Femtosecond Structural Dynamics Observed
in Semiconductor Crystals
Ultrafast x-ray experiments are an emerging area of research in chemistry, solid-state physics, and biology, where dynamic processes occur on a time scale dictated by the period of molecular or crystal lattice vibrations, typically about 100 femtoseconds. A group from Berkeley Lab; the University of California, Berkeley; and the University of Aarhus in Denmark has measured structural changes occurring over a few picoseconds or less by means of time-resolved x-ray diffraction with a novel femtosecond x-ray source.
At the Beam Test Facility, which makes use of the electron beam in the ALS linear accelerator, a high-power, femtosecond infrared laser crossing the tightly focused linac beam at 90 degrees generates femtosecond x-ray pulses by the process of Thomson scattering (left). X-ray diffraction measurements of indium antimonide crystals heated with the same infrared laser showed a shift in the diffraction peak to lower energies, as compared to the unheated sample, and an increase in the diffraction intensity (right). These data quantitatively demonstrate lattice expansion propagating over tens of picoseconds after laser excitation. The data also established, for the first time, a 10-picoseond delay in the onset of the expansion.
In a second set of measurements at an x-ray photon energy absorbed closer to the surface, the researchers found a decrease in diffraction intensity caused by a nonthermal lattice disordering at the surface in less than a picosecond. They attributed the disordering to a high concentration of photoexcited electrons near the surface.
Research conducted by R. W. Schoenlein, T. E. Glover, and W. P. Leemans (Berkeley Lab); A. H Chin and C. V. Shank (University of California, Berkeley, and Berkeley Lab); and P. Balling (University of Aarhus, Denmark), using the Beam Test Facility.
Funding: U. S. Department of Energy, Office of Basic Energy Sciences; National Science Foundation.
Publication about this experiment: A. H. Chin et al., Phys. Rev. Lett. 83, 336 (1999).
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