Peter Diehr
Advisor: Gerard Mourou, Roy Clarke

Research: Ultrafast time-resolved electron diffraction: A motion picture is made from a sequence of evenly spaced still shots. If you slow the playback, rapid events become clear. Eadweard James Muybridge first carried this out in 1872; the attached gif of a galloping horse is from 1877.  Separate cameras that can be seen in the background took the individual frames; the strings triggered the exposure.

By using an electron microscope you can directly see the structure of crystalline materials through their diffraction patterns. Since the diffraction pattern changes with conditions, a series of diffraction images provides information on structure changes that have taken place.

I am developing an instrument to generate and record diffraction patterns with sub-picosecond temporal resolution, potentially better than 30 femtoseconds. The events being recorded are the interactions between ultrafast laser pulses and various solid, liquid, or gaseous materials, or even plasmas. Effects that can be seen include temperature changes, melting and super-heating, crystal phase changes, ionization, and other structure changes.

The technique employed is to split the laser pulse into pump and probe pulses, which are thus exquisitely synchronized. The pump pulse is allowed to interact directly with the sample while the probe pulse is shifted in frequency to the UV, which then generates an electron pulse at the photocathode of the electron gun. The electron pulse passes through the sample, and the resulting diffraction pattern is recorded. Relative timing is controlled by an optical delay line between pump and probe pulses. Each still shot is repeated thousands of times, and the results averaged by extended exposure times. The current experiment is to determine electron-phonon coupling coefficients for gold samples.