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The recent development of femtosecond electron and x-ray techniques for diffraction and imaging allows for the direct observation of structural dynamics in the course of photo-induced chemical or physical processes with atomic spatial and femtosecond temporal resolution. We investigate ultrafast structural as well as electronic dynamics in bulk materials, two-dimensional semiconductors, one-dimensional nanowires, and nanoparticles. Such studies require femtosecond probes strongly interacting with small-volume samples. We employ two experimental approaches employing sub-keV single-electron pulses and 100 keV electron bunches, respectively.
Electrons with energies ranging from 50 to 1000 eV exhibit extremely large scattering cross sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum [1] so far prevented their use as femtosecond probe pulses in time-resolved experiments. Employing a laser-triggered point-like source of either divergent or collimated electron wave packets, we developed a hybrid approach for femtosecond point projection microscopy (fsPPM) and femtosecond low-energy electron diffraction (fsLEED) [2]. We investigate ultrafast electric currents in nanowires with sub-100 femtosecond temporal and few 10 nm spatial resolutions and demonstrate the potential of our approach for studying structural dynamics in crystalline single-layer materials.
The investigation of structural dynamics in thin film samples requires higher energy electrons. We developed a highly compact femtosecond electron diffractometer which allows for delivering 100 fs long pulses containing up to 5000 electrons at 100 keV to the sample [3]. We investigate the photo-induced structural dynamics in Ge2Sb2Te5 (GST), a popular phase change material exhibiting two metastable crystalline states which can be switched by light or current pulses. We observe distinct differences between the dynamics of optical properties and lattice which we explain in terms of the resonant bonding present in these phase change materials [4]. Finally, I will briefly discuss recent results on the electron-phonon coupling in the semiconducting transition metal dichalcogenide material WSe2.
[1] A. Paarmann et al, J. Appl. Phys. 112, 113109 (2012).
[2] M. Müller et al., Nature Communications 5, 5292 (2014).
[3] L. Waldecker et al., J. Appl. Phys. 117, 044903 (2015).
[4] L. Waldecker et al., arXiv:1412.0901v1
