Abstract

A new technique for imaging ultrafast processes will be described which has a time resolution determined by the duration of the excitation and illumination optical pulses. This technique has been applied to the problem of optically-induced phase transitions in Si. A motion picture having 100 femtosecond frame resolution has been made which shows a silicon surface undergoing melting and evaporation following intense excitation by an ultrashort laser pulse. The images clearly temporally and spatially resolve the increase in surface reflectivity caused by melting. In addition, at later times, material evaporated from the surface further alters the surface images by absorbing and scattering the illuminating laser light. Analysis of the imaged light strongly suggests a process in which evaporated material emerges as liquid droplets which atomize in hundreds of picoseconds.

© 1984 Optical Society of America