Acoustic, Thermal and Non-thermal Dynamics in Condensed Matter Studied by Time-Resolved X-ray Diffraction

Sammanfattning: The method of using short laser and X-ray pulses to investigate dynamics in materials (pump-probe) has been used for several decades. This thesis presents work that was carried out using these sources of light to reveal acoustic, thermal and non-thermal dynamics. The dynamics were induced in the samples by depositing energy with a pulsed laser, and probed with X-ray diffraction. The recent increase in the availability of ultrashort X-ray source facilities, such as the FemtoMAX beamline at the MAX IV laboratory, has made it possible to carry out increasing numbers of experiments with femtosecond time resolution. The main part of the experiments were carried out on femto- and picosecond timescales.When an intense laser pulse impinges on a semiconductor the large number of electrons excited can result in melting of the material within a time much shorter than the time taken to melt the material thermally. At lower intensities and on longer timescales the heat will instead lead to thermally expansion of the material, generating acoustic waves that propagate into the material. Part of the work presented in this thesis involves experiments on atomic disordering during non-thermal melting and modification of the propagating acoustic waves in the semiconductor indium antimonide (InSb). The other material studied in this work was graphite. The relaxation of the material thermally expanded by laser irradiation was studied to measure the thermal cross-plane conductivity of a graphite film. An accurate measurement of the film thickness was carried out at the same time. An intense laser pulse was also used to generate large amplitude pressure waves in graphite, which may provide a means of driving a direct phase transition from graphite to diamond.To summarize, this thesis presents experimental work and new findings that contribute to our understanding of the physics in semiconductors, and to the development and properties of materials such as thin films and nano diamonds formed in graphite.