Andrea Cavalleri

Abstract:

Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through nonequilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.

Abstract:

Femtosecond laser ablation of semiconductors was studied with a number of techniques, including time resolved microscopy and time-of-flight mass spectroscopy. The laser pulse excites a very dense electron-hole plasma in the semiconductor, leading to nonthermal melting on a sub-picosecond timescale. The liquid is left in a state of high temperature and high pressure.

Abstract:

The coherent lattice dynamics in semiconductor heterostructures were studied using an optical-pump X-ray probe. The data provided quantitative information on the propagation of optically generated picosecond acoustic pulses inside Germanium layers, across buried interfaces and into underlying Silicon substrates. The analysis of the time resolved data from Silicon, detect transmission of the acoustic pulse across the buried Ge/Si interface into the Silicon substrate. The peak magnitude of the compressive strain is of the order of 0.006% corresponding to an absolute change in interlayer spacing of approximately 20 femtometer. The magnitude of the measured compression feature is one order of magnitude smaller than in any previous reported data.