SFB 1238 | December 03, 14:30
Probing light-matter interactions and ultrafast dynamics at the angstrom scale
Recent advances in coupling ultrafast optical and THz pulses to low-temperature scanning tunnelling microscopes have opened up new possibilities for probing light-driven processes with simultaneous femtosecond temporal and nanometre-to-angstrom spatial resolution. In this talk, I introduce two approaches to exploring ultrafast surface dynamics with high spatial resolution: photon-assisted STM (phSTM) and THz-lightwave-driven STM (THz-STM) [1].
First, I will demonstrate coherent phonon spectroscopy in ultrathin ZnO/Ag(111) films with nanometer resolution using phSTM [2]. Here, the excitation of localized surface plasmons inside the STM enables the local excitation of coherent phonons and their efficient detection through resonant plasmon-assisted tunneling.
Second, I will present THz-STM measurements on the dynamical evolution of the commensurate charge density wave (CDW) phase in the layered material 1T-TaS₂ [3]. We track the ultrafast dynamics of coherent collective modes, including the CDW amplitude mode, in real space and time with a precision down to individual CDW superlattice sites. We further identify a low-frequency mode at 1.3 THz that emerges near a local CDW irregularity and whose frequency coincides with that predicted for interlayer phonon modes. In addition to these ultrafast responses, we find that tip-enhanced THz pulses can induce long-lived modifications of the local CDW order and local density of states, which we attribute to THz-driven changes in the stacking configuration.
Together, these results highlight the power of ultrafast STM to resolve and control non-equilibrium phenomena with unprecedented spatial resolution. This technique provides direct access to local excited-state physics and opens the door to ultrafast manipulation of emergent phases at the atomic scale. If time permits, I will conclude with an outlook, including first theoretical steps toward probing the local properties of light-induced Floquet states in graphene using ultrafast STM [4].
[1] M. Müller, Prog. Surf. Sci. 99, 1 (2024)
[2] S. Liu et al., Sci. Adv. 8, 42 (2022)
[3] L. E. Parra López et al., arXiv:2505.20541 (2025)
[4] N. Jacobsen et la., in preparation
FHI Berlin MPG
PH2
Contact: Achim Rosch / Matteo Cacco