Transient optical properties as a signature of structural changes within FEL irradiated solids

Abstract

Present-day XUV and X-ray free-electron lasers deliver the energy sufficient to drive solid systems out of equilibrium on an ultrashort time scale. Structural evolution of the material in non-equilibrium is then reflected in the modification of its complex dielectric function and subsequent changes of the observable optical coefficients. The optical probing of the material allows to measure the reflection or transmission coefficients with a femtosecond time resolution.

Due to the features of their band structure, semiconductors are of particular interest for the FEL-related research. X-ray FEL photons are capable to ionize atoms and excite valence band or core hole electrons to the conduction band in them. In case of a large density of carriers in the conduction band the potential energy surface of atoms significantly changes. The subsequent atomic dynamics leads to structural transformations and irreversible phase transitions on a time scale of a few hundred fs. On the other hand, laser pulses may also induce thermal phase transitions via electron-phonon coupling and, consequently, lattice heating on a time scale of 1 ps or longer.

The thesis studies three different materials - diamond, silicon and gallium arsenide - exposed to X-ray FEL radiation. The developed theoretical models evaluating the optical response of investigated materials are based on semi-empirical approaches, such as tight-binding scheme, which give an opportunity to treat the time evolution of the system with a large number of atoms. The optical properties, being affected by structural modifications, then set up the link between the microscopic parameters and experimental observables. Corresponding experiments with these materials were performed at such XUV/X-ray FEL facilities as FLASH, FERMI@Elettra, LCLS and SACLA. The obtained optical measurements allowed to verify the accuracy of the models.