main links are to project websites (internal access only) Strain Saturation in Semiconductors Description: When a short, intense laser pulse is absorbed in a semiconductor, several competing nonlinear processes occur as the material returns to equilibrium. One signature of these events is the nonlinear response of the lattice spacing to increasing laser fluence which occurs in some semiconductors below the damage threshold. We are using time-resolved high angular resolution dynamical x-ray diffraction to measure the depth-dependent strain profiles in different materials as a function of laser intensity. This information in turn informs our models of how charge, strain, and heat evolve during the first few nanoseconds following intense photoexcitation. Students: G. Jackson Williams, Thomas McManus, Michael Watson Collaborators: SooHeyong Lee, David Reis, Dohn Arms, Donald Walko Update: We have collected time-resolved x-ray diffraction on several diffrerent semiconductor crystals. Code is being written to analyze this data using dynamical diffraction theory while optical pump-probe experiments are being planned. Transient Strain Generated by the Momentum of Light Description: When light reflects or transmits at an interface, momentum must be conserved. One consequence is that the medium may recoil during the interaction. For short pulses of light, this radiation pressure can result in an impulsive strain wave. We are experimentally investigating several situations where these transient strains are expected to occur: upon reflection from a nearly perfect mirror, during transmission of light through a transparent material, and around the edges of a finite sized laser beam. Time-resolved x-ray diffraction will be used to provide a quantitative measurement of the direction and magnitude of these strain waves, which will allow for direct comparison with opposing theoretical predictions. Students: Jakub Kolacz, Thomas McManus, G. Jackson Williams Collaborators: David Reis, Dohn Arms, Donald Walko Update: Preliminary time-resolved x-ray diffraction data has shown on two separate occassions the transmission of a radiation-pressure induced strain impulse into a sapphire substrate following ultrashort laser reflection from a high-reflectivity optical coating. Code is being written to analyze this data using dynamical difraction theory while optical pump-probe experiments are being planned. Ultrafast X-ray Detectors Description: Solid-state x-ray detectors with picosecond rise times present an attractive alternative to expensive new x-ray sources and vacuum-tube based streak cameras for ultrafast x-ray science. Students: Pinyaphat Srikrishna, Michael Watson Collaborators: Steve Ross, David Cline Update: We are measuring the electrical response of the newest set of photodiodes to ultrafast laser pulses using the Byrne Hall laser lab. Ultrafast Phase Transitions Description: The rapid development of computers and mobile technology has increased the demand for nonvolatile memory with fast operation speed and high energy efficiency. In particular, phase change random access memory (PRAM) has attracted considerable attention. The appropriate characteristics for an ideal PRAM material is a fast reversible phase transition speed and a large difference in electrical conductivity in each transition state (e.g. amorphous and crystallized forms). Typically this phase transition can be induced by heating the sample beyond its activation energy. However an extremely fast (~nanoseconds) phase transition can also be induced by intense optical excitation. The mechanism of such a fast switching mechanism is still under investigation. Students: Abe Burleigh Collaborators: SooHeyong Lee Update: We are developing a continuum pump-probe apparatus to measure ultrafast reflectivity changes in new PRAM candidate materials. Time-resolved x-ray studies may be undertaken in the future. Structural Studies of Saturable Absorption by Quantum Dots in Glass Description: Colored glasses consisting of dense nanoparticles (e.g. CdTe) embedded in otherwise optically clear glass are finding application as optical limiters: materials which increase transparency with increasing light levels. This characteristic of fast saturable absorption is believed to occur as excited electronic states in the confined quantum dots are filled. The exact mechanism is still under discussion, and understanding of these materials has become increasingly important with the demand for optical limiters with a faster recovery time in high repetition rate lasers and optical information processing systems. We are approaching this problem by investigating the structure of these particles both before and after photoexcitation using the technique of time-resolved Extended X-Ray Absorption Fine Structure (EXAFS). This information should compliment the extensive time-resolved optical investigations in the literature. Students: Anthony Holoska Collaborators: Dale Brewe Update: Static EXAFS data on a series of commercial materials with different wavelength cutoffs (colors) has been taken, along with preliminary time-resolved data. Analysis is underway. Nanosecond Temperature-Jump Small Angle X-Ray Scattering Description: We are developing nanosecond time resolution temperature jump (T-jump) instrumentation for SAXS measurements to determine protein conformation in solution. Our primary motivation is to record molecular movies of protein folding/unfolding processes. This technique may also be used for structurally investigating other large-scale protein conformational changes in solution and in real time. T-jump SAXS will be complimentary to time-resolved protein crystallography approaches which seek to explore high-resolution, small motion, and early events following a light-initiated reaction. T-jump techniques have received increasing attention because of the availability of high-power lasers which can uniformly and suddenly increase the water temperature of a small sample tens of degrees C without the use of added dyes or other absorbers. This method has been combined with several time-resolved optical techniques to study protein folding, catalysis, and reaction kinetics. The time resolution is usually determined by the T-jump laser which is generally a few ns in duration. Protein conformation changes happen on ns to ms timescales, so this technique is quite useful. Unfortunately, visible light probes such as fluorescence or absorption spectroscopies yield a limited amount of structural information which at best is model dependent and furthermore interrogates only certain amino acids or dyes. A general structural probe that works on these timescales and is suitable for measuring proteins in solution is therefore highly desirable. To date, some results have been obtained using single-shot, non-laser based T-jump SAXS measurements during which slower components of the folding or unfolding process are observed by rapidly recording SAXS images following rapid mixing of protein into different temperature or pH solutions. These measurements are limited in time resoloution by ~100 us mixing deadtimes. We propose a 3-6 order of magnitude improvement by applying a pump/probe approach. Students: Margaret Elmer, Joseph Marcus, Katherine Butler, Christopher Asta Collaborators: Thomas Irving, Martin Gruebele, Liang Guo, David Gore Update: New publication on techniques for these experiments: Picosecond time-resolved laser pump/X-ray probe experiments using a gated single-photon-counting area detector, T. Ejdrup, H. T. Lemke, K. Haldrup, T. N. Nielsen, D. A. Arms, D. A. Walko, A. Miceli, E. C. Landahl, E. M. Dufresne and M. M. Nielsen, J. Synchrotron Rad. (2009). 16, 387-390 doi:10.1107/S0909049509004658 , Mechanisms of Cooperative Behavior in Interacting Molecular Motors Description: We observed the movement of microtubules by mixtures of slow and fast kinesin motors attached to a glass coverslip in a classic sliding filament assay. The motors are identical, except that the slow ones contain five point mutations that collectively reduce their velocity  15-fold
without compromising maximal ATPase activity. Our results indicate that
a small fraction of fast motors are able to accelerate the dissociation
of slow motors from microtubules. Because of this, a sharp, highly
cooperative transition occurs from slow to fast microtubule movement as
the relative number of fast motors in the assay is increased.
Microtubules move at half-maximal velocity when only 15% of the motors
in the assay are fast. Our model indicates that this behaviour depends
primarily on the relative motor velocities and the asymmetry between
their forward and backward dissociation forces. It weakly depends on
the number of motors and their processivity. We predict that movement
of cargoes bound to two types of motors having very different
velocities will be dominated by one or the other motor. Therefore,
cargoes can potentially undergo abrupt changes in movement in response
to regulatory mechanisms acting on only a small fraction of motors Students: none yet Collaborators: Sarah Rice, Adam Larson Update: New publication: Mechanism of cooperative behaviour in systems of slow and fast molecular motors, Adam G. Larson, Eric C. Landahl and Sarah E. Rice, Phys. Chem. Chem. Phys., 2009, 11, 4890 DOI: 10.1039/b900968j |