Biomedical subjects

E M Dufresne

Publications and source records attributed to E M Dufresne.

3 recordsLinked to original sources

PubMed2006

X-ray microprobe of orbital alignment in strong-field ionized atoms.

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

Journal Article↗

PubMed2003

Transient strain driven by a dense electron-hole plasma.

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

Journal Article↗

PubMed2001

Coherent control of pulsed X-ray beams.

Synchrotrons produce continuous trains of closely spaced X-ray pulses. Application of such sources to the study of atomic-scale motion requires efficient modulation of these beams on timescales ranging from nanoseconds to femtoseconds. However, ultrafast X-ray modulators are not generally available. Here we report efficient subnanosecond coherent switching of synchrotron beams by using acoustic pulses in a crystal to modulate the anomalous low-loss transmission of X-ray pulses. The acoustic excitation transfers energy between two X-ray beams in a time shorter than the synchrotron pulse width of about 100 ps. Gigahertz modulation of the diffracted X-rays is also observed. We report different geometric arrangements, such as a switch based on the collision of two counter-propagating acoustic pulses: this doubles the X-ray modulation frequency, and also provides a means of observing a localized transient strain inside an opaque material. We expect that these techniques could be scaled to produce subpicosecond pulses, through laser-generated coherent optical phonon modulation of X-ray diffraction in crystals. Such ultrafast capabilities have been demonstrated thus far only in laser-generated X-ray sources, or through the use of X-ray streak cameras.

Journal Article↗