X rays have long been used as structural probes of complex molecules and solids on the atomic scale, but only recently have these techniques been extended into the time domain. At the Advanced Light Source (ALS), an international collaboration led by researchers from the University of California, Berkeley, developed and then applied a high-speed x-ray streak camera to watch--in real time--the motion of atoms in the semiconductor indium antimonide on picosecond time scales. Following a "kick" from a short laser pulse, they directly observed large-amplitude coherent atomic vibrations, in which the atoms collectively oscillated about their equilibrium positions. At higher laser powers, they were able to follow the structural phase transition from an ordered to a disordered state in real time. The investigators performed their experiment on Beamline 7.3.3. A femtosecond laser pulse synchronized to the individual electron bunches in the storage ring with a jitter of less than 5 picoseconds was made to overlap--in both space and time--a single x-ray pulse on the crystal. A monochromator selected x rays of wavelength 2.4 Å that illuminated a crystal oriented to diffract from the (111) planes onto the detector, a streak camera with 3-picosecond resolution. They then followed the structural dynamics of the crystal by monitoring the intensity of the diffracted x rays as a function of time with the camera. Since the length of the x-ray pulse was hundreds of times longer than the laser pulse, it served as a "continuous" source of x rays for diffraction before, during, and after the laser excitation of the sample. 
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Impulsive laser excitation of the crystal initially created "hot" electrons whose energy quickly was transferred to lattice vibrations, thereby heating the crystal. Following the excitation, the intensity of diffracted x rays decreased as expected, owing to a shift in the wavelength of the Bragg peak as the heated lattice expanded. But following the decrease, the team observed distinct temporal oscillations in the diffracted intensity, indicative of coherent lattice motion, in contrast to the incoherently excited lattice vibrations of a crystal in thermal equilibrium. By slightly changing the angle of the crystal with respect to the incident x rays, different vibrational modes, or phonons, could be selected, thus mapping out part of the acoustic phonon dispersion relation. This new technique thus allows one to probe the vibrational properties of solids at frequencies up to 0.1 THz, even under extreme, highly nonequilibrium conditions, by directly watching the atoms collectively ring. 
Because the investigators were able to resolve in real time the transfer of energy from the carrier system to the lattice, important physical parameters such as the electron-acoustic phonon coupling time could be extracted by quantitatively fitting the data to models of the processes involved. In addition, the researchers found that a significant contribution to the excitation of the coherent phonon state was also due to a direct coupling between the carriers and the acoustic phonons through the deformation potential interaction. Furthermore, the team found a close relationship between the excitation of this coherent phonon state and the disordering transition that occurred above a critical laser fluence. In particular, the diffracted x-ray signal disappeared on a time scale determined by a vibrational period, implying that each mode took one final collective swing in one direction before disordering. Further time-resolved observations of phase transitions in other materials (for example, strongly correlated systems) should lead to greater understanding of the driving mechanisms behind them. Research conducted by A.M. Lindenberg, I. Kang, and S.L. Johnson (University of California, Berkeley); T. Missalla (ALS and Lawrence Livermore National Laboratory); P.A. Heimann and H.A. Padmore (ALS); Z. Chang and P.H. Bucksbaum (University of Michigan); J. Larsson (Lund Institute of Technology, Sweden); H.C. Kapteyn (University of Colorado); R.W. Lee (Lawrence Livermore National Laboratory); J.S. Wark (University of Oxford); and R.W. Falcone (University of California, Berkeley, and Berkeley Lab). Research funding: Office of Basic Energy Sciences (BES), U.S. Department of Energy; ILSA at Lawrence Livermore National Laboratory; Lawrence Berkeley National Laboratory; and National Science Foundation. Operation of the ALS is supported by BES. Publication about this research: A.M. Lindenberg, I. Kang, S.L. Johnson, T. Misalla, P.A. Heimann, Z. Chang, J. Larsson, P.H. Bucksbaum, H.C. Kapteyn, H.A. Padmore, R.W. Lee, J.S. Wark, and R.W. Falcone, "Time-Resolved X-Ray Diffraction from Coherent Phonons during a Laser-Induced Phase Transition," Phys. Rev. Lett. 84, 111 (2000). | |||
ROSSANA HERNANDEZ
C.I 19234948
ELECTRONICA DE ESTADO SOLIDO
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