Studies of Phonons and Phase Transitions by Time-Resolved X-ray Diffraction
Abstract: When light is used to study structures, the wavelength limits the size of the details that can be resolved. Visible light can be used to investigate structures as small as a micrometre in size. To study smaller structures a shorter wavelength is required. The wavelength of hard X-ray radiation is much shorter than that of visible light, and is comparable to the distance between atoms in solids and liquids, which is a few tenths of a nanometre. X-ray diffraction can thus be used to study structures on the atomic scale, and by conducting the measurements with high time resolution the atomic motion can be mapped. The absorption of intense ultrashort laser pulses in solid materials can trigger a multitude of processes. Heating arising from the deposition of energy leads to rapid expansion, creating coherently excited phonons. The properties of the material are changed by the large number of excited carriers, for instance enhancement of the heat conductivity. Very intense pulses may even induce melting. Time-resolved X-ray diffraction can then be used to directly measure the changes in the structure. As the material cools down the molten material solidifies, and the surface can develop sub-micrometre periodic structures. Not only laser pulses induce structural changes. Electric pulses can generate largeamplitude acoustic pulses in piezoelectric materials, which can then trigger phase transitions between different solid structures. Using time-resolved X-ray diffraction it is possible to follow the transition in real time. The work described in this thesis has been focused mainly on experimental studies of structural dynamics with picosecond time resolution. The results obtained have, among other things, helped in the understanding of the various processes involved in the melting and subsequent regrowth of semiconductors, as well as the dynamics of the photocarriers following intense laser excitation.
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