Acoustic phonon dispersion measured with time-resolved x-ray diffraction

Summary form only given. The experiments reported were conducted at the Advanced Photon Source. An undulator/cryogenically cooled Si monochromator pair produced 10 keV x-rays for the experiment. A commercial Ti:sapphire 840 nm, 70 fs, 1 kHz laser system was phase locked to the electron bunches in the storage ring. The laser is used to impulsively excite a two side polished Ge [100] single crystal. The absorption of the ultrafast optical light on the crystal surface generates a coherent acoustic pulse. The symmetric 400 diffraction plane was used to diffract the X-rays. The time-resolved x-ray detector was an avalanche photodiode. Acoustic pulses generated at the crystal surface propagate into the crystal bulk in a direction parallel to the surface normal. Because both crystal faces are highly polished, the acoustic pulse traverses the crystal bulk many times. The time-resolved x-ray diffraction measurement of an impulsively strained Ge crystal is shown. The crystal is oriented such that the expansion component was measured. The slow decaying background represents the static heated layer at the crystal surface, which thermally diffuses in /spl sim/ 200 ns. Each time the acoustic pulse is in the vicinity of the crystal face an increase in diffraction efficiency is observed. The period of the diffraction spikes represents the acoustic pulse round trip time through the 280 /spl mu/m thick crystal.