Ab-initio analysis of the structural response of solids after femtosecond-laser-pulse excitation

After an intense femtosecond-laser excitation extreme non-equilibrium conditions are induced in materials. The high photon density in such an ultrashort-laser pulse interacts directly with the electronic system of the material. By absorbing the photon energy the electrons are excited in high energy states, where they form a non-equilibrium distribution shortly after the excitation. Due to fast electron-electron scattering and cascading processes equilibrates the electronic system within several femtoseconds to a common temperature of several thousand K. In contrast, the much heavier ionic systems remains unaffected near room temperature after the excitation. This highly non-equilibrium state between ions and electrons induces new ultrafast phenomena, in which the ions follow atomic pathways that are not accessible under thermodynamic conditions. In particular, the changed interatomic bonding due to the dramatic redistribution of the electronic system after a femtosecond-laser pulse causes these new phenomena. Induced phenomena are, e.g., coherent phonons, solid-to-solid phase transitions, thermal phonon squeezing, and nonthermal melting. In this work we analyzed the structural response of solids after femtosecond-laserpulse excitation, by performing ab initio molecular dynamics simulations of laserexcited silicon and antimony using our Code for Highly excIted Valence Electron Systems (CHIVES). It is an electronic-temperature-dependent density-functional-theory code, which high computational performance allows to treat the largest used ab initio supercells so far containing up to 1200 silicon atoms and 1440 antimony atoms, respectively. In particular, we unraveled the over 30 years existing mystery of the underlying atomic pathways during laser-induced nonthermal melting. Our results indicate that the atoms undergo three different stages of atomic motion in the excitation regime of non-reversible structural changes. Those melting stages are independent on the laser excitation strength. First, the atoms are accelerated due to the laser-changed interatomic bonding, which is characteristic for a super diffusive atomic behavior. Second, the increasing scattering rate of atoms that were not nearest neighbours decelerates the atomic motion, the atoms move fractionally diffusive, which is a behavior that has never been reported in solids before. Third, the atoms behave diffusively, like in a liquid, after the scattering rate saturates. Furthermore, the obtained results on laser-excited silicon were used to relate microscopic signatures of nonthermal melting to experimentally accessible quantities, like, decaying Bragg peak intensities, the total structure function, diffuse scattering, or the pair-correlation function. Our findings give practical indications to distinguish nonthermal melting from thermally driven phenomena in experiments. The large used supercells allowed to additionally investigate nonthermal melting in reciprocal space. With those results, we were enabled to identify the important phonon modes that cause predominantly nonthermal melting. Furthermore, we studied the existence of a fundamental speed limit of nonthermal melting in silicon and antimony at very high excitation densities. The obtained results give reasonable indications for a fundamental speed limit present in both materials. The combination of our findings on the microscopic nonthermal melting process enabled us to propose a control scheme, that induces a different structural response of the silicon crystal at an excitation density that normally would cause nonthermal melting. We found that the structural response can be controlled by light, if an intermediate state is used that shows predominantly thermal phonon squeezing. The proposed control scheme reduces the structure factor intensity decay after an intense femtosecond-laser excitation by almost 54 % compared to a single excitation to the same excitation level. In addition, we could identify coherent and incoherent electron-phonon scattering in moderate laser-excited antimony by combining experimental results obtained by L. Waldecker and coworkers, our ab initio molecular-dynamics simulations, and the ab initio computed electron-phonon coupling strength from F. H. Valencia. As a results we found that the incoherent electron-phonon coupling is insufficient explained by a Two-Temperature Model that considers an unified energy transfer between electrons and ions. Therefore, an approach that accounts at least for a different coupling strength for acoustic and optical phonons is needed in order to describe incoherent energy transfer properly. Finally, we studied ultrafast phenomena near a surface by performing the first ab initio simulations on laser-excited thin film silicon. We found that the film expansion, in particular the ensuing pressure release, stabilizes the crystalline structure against intense femtosecond-laser excitations. Furthermore, we found that the nonthermal melting process starts in the thin film center, although it is supposed to start at the surface due to the less bounded surface atoms.