Femtosecond X-ray Sources and Their Applications

Laboratoire d’Optique Appliquée, ENSTA/Ecole Polytechnique, France

Friday 21^st October 2005, 11.00AM, Seminar Room AR103, Graduate Research Centre.

The sub-picosecond time scale is of capital importance for the study of the dynamical evolution in the matter, because it represents the first instants of the material responses to external excitations. When such dynamics can be triggered by an ultra-short laser pulse, a pump-probe experiment allow the measurement of transient state during the photo-induced transition in the material. The new tool of femtosecond X-ray diffraction [1] allowed a direct monitoring of the atomic position during a phase transition [2] following the change in time of one or more Bragg peaks after the excitation pulse. In this seminar, I will first present the principles and the technique to produce femtosecond X-ray pulses from laser-plasma interaction on solid target, which was the first technique to allow time-resolved X-ray diffraction on the femtosecond time-scale. Such a source is monochromatic (K-line emission), fully divergent, and the X-ray pulse duration is about 100 fs. Two of the main results obtained from this source will be shown: the non-thermal melting and the optical phonon detection. The ultra-fast atomic displacements were directly measured by the variation of the X-ray diffraction signal due to the change in the structure factor. The ability to detect fine coherent atomic motion paves the way to a number of applications in studying ultra-fast laser-induced dynamics, from solid-state physics to biochemistry. In particular, we pay attention to the “charge-transfer dynamics”, which are characterized by a photo-induced cooperative molecular switching. Such a dynamics could have a significant impact on the realization of an ultra-fast molecular transistor. In the final part of this seminar, I will present the development of a new femtosecond X-ray source produced by laser-plasma interaction on gas target. This source is monochromatic, fully collimated and can deliver X-ray pulses of only 30 fs, in a spectral range from 1 to 10 keV. This source opens the possibility to realize femtosecond time-resolved EXFAS experiments.

[1] C. Rischel et al., Nature 390, 490 (1997).
[2] A. Rousse et al., Nature 410, 65 (2001).

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