Ola Synnergren

Projecting picosecond lattice dynamics through x-ray topography

A method for time-resolved x-ray diffraction studies has been demonstrated. As a test case, coherent acoustic phonon propagation into crystalline InSb is observed using a laser plasma x-ray source. An extended x-ray topogram of the semiconductors surface was projected onto a high spatial resolution x-ray detector and acoustic phonons were excited by rapidly heating the crystals surface with a femtosecond laser pulse. A correlation between the spatial position on the x-ray detector and the time of arrival of the laser pulse was encoded into the experimental geometry by tilting the incident laser pulse with an optical grating. This approach enabled a temporal window of 200 ps to be sampled in a single topogram, thereby negating the disadvantages of pulse-to-pulse fluctuations in the intensity and spectrum of the laser-plasma source

Coherent Phonon Control

Trains of ultrashort laser pulses have been used to generate and to coherently control acoustic phonons in bulk InSb. The coherent acoustic phonons have been probed via time-resolved x-ray diffraction. The authors show that phonons of a particular frequency can either be enhanced or canceled. They have carried out simulations to understand the size of the effects and the levels of cancellation. (c) 2007 American Institute of Physics.

Picosecond x-ray studies of coherent folded acoustic phonons in a periodic semiconductor heterostructure

Zone folded coherent acoustic phonons were generated in a multilayered GaSb/InAs epitaxial heterostructure via rapid heating by femtosecond laser pulses. These phonons were probed by means of ultrafast x-ray diffraction. Phonons both from the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the absorption index due to photogenerated free carriers.

Opportunities and challenges using short-pulse x-ray sources

Free-electron lasers will change the way we carry out time-resolved X-ray experiments. At present date, we use laser-produced plasma sources or synchrotron radiation. Laser-produced plasma sources have short pulses, but unfortunately large pulse-to-pulse fluctuations and large divergence. Synchrotron radiation from third generation source provide collimated and stable beams, but unfortunately long pulses. This means that either the time-resolution is limited to 100 ps or rather complex set-ups involving slicing or streak cameras are needed. Hard X-ray free-electron lasers will combine the best properties of present-day sources and increase the number of photons by many orders of magnitude. Already today, a precursor to the free-electron lasers has been built at Stanford Linear Accelerator Centre (SLAC). The Sub-Picosecond Photon Source (SPPS) has already shown the opportunities and challenges of using short-pulse X-ray sources. (Less)