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O: Fachverband Oberflächenphysik
O 20: Poster Scanning Probe Microscopy: Light-Matter Interactions at the Atomic Scale
O 20.4: Poster
Montag, 17. März 2025, 18:00–20:00, P2
Two-Color Pump-Probe STM of Coherent Phonon Dynamics in Ultrathin ZnO/Ag(111) — •Henrik Wiedenhaupt1, Shuyi Liu2, Akitoshi Shiotari1, Adnan Hammud1, Daniel Wegkamp1, Martin Wolf1, Takashi Kumagai3, and Melanie Müller1 — 1Fritz Haber Institute of MPG, Berlin, Germany. — 2Huazhong University of Science & Technology, Wuhan, China. — 3Institute for Molecular Science, Okazaki, Japan.
We use photon-assisted ultrafast scanning tunneling microscopy (ph-USTM) to study coherent phonon (CP) dynamics in optically excited ZnO/Ag(111). In recent work, we have shown that resonant single-color ph-USTM enables CP spectroscopy on 3ML-ZnO/Ag(111) with nanometer spatial resolution [1], where the optical resonance between an interface state (IS) and the ZnO conduction band edge (CBE) appears to play an important role, as also observed in tip-enhanced Raman spectroscopy (TERS) [2]. However, the detailed mechanisms by which the CPs modulate the photocurrent and of their local excitation remain unclear. To gain further insight, we implement two-color ph-USTM for the selective on- and off-resonant ultrafast excitation of the IS-CBE transition on 2ML-ZnO and 3ML-ZnO. Besides demonstrating local CP spectroscopy also on 2ML-ZnO, our results show that CPs can also be excited off-resonantly, while optically resonant photon-assisted tunneling seems to be more crucial for local CP detection via ph-USTM. We explain our observations by phonon-induced transient changes in the local dielectric response. [1] S. Liu et al., Sci. Adv. 8, 42, eabq5682 (2022) [2] S. Liu et al., Nano Lett. 19, 8, 5725 (2019)
Keywords: Coherent Phonon; Ultrafast STM; Ultrathin ZnO; CP spectroscopy; Two color pump-probe