We are interested in the spatio-temporal response of solid state surfaces, quantum materials, and nanostructures following ultrafast optical or electronic excitation. Our goal is to obtain an atomistic understanding of photoinduced processes on (sub-) nanometer length and femtosecond time scales by employing novel pump-probe schemes for ultrafast scanning probe microscopy (SPM). We envision to study, drive and control solid state matter in highly non-equilibrium states and probe its dynamics locally at atomic scales.

To achieve this goal, we are putting efforts into the development of ultrafast scanning probe methods, with a focus on broadband THz-gated STM combined with femtosecond optical excitation and optical photon-assisted femtosecond STM. Besides detecting the localized tunneling signals, we complement our methodological approach by local light detection and optical near-field techniques. For detailed information and updates, please follow our news below.

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Spintronic terahertz emitters are powerful sources of ultra-broadband single-cycle terahertz (THz) fields, and are of particular interest for field-driven applications such as THz-STM. They work with any pump wavelength, and their polarity and polarization direction are easily adjustable. However, at high pump powers and high repetition rates, as required for THz-STM operation at high THz bias and measurable currents, STE operation is hampered by a significant increase in the local temperature. We resolved this long-standing issue by rotating the STE at a few 100 Hz. This distributes the absorbed power over a larger area and allows us to operate the STE at power densities far above the melting threshold of metallic films! We can now sent THz pulses with peak fields of 10 kV/cm into our THz-STM at 1 MHz repetition rate. (A. Vaitsi et al., arXiv:2404.16976)

There has been incredible progress in the field of ultrafast STM (USTM) in recent years, including the emergence of new technological concepts such as single-cycle lightwave-driven tunneling and rectification or plasmon-driven tunneling in optical USTM. This review provides a comprehensive summary of the field, focusing on the classification of light-matter interaction in tunnel junctions and approaches to image ultrafast electronic and structural excitations on solid surfaces. Questions addressed include: What are useful approaches to classify the interaction of tunnel junctions with time-varying electromagnetic fields? What types of ultrafast tunnel currents can occur in USTM? What information about the transient state of the sample, and what physical insights can be gained? By discussing these questions, the article aims to provide a solid foundation to the further establishment of USTM as a valuable tool for ultrafast surface science. [more...]

Nonlinear optical processes require high light intensities and are typically induced by ultrashort pulsed laser excitation exploiting the temporal confinement of the laser electric field. In this paper we show that strong spatial confinement of continuous-wave electromagnetic fields can also induce multiphoton photocurrents in a plasmonic STM junction. The observation of multiple photon steps in the current-voltage curve of the tunnel junction reveals that the electron transfer originates from nonthermal photogenerated electrons and not from heating. The results raise the fundamental question whether such transfer originates from hot electrons or from coherent photo-assisted tunneling. The results are now published in ACS Photonics 10, 10, 3637-3646 (2023).

Exciting! Femtosecond laser-excited STM allows us to excite and probe coherent phonons with few nanometer spatial resolution. This work brings us a big step closer to our research goal of understanding the coupling between microscopic degrees of freedom – such as electrons and phonons – in spatially inhomogeneous crystalline solids at the nanoscale. One of our open questions has been: Can we use STM to probe lattice vibrations in real-time? In this collaborative study, we demonstrate that femtosecond laser-excited STM can measure coherent phonons in ultrathin ZnO films with ~2 nm spatial resolution. Our work introduces a new approach to locally probe the ultrafast structural response and electron-phonon coupling in a photoexcited solid with STM. For more details read our work: DOI: 10.1126/sciadv.abq5682.