The role of thermal and electronic pressure in the picosecond acoustic response of femtosecond laser-excited solids
- PDF / 289,930 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 154 Views
1230-MM06-03
The role of thermal and electronic pressure in the picosecond acoustic response of femtosecond laser-excited solids Uladzimir Shymanovich1, Matthieu Nicoul2, 1, Stefan Kähle1, Wei Lu1, Alexander Tarasevitch1, Ping Zhou1, Tobias Wietler3, Michael Horn von Hoegen1, Dietrich von der Linde1, Klaus Sokolowski-Tinten1 1 University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany. 2 University of Cologne, Zülpicher Straße 77, 50937 Köln, Germany. 3 University of Hannover, Hannover, Germany. ABSTRACT In this work we apply ultrafast time-resolved X-ray diffraction to study the dynamics of coherent acoustic phonons in laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal driving forces. For Ge our measurements reveal that the relative strength of the electronic pressure decreases with increasing laser fluence. For larger laser fluences the thermal pressure exceeds the electronic one, and only at low excitation strength the electronic pressure becomes the dominant driving force, as predicted by theory [1]. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Grüneisen parameters. INTRODUCTION Upon excitation of a solid with ultrashort laser-pulses the optical energy is initially stored in the electronic subsystem and subsequently transferred to the lattice in a few picoseconds. Both, electronic excitation, as well as the quasi isochoric heating of the material, lead to a nearly instantaneous increase in pressure. Relaxation of the pressure triggers strain waves which can be regarded as a coherent superposition of acoustic phonons. Laser generated coherent acoustic phonons (CAPs) have been extensively studied in the past using time-resolved optical spectroscopy and, more recently, time-resolved diffraction techniques. The general behavior of CAPs can be qualitatively understood within the phenomenological model of Thomson et al. [1]. However, there are significant quantitative discrepancies among different studies with respect to the role of the electronic and thermal contributions to the driving pressure. In this work we apply ultrafast time-resolved X-ray diffraction to study CAPs in laser-excited thin films of Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal driving forces. TIME-RESOLVED DIFFRACTION WITH LASER-PLASMA KEV X-RAY SOURCES Time-resolved X-ray diffraction with femtosecond X-ray pulses combines atomic scale temporal and spatial resolution and allows, therefore, to directly follow atomic motion in real time. For the particular case of CAPs in laser-excited solids the associated time-dependent expansion and compression of the lattice leads to transient changes of the angular distribution of the diffracted X-rays (the so-called rocking curves).
In our experiment we have used ultrashort Kα X-ray pulses f
Data Loading...
