arXiv subjects

Tobias Eul

Publications and source records attributed to Tobias Eul.

8 recordsLinked to original sources

arXiv2026

Reflective Metastructure Q-plate for Ultrashort Laser Pulses

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.

cond-mat.mes-hall↗

arXiv2025

Exploring the third dimension in quantum confinement of surface electrons

Quantum confinement of surface electrons in two-dimensional metal-organic porous networks offers a powerful route to engineer electronic states for emerging quantum and spintronic technologies at the molecular scale. To date, such confinement has been understood primarily in terms of lateral, two-dimensional surface potential landscapes. Here, we demonstrate the pivotal role of vertical variations in the surface potential on the quantum confinement of surface electrons. We investigate the confinement behavior of both Shockley surface state and image potential state electrons with their distinct vertical electron density distributions in an exemplary Cu-T4PT metal-organic porous network on Cu(111). Advanced spectroscopic techniques reveal a substantial band renormalization for the image state electrons, manifested as a significant increase in their effective mass, whereas the Shockley state electrons remain almost unchanged. Such striking divergence is attributed to 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 underneath the Cu coordination spheres. Our findings demonstrate the essential role of vertical potential engineering in designing quantum confined states and thus advance the control of electronic properties and quantum phenomena on the nanoscale.

cond-mat.mtrl-sci↗

arXiv2025

Photoemission electron microscopy of exciton-polaritons in thin WSe$_2$ waveguides

Exciton-polaritons emerging from the interaction of photons and excitons in the strong coupling regime are intriguing quasiparticles for the potential exchange of energy during light-matter interaction processes such as light harvesting. The coupling causes an energy anti-crossing in the photon dispersion centered around the exciton resonance, i.e., a Rabi splitting between a lower and upper energetic branch. The size of this splitting correlates with the coupling strength between the exciton and the photonic modes. In this work, we investigate this coupling between excitons and photonic waveguide modes excited simultaneously in thin-film flakes of the transition-metal dichalcogenide WSe$_2$. Using a Photoemission electron microscope, we are able to extract the dispersion of the transversal electric and magnetic modes propagating through these flakes as well as extract the energy splitting. Ultimately, our findings provide a basis for the investigation of the propagation of exciton-polaritons in the time-domain via time-resolved photoemission.

cond-mat.mtrl-sci↗

arXiv2023

Temperature-driven confinements of surface electrons and adatoms in a weakly interacting 2D organic porous network

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.

cond-mat.mtrl-sci↗

arXiv2021

Energy and Momentum Distribution of Surface Plasmon-induced Hot Carriers Isolated via Spatiotemporal Separation

Understanding the differences between photon-induced and plasmon-induced hot electrons is essential for the construction of devices for plasmonic energy conversion. The mechanism of the plasmonic enhancement in photochemistry, photocatalysis, and light-harvesting and especially the role of hot carriers is still heavily discussed. The question remains, if plasmon-induced and photon-induced hot carriers are fundamentally different, or if plasmonic enhancement is only an effect of field concentration producing these carriers in greater numbers. For the bulk plasmon resonance, a fundamental difference is known, yet for the technologically important surface plasmons this is far from being settled. The direct imaging of surface plasmon-induced hot carriers could provide essential insight, but the separation of the influence of driving laser, field-enhancement, and fundamental plasmon decay has proven to be difficult. Here, we present an approach using a two-color femtosecond pump-probe scheme in time-resolved 2-photon-photoemission (tr-2PPE), supported by a theoretical analysis of the light and plasmon energy flow. We separate the energy and momentum distribution of the plasmon-induced hot electrons from the one of photoexcited electrons by following the spatial evolution of photoemitted electrons with energy-resolved Photoemission Electron Microscopy (PEEM) and Momentum Microscopy during the propagation of a Surface Plasmon Polariton (SPP) pulse along a gold surface. With this scheme, we realize a direct experimental access to plasmon-induced hot electrons. We find a plasmonic enhancement towards high excitation energies and small in-plane momenta, which suggests a fundamentally different mechanism of hot electron generation, as previously unknown for surface plasmons.

cond-mat.mes-hall↗

arXiv2021

Spectroscopic evidence for a new type of surface resonance at noble metal surfaces

We investigate the surface- and bulk-like properties of the pristine (110)-surface of silver using threshold photoemission by excitation with light of 5.9 eV. Using a momentum microscope, we identified two distinct transitions along the $\overlineΓ\,\overline{\textrm{Y}}$-direction of the crystal. The first one is a so far unknown surface resonance for the (110) noble metal surface, exhibiting an exceptionally large bulk character, that has so far been elusive in surface sensitive experiments. The second one stems from the well known bulk-like Mahan cone oriented along the $ΓL$-direction inside the crystal but projected onto the (110)-surface cut. The existence of the new state is confirmed by photocurrent calculations and its character analyzed.

cond-mat.mtrl-sci↗

arXiv2021

Adsorption-induced modification of the hot electron lifetime in a Pb/Ag111 quantum well system

The interfacial band structures of multilayer systems play a crucial role for the ultrafast charge and spin carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited states of a lead monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy, we show that the electron dynamics of the bare Pb/Ag(111) bilayer system is dominated by isotropic intraband scattering processes within the quantum well state as well as interband scattering processes from the QWS into the Pb sideband. After the adsorption of PTCDA on the Pb monolayer, the interband scattering is suppressed and the electron dynamics is solely determined by intraband or inelastic scattering processes. Our findings hence uncover a new possibility to selectively tune and control scattering processes of quantum well systems by the adsorption of organic molecules.

cond-mat.mes-hall↗

arXiv2019

Broadband enhancement of the magneto-optical activity of hybrid Au loaded Bi:YIG

We unravel the underlying near-field mechanism of the enhancement of the magneto-optical activity of bismuth-substituted yttrium iron garnet films (Bi:YIG) loaded with gold nanoparticles. The experimental results show that the embedded gold nanoparticles lead to a broadband enhancement of the magneto-optical activity with respect to the activity of the bare Bi:YIG films. Full vectorial near- and far-field simulations demonstrate that this broadband enhancement is the result of a magneto-optically enabled cross-talking of orthogonal localized plasmon resonances. Our results pave the way to the on-demand design of the magneto-optical properties of hybrid magneto-plasmonic circuitry.

physics.app-ph↗