My group is interested in the coherent control of the structure of matter by the use of ultrashort pulses of laser radiation. In order to initiate and monitor such material dynamics, we develop lasers with ultrashort pulse length (measured in femtoseconds, 10-15 s) and time-resolved scattering techniques (using intense synchrotron x-ray sources). We record atomic motion on the femtosecond time scale and Angstrom length scale associated with fundamental motions of bound atoms. Research projects involve condensed matter, atomic, and plasma physics. Applications to induced structural changes in biological and nanoscale systems are also possible.
The interaction of light with matter is traditionally studied at intensities and time-scales such that perturbation theory accurately predicts absorption or scattering of photons, and the response of the material. The technology of advanced lasers with ultrashort pulse length has enabled physics in a new regime. For example, we have studied (1) laser propagation in plasmas where the response of the medium is nonlinear with intensity, (2) the exclusion of electrons from regions of high-intensity light, leading to coherent emission of single-cycle pulses of terahertz radiation, (3) the generation of short wavelength harmonics from mico-clusters of atoms illuminated by high intensity laser pulses, and (4) the focusing of intense pulses of light onto nanostructured materials, leading to the generation of high-temperature plasmas and ultrashort-pulses of x-rays.
An important current project involves the use of short-pulse x-rays to probe materials undergoing rapid dynamics. We are using a synchrotron x-ray source to resolve the fundamental time scale (about 100 femtoseconds) for lattice motion in materials undergoing phase transitions. One simple phase transition is melting, or the crystal to liquid transition. Dynamics of laser-pulse-excited materials are observed through changes in x-ray scattering patterns or absorption. Both incoherent (thermally driven) and coherent (driven phonon) lattice motions may be observable. Site specific diffraction changes are observable following excitation of local modes by tunable, ultrashort-pulse lasers used to excite atomic bonds. Short-lived structures in heated or strained materials generate transient x-ray diffraction patterns observable in pump-probe experiments. It might then be possible to use rapid quench techniques to “freeze-in” such structures and form novel materials with interesting and useful properties.
Graduate students in my group work with a small, international research team of undergraduate and graduate students, postdoctoral students, and research scientists. Graduating students have followed a variety of career paths. Several are faculty members at colleges and universities; others work in research and management for high-technology companies and at the national laboratories.
M. M. Murnane, et al., “Ultrafast x-ray pulses from laser-produced plasmas,” Science 251, 531 (1991).
H. Hamster, et al., “Subpicosecond, electromagnetic pulses from intense laser-plasma interaction,” Phys. Rev. Lett. 71, 2725 (1993).
A. Sullivan, et al., “Propagation of intense, ultrashort laser pulses in plasmas,” Opt. Lett. 19, 1544 (1994).
T. D. Donnelly, R. W. Lee, and R. W. Falcone, “Dynamics of optical-field ionized plasmas for x-ray lasers,” Phys. Rev. A 51, R2691 (1995).
T. Ditmire, et al., “Strong x-ray emission from high-temperature plasmas produced by intense irradiation of clusters,” Phys. Rev. Lett. 75, 3122 (1995).
T. D. Donnelly, et al., “High-order harmonic generation in atom clusters,” Phys. Rev. Lett. 76, 2472 (1996).
J. Larsson, et al., “Ultrafast structural changes measured by time-resolved x-ray diffraction,” Appl. Phys. A 66, 587 (1998).
A. M. Lindenberg, et al., “Time-resolved x-ray diffraction from coherent phonons during a laser-induced phase transition,” Phys. Rev. Lett. 84, 111 (2000).