Electron and lattice response to ultrafast laser excitation: Exploring temperature dynamics in transition metals

This thesis explores the interaction of ultrafast light pulses with metals at moderate irradiation intensities, leading to pronounced excitation and heating without phase transitions or damage. It examines the ultrafast heating dynamics in ruthenium (Ru) thin films through pump-probe measurements, alongside a detailed analysis of the changes on the Ru surface. Theoretically, it investigates the processes following light absorption by transition metals, focusing on the dielectric response of electrons and their coupling with lattice vibrations.

The study of heating in Ru thin films, induced by femtosecond near-infrared laser irradiation, is a cornerstone of this thesis. It traces the transient electron and lattice temperatures, revealing unexpected thermoreflectance profiles that challenge the separate stages of two-temperature relaxation in Ru. Additional post-mortem analysis of Ru surfaces indicates that heat-induced film cracking is a primary degradation process under multi-shot, low-intensity irradiation.

A significant portion of the thesis investigates the electron-phonon coupling mechanism in laser-excited transition metals. It establishes a strong link between the electron-phonon coupling strength and transient dynamics, suggesting that peculiarities in the thermoreflectance signal are due to a specific form of electron-phonon coupling, which varies with electron and lattice temperatures. Extensive first-principles simulations highlight the necessity of taking into account the often-overlooked effect of coupling different electron states with phonons, as it might significantly change the coupling values in the considered metals.

Employing methods of density functional theory, the thesis also tackles the optical response of excited metals, which defines the observable thermoreflectance. It considers both electron-phonon and electron-electron scattering when studying fluence-dependent heating dynamics in metals. With Ru’s temperature-dependent optical properties at hand, the thesis reconciles experimental findings with theoretical models, proposing rapid electron-lattice equilibration in Ru due to strong electron-phonon coupling. This assumption allows the reproduction of experimental thermoreflectance dynamics by considering only lattice temperature dynamics and equilibrium-temperature-dependent optical properties. The peculiar heating dynamics in Ru is resolved: the experiment captures the lattice response but not the electron heating and two-temperature relaxation process, prompting further research to fully understand the absence of the two-temperature picture in Ru.