the inverse Faraday effect and the magnetic circular dichroism in stochastic all-optical switching. In this study, $\mathrm{Cr}$ was found to be a promising dopant, which can almost double the relative magnetization change of the partially magnetized states compared to pure $\mathrm{Fe}\mathrm{Pt}$.

One of the key challenges for spintronic and novel quantum technologies is to achieve active control of the spin angular momentum of electrons in nanoscale materials on ultrafast, femtosecond timescales. While conventional ferromagnetic materials and materials supporting spin texture suffer both from conceptional limitations in miniaturization and in efficiency of optical and electronic manipulation, non-magnetic centrosymmetric layered materials with hidden spin polarization may offer an alternative pathway to manipulate the spin degree of freedom by external stimuli. Here we demonstrate a novel approach to generate transient spin polarization on a femtosecond timescale in the otherwise spin-unpolarized band structure of the centrosymmetric 2H-stacked group VI transition metal dichalcogenide WSe$_{2}$. Using ultrafast optical excitation of a fullerene layer grown on top of WSe$_{2}$, we trigger an ultrafast interlayer electron transfer from the fullerene layer into the WSe$_{2}$ crystal. The resulting transient charging of the C$_{60}$/WSe$_{2}$ interface leads to a substantial interfacial electric field that by means of spin-layer-valley locking ultimately creates ultrafast spin polarization without the need of an external magnetic field. Our findings hence open a novel pathway for optically engineering spin functionalities such as the sub-picosecond generation and manipulation of ultrafast spin currents in 2D heterostructures.

Excitons are realizations of a correlated many-particle wave function, specifically consisting of electrons and holes in an entangled state. Excitons occur widely in semiconductors and are dominant excitations in semiconducting organic and low-dimensional quantum materials. To efficiently harness the strong optical response and high tuneability of excitons in optoelectronics and in energy-transformation processes, access to the full wavefunction of the entangled state is critical, but has so far not been feasible. Here, we show how time-resolved photoemission momentum microscopy can be used to gain access to the entangled wavefunction and to unravel the exciton’s multiorbital electron and hole contributions. For the prototypical organic semiconductor buckminsterfullerene (C 60 ), we exemplify the capabilities of exciton tomography and achieve unprecedented access to key properties of the entangled exciton state including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.

The recently discovered altermagnets exhibit collinear magnetic order with zero net magnetization but with unconventional spin-polarized d/g/i-wave band structures, expanding the known paradigms of ferromagnets and antiferromagnets. In addition to novel current-driven electronic transport effects, the unconventional time-reversal symmetry breaking in these systems also makes it possible to obtain a spin response to linearly polarized fields in the optical frequency domain. We show through ab-initio calculations of the prototypical d-wave altermagnet RuO_2, with a symmetry combining twofold spin rotation with fourfold lattice rotation, [C_2C_4z], that there is an optical analogue of a spin splitter effect, as the coupling to a linearly polarized exciting laser field makes the d-wave character of the altermagnet directly visible. By magneto-optical measurements on RuO_2 films of different thicknesses ranging from 2 to 8nanometers, we demonstrate the predicted connection of the linear polarization of an ultrashort pump pulse to the sign and magnitude of the optically excited electronic spin polarization in the ultrathin RuO_2 films. The possibility of exciting and controlling an electronic spin polarization by linearly polarized optical pulses in a compensated system is a unique consequence of the altermagnetic material properties. Our experimental results therefore establish an optical pump-probe based protocol for detection of altermagnetic characteristics in ultrathin RuO_2 films, but our all-optical approach should apply more generally to materials in this altermagnetic symmetry class.

Optical manipulation of the magnetization of thin films opens up exciting possibilities for ever -faster magnetic storage applications. In this context, L1 0 chemically ordered FePt thin films are of particular interest due to their high perpendicular magnetic anisotropy and their use as a storage material for heatassisted magnetic recording devices. However, these materials are difficult to manipulate with external fields due to their high coercivity field. Thus, we want to explore the possibility of tailoring the properties of these materials to enable switching using all -optical techniques. While stochastic all -optical switching between partially magnetized states has been reported for undoped FePt thin films, we have investigated to what extent doping with third elements can influence the switching behavior. Reducing the saturation magnetization may be one way to facilitate all -optical switching. While this is expected with the introduction of additional elements, we also want to highlight the role of the inverse Faraday effect and the magnetic circular dichroism in stochastic all -optical switching. In this study, Cr was found to be a promising dopant, which can almost double the relative magnetization change of the partially magnetized states compared to pure FePt.

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the magnitude of demagnetization-the so-called quenching-affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect technique. The largest coherent precession amplitude was observed for strongly quenched systems, indicating a well-defined precession phase for all pump pulses at a demagnetization of up to 90% in this system. Moreover, the phase of the excited spin-waves in Ni increases with the pump fluence, indicating a delayed start of the precession during the remagnetization. We compare these findings to recent studies in Ni80Fe20(permalloy), to evaluate the influence of the magneto-elastic coupling and non-linear spin-wave dynamics on the magnetization dynamics.

Surface templating by electrostatic surface potentials is the least invasive way to design large-scale artificial nanostructures. However, generating sufficiently large potential gradients remains challenging. Here, we lay the groundwork for significantly enhancing local electrostatic fields by chemical modification of the surface. We consider the hexagonal boron nitride (h-BN) nanomesh on Rh(111), which already exhibits small surface potential gradients between its pore and wire regions. Using photoemission spectroscopy, we show that adding Au atoms to the Rh(111) surface layer leads to a local migration of Au atoms below the wire regions of the nanomesh. This significantly increases the local work function difference between the pore and wire regions that can be quantified experimentally by the changes in the h-BN valence band structure. Using density functional theory, we identify an electron transfer from Rh to Au as the microscopic origin for the local enhancement of potential gradients within the h-BN nanomesh.