Imaging with Femtosecond Optical Pulses

Abstract

The interaction of short laser pulses with semiconductors has been studied by a variety of techniques including time-resolved reflectivity, [1–5] transmission, ]1–5] photoluminescence, [1–5] surface ellipsometry, [6] and surface second harmonic generation. [7] In the present work, we report an imaging technique used to obtain the first time-resolved photographs of a silicon surface at fixed time delays ranging from 100 fsec. to 600 psec following excitation with an intense ultrashort optical pulse. When the fluence E of 4he excitation pulse exceeds a threshold value E_TH (approximately 0.1 J/cm², under our experimental conditions) a rapid increase in surface reflectivity occurs which has been widely interpreted [8] as thermal melting. [1–5,9] The photographs depict the evolution of the surface reflectivity during and following melting with a time resolution of 100 fsec. and a spatial resolution of 5 µm. Using a movie camera and elementary synchronization electronics, we have also made a motion picture which shows the continuous sequence of melting, boiling, and material e fiction over a 600 psec period slowed in time by as much as a factor of 10¹³. The still photographs presented here depict the major events in this sequence.