Ultrafast phase transitions and lattice dynamics probed using laser-produced x-ray pulses

When intense femtosecond laser pulses are focused on solid targets short-lived microplasmas are formed which emit bursts of x-rays with kilovolt photon energies. Under the proper conditions x-ray pulses as short as a few hundred femtoseconds can be produced. These x-ray pulses enable ultrafast x-ray spectroscopy using pump-probe schemes where the x-ray pulses serve as probe pulses. This article describes time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds. In particular, we have studied solid-to-liquid phase transitions in semiconductors induced by femtosecond photoexcitation and the accompanying thermoacoustic phenomena. We were able to monitor the changes in the atomic position underlying a coherent optical phonon mode. These and a number of other lattice dynamics experiments discussed here demonstrate the feasibility and usefulness of ultrafast time-resolved x-ray diffraction. Future applications in many other fields of science can be foreseen.