Regensburg 2025 – wissenschaftliches Programm

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HL: Fachverband Halbleiterphysik

HL 29: Poster II

HL 29.95: Poster

Dienstag, 18. März 2025, 18:00–20:00, P1

Determination of Arrival Time in Ultrafast Electron Diffraction in Specimen Close Proximity with High Accuracy — •Ahmed Hassanien, Mashood Tariq Mir, Arne Ungeheuer, Lukas Nöding, Arne Senftleben, and Thomas Baumert — University of Kassel, Institute of Physics, 34132 Kassel, Germany

The ability to determine time zero in a pump-probe experiment qualitatively indicates its ability to capture dynamics and quantitatively serves as a measure of its temporal resolution. There are only a few methods for determining the time zero in ultrafast electron diffraction (UED) [1,2]. Here, we propose a robust and easy-to-implement method to determine time zero in the immediate vicinity of the sample with sub-picosecond accuracy using Fourier-transformed electron micrography (FT-EM). In the same analogy as the Debye-Weller effect in crystalline solids, the attenuation of the peak amplitude in FT-EM patterns for a metal grid blurred by femtosecond optically-induced transient electrostatic lensing (TEL) allowed us to determine time zero to better than 500 fs. To demonstrate our method, we measured the time zero in the vicinity of a graphite flake, for which we also performed a routine UED measurement by exciting with the same optical pump fluence, well below the damage threshold for either material. Using the accurately determined time zero, we were able to identify the earlier onset of the out-of-plane coherent optical phonon mode in graphite. References: [1] Olshin, Pavel K., et al. APL 120.10 (2022). [2] Dwyer, Jason R., et al. Philos. Trans. of the Royal Society A: Math., Phys. and Eng. Sci. 364.1840 (2006): 741-778.

Keywords: Structural Dynamics; Time Zero; Ultrafast Electron Diffraction; Electron Emission; Coherent Phonons