Andrea Cavalleri

Abstract:

A study of lattice dynamics in fs-laser-excited crystalline bulk Ge and Ge/Si-heterostructures, using time-resolved x-ray diffraction, was presented. Fast energy transport deep into the bulk of the material, coherent acoustic phonon dynamics, lattice anharmonicity, and vibrational transport across a buried interfaces were observed. Graphs showing time-dependent shifts of the centroid of the measured rocking curves in the Ge-overlayer and in the Si-substrate for different fluences, were presented.

Abstract:

Time-resolved x-ray diffraction using femtosecond, multi-keV x-ray pulses from a laser-produced plasma directly reveals non-thermal structural changes in a semiconductor and, for the first time, in a metal.

Abstract:

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).

Abstract:

Recent advances in femtosecond laser plasma x-rays sources have resulted in several experiments to explore the dynamics of physical and chemical processes on the femtosecond time scale. We present our most recent progresses on the development of an intense broadband x-ray source in the multi-keV range, for application to time-resolved EXAFS experiments. Experiments have been realized with two different CPA laser systems having different pulse durations and characteristics. X-ray emissions in the 5KeV range generated from solid targets with the MRS Nd:Glass laser (400fs, high contrast) and the UCSD Ti:Sapphire laser (20fs, 20Hz) have been characterized through high resolution and time resolved x-ray spectroscopy. The application of this source to time resolved EXAFS measurements with a subpicosecond time resolution will also be discussed.