Imaging with Femtosecond Optical Pulses
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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,  and surface second harmonic generation.  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 ETH (approximately 0.1 J/cm2, under our experimental conditions) a rapid increase in surface reflectivity occurs which has been widely interpreted  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.
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