Femtosecond spectroscopy of the electronic distribution function in Copper
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
Ultrafast spectroscopy is the method of choice to study carrier dynamics and coupling strengths between electrons and other degrees of freedom (e.g. phonons, magnons, ...) in solids. Many materials are investigated with time-resolved spectroscopy, ranging from well understood systems, such as metals and semiconductors, to advanced materials with complex low temperature orders, like high temperature superconductors, ferromagnets or multiferroics. Although the physics of simple metals like Cu and Au is well understood and many theoretical and experimental studies of the electronic band structure, optical- and transport-properties and their temperature dependencies are reported, some important parameters are still difficult to determine with adequate precision. One such parameter is the electron-phonon coupling strength, which is a key parameter in describing the phonon-mediated interaction between two electrons forming a Cooper pair in a superconductor. In this thesis, a broadband ultrafast spectroscopy setup is designed to record the dynamics of optical constants in thin Cu films upon excitation with a femtosecond optical pulse. The analysis of experimental data is performed with a particularly developed model, which relates the optical conductivity of thin Cu films to the electronic distribution function around the Fermi level. Moreover, this model allows to determine the time-evolution of the electronic distribution function from the time-resolved data and provides access to quantitatively follow the electronic thermalization and lattice heating processes. Important parameters such as the electron thermalization time and the electron-phonon coupling constant are determined. Further, a method is developed, which allows to read out the electron-phonon coupling constant directly from the unprocessed data by using simple analytical modeling. With the experimental data on the time evolution of the changes in the electronic distribution function at hand we are able to test the existing models of ultrafast carrier relaxation in metals, and provide clues for the description of dynamics at times where the electronic distribution is highly athermal. Importantly, the presented approach could also be extended to advanced solids, like high temperature superconductors, where the consensus on the coupling strengths between the different subsystems and - correspondingly - on the nature of coupling bosons is still missing.