The need of sub-nanosecond resolution to reveal new features during laser induced solidification

This work reports the achievement of real-time optical reflectivity measure ments with sub-nanosecond time resolution during pulsed laser induced struc tural transformations. The experimental set-up uses a streak camera and pro vides both, excellent time resolution in single exposure and high versatili ty. This is illustrated through the results obtained upon picosecond laser pulse melting of thin amorphous Ge films, which lends to processes that are completed in a time window of a few tens of nanoseconds. By means of a nov el set-up we could optically resolve film recalescence after solidification and demonstrate the presence of surface initiated solidification. The fact that both processes could not be resolved by measurements performed with n anosecond resolution confirms the need of sub-nanosecond resolution. (C) 20 00 Published by Elsevier Science B.V.