Femtosecond tunneling spectroscopy of ultrafast band bending dynamics at the atomic limit
Atomic-scale disorder shapes the potential energy landscape traversed by photocarriers, while the carriers themselves also dynamically reshape this landscape. However, resolving ultrafast photocarrier motion at atomic length scales has remained a central challenge in materials science. Here, we demonstrate that lightwave-driven terahertz scanning tunnelling microscopy (THz-STM) provides access to these dynamics by probing the ultrafast evolution of local electronic structure following resonant interband excitation. We introduce a generalized inversion protocol that reconstructs the time-dependent differential conductance directly from measured THz-STM data, enabling quantitative analysis of the photoexcited electronic structure without prior knowledge of the sample's properties. Applying this approach to the photoexcited GaAs(110) surface, we image the resulting femtosecond carrier dynamics by tracking the transient photocurrents produced by ultrafast shifts in the energy alignment of surface and bulk electronic states near individual surface defects. The reconstructed dynamics reveal time-dependent band bending induced by the photocarriers and highlight three key mechanisms that contribute to the ultrafast tunnel current response. Critically, terahertz time-domain sampling of the tip near-field, which both captures the waveform and demonstrates its invariance under sample photoexcitation, allows the intrinsic sample dynamics to be disentangled from the coherent sub-cycle dynamics induced by the terahertz driving field. We establish a new regime of ultrafast tunnelling spectroscopy that captures transient electronic structure and dynamic band alignment with unprecedented spatio-temporal resolution, which has significant implications for understanding carrier transport, defect-mediated processes, and the development of optoelectronic technologies based on dynamically tunable materials.