Ultrafast scanning tunneling microscopy and spectroscopy of solid surfaces

Place: conference room, IMDEA Nanociencia.

Abstract: 

Recent advances in the coupling of broadband optical and terahertz (THz) radiation with low-temperature scanning tunneling microscopes (STM) have greatly expanded our ability to probe ultrafast processes and the optical response of surfaces at the atomic scale [1]. In this talk, I will introduce optical photon-driven STM (ph-STM) and THz-lightwave-driven STM (THz-STM) as two approaches for imaging light-induced processes at surfaces with simultaneous femtosecond temporal and nanometer to angstrom spatial resolution.
First, I will demonstrate the local detection of coherent phonons (CP) in ultrathin ZnO films on Ag(111) with 1 nm spatial resolution by femtosecond ph-STM using plasmonic nanotips [2]. Our results show that resonant optical excitation of the ZnO/Ag(111) by the localized surface plasmon (LSP) is crucial for efficient photocurrent generation and the detection of CPs. Combined with tip-enhanced Raman spectroscopy (TERS) [3] and STM-induced luminescence [4], our results show that the ZnO/Ag(111) couples strongly to the LSP inside the plasmonic nanocavity .
Second, I will discuss our recent results on probing the photoinduced dynamics of the charge density wave insulator 1T-TaS2 by THz-STM [5]. Upon photoexcitation, we observe a periodic modulation of the tunneling current at a frequency of 2.4 THz, corresponding to the CDW amplitude mode in 1T-TaS2. By variation of the DC and THz bias in the STM, we find that the AM coherently modulates the local density of states. In addition, we observe an ultrafast collapse and recovery of the insulating Mott gap, which modulates the current on THz sub-cycle time scales. Our results are an important step towards ultrafast atomic-scale imaging and spectroscopy of light-induced phenomena in complex quantum materials.
References
[1] M. Müller, Prog. Surf. Sci. 99, 1 (2024)
[2] S. Liu et al., Sci. Adv. 8, 42 (2022)
[3] S. Liu et al., Nano Lett. 19, 8 (2019)
[4] H. Wiedenhaupt et al. (in preparation)
[5] L. Parra Lopéz et al. (in preparation)