arXiv subjects

Baerbel Rethfeld

Publications and source records attributed to Baerbel Rethfeld.

21 records · Page 2Linked to original sources

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↗

arXiv2018

Phonon-induced long-lasting nonequilibrium in the electron system of a laser-excited solid

Electron-electron thermalization and electron-phonon relaxation processes in laser-excited solids are often assumed to occur on different timescales. This is true for the majority of the conduction band electrons in a metal. However, electron-phonon interactions can influence the thermalization process of the excited electrons. We study the interplay of the underlying scattering mechanisms for the case of a noble metal with help of a set of complete Boltzmann collision integrals. We trace the transient electron distribution in copper and its deviations from a Fermi-Dirac distribution due to the excitation with an ultrashort laser pulse. We investigate the different stages of electronic nonequilibrium after an excitation with an ultrashort laser-pulse of 800 nm wavelength and 10 fs pulse duration. Our calculations show a strong nonequilibrium during and directly after the end of the laser pulse. Subsequently, we find a fast thermalization of most electrons. Surprisingly, we observe a long-lasting nonequilibrium, which can be attributed to the electron-phonon scattering. This nonequilibrium establishes at energies around peaks in the density of states of the electrons and persists on the timescale of electron-phonon energy relaxation. It influences in turn the electron phonon coupling strength.

cond-mat.mes-hall↗

arXiv2017

Relaxation of a nonequilibrium phonon distribution induced by femtosecond laser irradiation

Ultrafast laser irradiation of solids leads to a thermodynamic nonequilibrium within and between the electron and phonon subsystems of the material. Due to electron-electron and phonon-phonon collisions, both subsystems relax into respective new thermodynamic equilibria within a characteristic thermalization time, which is different for each one of them. Moreover, they equilibrate their temperatures by electron phonon coupling. The relaxation of the electronic nonequilibrium and its effect on the electron phonon coupling was subject to a number of studies and it is comparably well understood, while the nonequilibrium within the phononic subsystem is usually neglected and its influence of the nonequilibrium phonons on other relaxation processes is unclear. Our calculations show significant differences in the energy transfer rate between the electrons and the phonons depending whether a nonequilibrium distribution is assumed for the phonons or not. Here, we present a model to study the relaxation of the nonequilibrium phonon subsystem. Collisions between phonons are described within the frame of Boltzmann integrals. From this, an energy-dependent relaxation time can be extracted and inserted into a relaxation-time approach. Within the frame of this model, we study the thermalization of a phonon distribution induced by ultrafast laser irradiation. We show, that the thermalization time of such a distribution is of the order of some hundreds of picoseconds. Moreover, we discuss the energy transfer between Fermi-distributed electrons and nonequilibrium phonons and compare this to the energy transfer for equilibrium distributions in both subsystems.

cond-mat.mtrl-sci↗