Electron and lattice response to ultrafast laser excitation - Exploring temperature dynamics in transition metals
Fedor Akhmetov is a PhD student in the department XUV Optics. (Co)Promotors are prof.dr. M.D. Ackermann, dr.ir. I.A. Makhotkin and dr. I. Milov from the faculty of Science & Technology.
The work presented in this thesis focuses on deepening and expanding our understanding of the physics behind the interaction of light with metals on ultrashort timescales. It specifically covers the region of moderate radiation intensities, where matter experiences pronounced excitation and eventual heating but does not undergo phase transitions or severe damage. On the experimental side, pump-probe measurements of ultrafast heating dynamics in ruthenium (Ru) thin films are conducted, combined with in-depth analysis of the exposed Ru surface. Meanwhile, on the theoretical side, significant effort has been devoted to understanding the separate processes that occur following ultrafast light absorption by transition metals. These processes include the dielectric response of hot conduction band electrons and the coupling of electrons to lattice vibrations. These insights are crucial for comprehending the observable heating dynamics within the two-temperature framework.
The study of heating in Ru thin films induced by femtosecond near-infrared laser irradiation serves as a cornerstone of this thesis. The time-domain thermoreflectance technique is employed to trace the transient evolution of electron and lattice temperatures, depending on the laser fluence. Surprisingly, the recorded temporal thermoreflectance profiles contradict our expectations, making it challenging to resolve the separate stages of two-temperature relaxation in Ru. Consequently, the rest of this thesis is dedicated to unraveling the mysteries of ultrafast dynamics in Ru using various theoretical approaches. Additionally, extensive post-mortem analysis of surface modifications in Ru provides insights into degradation processes occurring under multi-shot, low-intensity irradiation below the melting threshold. This thesis reveals that the primary degradation process in this regime is heat-induced film cracking. Unfortunately, this significantly limits the operational conditions for potential applications of Ru thin films in short-wavelength optics and semiconductor industry. However, the proposed mechanism for crack generation still requires comprehensive theoretical and experimental confirmation.
The investigation of the electron-phonon coupling mechanism in laser-excited transition metals constitutes a significant part of this thesis. A tight relationship between the strength of electron-phonon coupling and the transient dynamics pathways has been established, suggesting that any peculiarities in the thermoreflectance signal must be attributed to a specific form of electron-phonon coupling, which varies as a function of electron and lattice temperatures. To derive this form for a particular metal, extensive first-principles simulations are employed based on the principles of Boltzmann transport theory and density functional perturbation theory. Notably, the importance of coupling different electron states with phonons -- an effect that is often neglected -- is highlighted. Our findings reveal that accounting for this effect changes coupling values by up to a factor of two, depending on temperature, for the considered transition metals. The first key component for understanding ultrafast heat dynamics in Ru -- fine electron-phonon coupling parameterization -- has been derived.
When, with an obtained in-depth understanding of electron-phonon relaxation, the limitations of previous simulation methods were revealed, the study shifted to the problem of the optical response of excited matter, which, in turn, defines the observable thermoreflectance. The scattering of conduction band electrons is a key mechanism explaining the behavior of optical properties of metals in the near-infrared regime. While electron-phonon scattering dominates in the low-temperature limit, electron-electron scattering becomes more pronounced as the temperature rises. Therefore, both channels should be taken into account when studying fluence-dependent heating dynamics in metals. By employing methods of density functional theory, the optical properties of Ru were obtained over a broad range of temperatures and photon energies. In this way, the second key ingredient was derived.
With electron-phonon coupling and optical properties at hand, it finally became possible to test the extent to which the measured heating dynamics in Ru aligns with the two-temperature framework. Theoretical optical reflectance suggested the presence of a narrow peak in the experimental thermoreflectance curve associated with ultrafast electron thermalization, yet this peak was not observed. Addressing this contradiction, exceptionally fast electron-lattice equilibration in Ru was assumed, stemming from a significant electron-phonon coupling parameter. Under this assumption, the experimental thermoreflectance dynamics could be reproduced by considering only the lattice temperature dynamics and equilibrium-temperature-dependent optical properties. Consequently, the mystery of ultrafast heating dynamics in Ru was unraveled: the experiment captures the lattice response but misses the process of electron heating and two-temperature relaxation. However, further research is needed to understand why the two-temperature picture in Ru is absent and whether it truly is missing.
