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.

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 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.

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.

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.

ZusammenfassungDie Umwandlung von Licht in Ladungsträger und deren Transport zu elektrischen Kontakten sind die Hauptaufgaben organischer Halbleiter in Bauelementen der Informationstechnologie und Photovoltaik. Doch bislang ist wenig über die gegenseitige Beeinflussung beider Prozesse bekannt. Die zeitaufgelöste Photoelektronen‐Spektroskopie mit ultrakurzen Pulsen im extremen UV‐Bereich liefert Einblicke in die Kopplung von optischen und Transporteigenschaften in organischen Halbleitern.

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient spintronic devices. Here, we experimentally investigate the coherent spin dynamics after ultrashort laser excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin Ni80Fe20 films. We provide a detailed study of the magnetic field and pump fluence dependence of the coherent precessional dynamics. We show that the coherent precession lifetime increases with the applied external magnetic field which cannot be understood by viscous Gilbert damping of the coherent magnons. Instead, it can be explained by nonlinear magnon interactions and by the change in the fraction of incoherent magnons. This interpretation is in agreement with the observed trends of the coherent magnon amplitude and lifetime as a function of the exciting laser fluence. Our results provide a new insight into the magnetization relaxation processes in ferromagnetic thin films, which is of great importance for further spintronic applications.

Altermagnets are an emerging elementary class of collinear magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of time-reversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO2 by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic structure basis for a family of interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

We present a study of the damping and spin pumping properties of CoFeB/X/Pt systems with X = Al,Cr and Ta. We show that the total damping of the CoFeB/Pt systems is strongly reduced when an interlayer is introduced independently of the material. Using a model that considers spin relaxation, we identify the origin of this contribution in the magnetically polarized Pt formed by the magnetic proximity effect (MPE), which is suppressed by the introduction of the interlayer. The induced ferromagnetic order in the Pt layer is confirmed by transverse magneto-optical Kerr spectroscopy at the M 2,3 and N 7 absorption edges as an element-sensitive probe. We discuss the impact of the MPE on parameter extraction in the spin transport model.

The antiferromagnetic order in heterostructures of NiO/Pt thin films can be modified by optical pulses. After the irradiation with laser light, the optically induced creation of antiferromagnetic domains can be observed by imaging the created domain structure utilizing the X‐ray magnetic linear dichroism effect. The effect of different laser polarizations on the domain formation can be studied and used to identify a polarization‐independent creation of 180° domain walls and domains with 180° different Néel vector orientation. By varying the irradiation parameters, the switching mechanism can be determined to be thermally induced. This study demonstrates experimentally the possibility to optically create antiferromagnetic domains, an important step towards future functionalization of all optical switching mechanisms in antiferromagnets.

Optically driven intersite and interlayer spin transfer are individually known as the fastest processes for manipulating the spin order of magnetic materials on the sub 100 fs time scale. However, their competing influence on the ultrafast magnetization dynamics remains unexplored. In our work, we show that optically induced intersite spin transfer (also known as OISTR) dominates the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy (Ni80Fe20) only in the absence of interlayer spin transfer into a substrate. Once interlayer spin transfer is possible, the influence of OISTR is significantly reduced and interlayer spin transfer dominates the ultrafast magnetization dynamics. This provides a new approach to control the magnetization dynamics of alloys on extremely short time scales by fine-tuning the interlayer spin transfer.

Two-dimensional metal-organic porous networks (2D-MOPNs) have been identified as versatile nanoarchitectures to tailor surface electronic and magnetic properties on noble metals. In this context, we propose a protocol to redecorate a ferromagnetic surface potential landscape using a 2D-MOPN. Ultrathin cobalt (Co) films grown on Au(111) exhibit a well-ordered surface triangular reconstruction. On the ferromagnetic surface, the adsorbed 2,4,6-tris(4-pyridyl)-1,3,5triazine (T4PT) molecules can coordinate with the native Co atoms to form a large-scale Co-T4PT porous network. The Co-T4PT network with periodic nanocavities serves as a templating layer to reshape the ferromagnetic surface potential. The subsequently deposited C60 molecules are steered by the network porous potential and the neighboring C60 interactions. The prototype of the ferromagnetic-supported 2D-MOPN is a promising template for the tailoring of molecular electronic and spin properties.

Two-dimensional organic porous networks (2DOPNs) have opened new vistas for tailoring the physicochemical characteristics of metallic surfaces. These typically chemically bound nanoporous structures act as periodical quantum wells leading to the 2D confinements of surface electron gases, adatoms and molecular guests. Here we propose a new type of porous network with weakly interacting 2,4,6-triphenyl-1,3,5-triazine (TPT) molecules on a Cu(111) surface, in which a temperature-driven (T-driven) phase transition can reversibly alter the supramolecular structures from a close-packed (CP-TPT) phase to a porous-network (PN-TPT) phase. Crucially, only the low-temperature PN-TPT exhibits subnano-scale cavities that can confine the surface state electrons and metal adatoms. The confined surface electrons undergo a significant electronic band renormalization. To activate the spin degree of freedom, the T-driven PN-TPT structure can additionally trap Co atoms within the cavities, forming highly ordered quantum dots. Our theoretical simulation reveals a complex spin carrier transfer from the confined Co cluster to the neighbouring TPT molecules via the underlying substrate. Our results demonstrate that weakly interacting 2DOPN offers a unique quantum switch capable of steering and controlling electrons and spin at surfaces via tailored quantum confinements.

Harnessing the optoelectronic response of organic semiconductors requires a thorough understanding of the fundamental light-matter interaction that is dominated by the excitation of correlated electron-hole pairs, i.e. excitons. The nature of these excitons would be fully captured by knowing the quantum-mechanical wavefunction, which, however, is difficult to access both theoretically and experimentally. Here, we use femtosecond photoemission orbital tomography in combination with many-body perturbation theory to gain access to exciton wavefunctions in organic semiconductors. We find that the coherent sum of multiple electron-hole pair contributions that typically make up a single exciton can be experimentally evidenced by photoelectron spectroscopy. For the prototypical organic semiconductor buckminsterfullerene (C$_{60}$), we show how to disentangle such multiorbital contributions and thereby access key properties of the exciton wavefunctions including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient devices. On short time scales, the inertia term in the LLG equation becomes important, leading to the nutation of spins. However, experimental observation of nutation is still in its infancy. Here, we experimentally demonstrate a non-resonant excitation of inertial spin dynamics in ultrathin Ni 80 Fe 20 films leading to nutation at a frequency of ∼ 0.1 THz, measured by time-resolved magneto optical Kerr effect (TR-MOKE). The inertial regime has a lifetime of more than 300ps.

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 its angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the strength of demagnetization affects the coherency 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 (tr-MOKE) technique. The largest coherent oscillation amplitude was observed for strongly quenched systems, showing the conservation of coherency for demagnetizations of up to 90%. Moreover, the change of the spin-wave phase with pump power shows a delay of the precession, which is related to the remagnetization time.