Andrea Cavalleri

Abstract:

Using time-resolved x-ray diffraction the ultafast strain dynamics in fs-laserexcited Ge/Si-heterostructures has been studied. A fluence dependent, anharmonic damping of the impulsively generated acoustic phonons and vibrational transport across the buried Ge/Si-interface are observed.

Abstract:

Recently it has been shown that near-threshold ablation of absorbing solids with femto- and picosecond pulses, exhibits a material-independent behavior, which is characterized by a distinct transient interference phenomenon in the nanosecond time range. This presents a combined experimental and theoretical study of ultrashort laser pulse ablation of graphite, emphasizing how the observed universal ablation behavior is influenced by the specific properties of the irradiated material.

Abstract:

Many fundamental processes in physics, such as heat transport and phase transitions in solids involve movement of the constituent atoms. Such changes cannot be directly measured with visible light and occur transiently on time-scales comparable with the natural oscillation periods of atoms (femtoseconds to picoseconds). This paper presents experiments on crystalline semiconductors irradiated with short visible pulses in two fluence regimes.

Abstract:

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.