Real-time optical measurements with picosecond resolution during laser induced transformations

This work describes a novel technique allowing to measure with picosecond resolution the time evolution of the optical properties (reflectivity/transmission) of a material surface exposed to a single laser pulse. The experimental setup is based on the use of a streak camera in combination with a single-mode probe cw laser. The maximum achievable time resolution is about 1 ps for the acquisition of a single event. Results concerning melting of Ge films under ps pulses will be used to illustrate the potential of this measurement technique for resolving single optical transient events in the ps time scale. The advantages of the system with respect to the use of pump-and-probe optical measurements are especially remarkable for the study of events occurring in time windows with full widths from 1 to 10 ns, as it is the case for rapid solidification phenomena induced by ultrashort laser pulses. This has allowed us to investigate in detail, for instance, the occurrence of bulk and surface initiated solidification phenomena in thin amorphous Ge films.