arXiv subjects

S. T. Weber

Publications and source records attributed to S. T. Weber.

5 recordsLinked to original sources

arXiv2025

Capturing non-equilibrium electron dynamics in metals accurately and efficiently

The simulation of non-equilibrium electron distributions is essential for capturing light-metal interactions and therefore the study of photoabsorption, photocatalysis, laser ablation, and many other phenomena. Current methodologies, such as the Boltzmann equation using full collision integrals, describe non-equilibrium electron dynamics in great detail but at often prohibitive computational expense. In contrast, the simplification via a relaxation time approach can hinder the description of important features or, even worse, lead to nonphysical behavior due to the lack of particle and energy conservation. We propose a model that bridges the gap between the Boltzmann equation and two-temperature models to trace non-equilibrium distributions efficiently. This Athermal Electron Model (AthEM) separately captures the dynamics of thermal and athermal electrons and describes the energy and particle flow between two electronic systems and phonons. We show that the results align well with the results of Boltzmann equation and data from photoemission experiments. The AthEM enables the rapid generation of qualitatively accurate non-equilibrium electron distributions and provides a good starting point for further extensions.

cond-mat.mtrl-sci↗

arXiv2023

An adaptive model for the optical properties of excited gold

We study the temperature-dependent optical properties of gold over a broad energy spectrum covering photon energies below and above the interband threshold. We apply a semi-analytical Drude-Lorentz model with temperature-dependent oscillator parameters. Our approximations are based on the distribution of electrons over the active bands with a density of states provided by density functional theory. This model can be easily adapted to other materials with similar band structures and can also be applied to the case of occupational nonequilibrium. Our calculations show a strong enhancement of the intraband response with increasing electron temperature while the interband component decreases. Moreover, our model compares well with density functional theory-based calculations for the reflectivity of highly excited gold and reproduces many of its key features. Applying our methods to thin films shows a sensitive nonlinear dependence of the reflection and absorption on the electron temperature. These features are more prominent at small photon energies and can be highlighted with polarized light. Our findings offer valuable insights for modeling ultrafast processes, in particular, the pathways of energy deposition in laser-excited samples.

cond-mat.mtrl-sci↗

arXiv2019

Influence of surface plasmon polaritons on laser energy absorption and structuring of surfaces

The accurate calculation of laser energy absorption during femto- or picosecond laser pulse experiments is very important for the description of the formation of periodic surface structures. On a rough material surface, a crack or a step edge, ultrashort laser pulses can excite surface plasmon polaritons (SPP), i.e. surface plasmons coupled to a laser-electromagnetic wave. The interference of such plasmon wave and the incoming pulse leads to a periodic modulation of the deposited laser energy on the surface of the sample. In the present work, within the frames of a Two Temperature Model we propose the analytical form of the source term, which takes into account SPP excited at a step edge of a dielectric-metal interface upon irradiation of an ultrashort laser pulse at normal incidence. The influence of the laser pulse parameters on energy absorption is quantified for the example of gold. This result can be used for nanophotonic applications and for the theoretical investigation of the evolution of electronic and lattice temperatures and, therefore, of the formation of surfaces with predestined properties under controlled conditions.

physics.app-ph↗

arXiv2018

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto $j_\textrm{s}$ because their spins have a correlation time of only ~4fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

cond-mat.mes-hall↗

arXiv2018

Wavelength-dependent reflectivity changes on gold at elevated electronic temperatures

Upon the excitation by an ultrashort laser pulse the conditions in a material can drastically change, altering its optical properties and therefore the relative amount of absorbed energy, a quan- tity relevant for determining the damage threshold and for developing a detailed simulation of a structuring process. The subject of interest in this work is the d-band metal gold which has an absorption edge marking the transition of free valence electrons and an absorbing deep d-band with bound electrons. Reflectivity changes are observed in experiment over a broad spectral range at ablation conditions. To understand the involved processes the laser excitation is modeled by a com- bination of first principle calculations with a two-temperature model. The description is kept most general and applied to realistically simulate the transfer of the absorbed energy of a Gaussian laser pulse into the electronic system at every point in space at every instance of time. An electronic temperature-dependent reflectivity map is calculated, describing the out of equilibrium reflectivity during laser excitation for photon energies from 0.9 - 6.4 eV, including inter- and intra-band transi- tions and a temperature-dependent damping factor. The main mechanisms are identified explaining the electronic temperature-dependent change in reflectivity: broadening of the edge of the occu- pied/unoccupied states around the chemical potential $μ$, also leading to a shift of the $μ$ and an increase of the collision rate of free s/p-band electrons with bound d-band holes.

cond-mat.mtrl-sci↗