Femtosecond tunneling spectroscopy of ultrafast band bending dynamics at the atomic limit
Atomic-scale disorder shapes the potential energy landscape traversed by photoexcited charge carriers, 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 tunneling microscopy (THz-STM) provides access to these dynamics by probing the ultrafast evolution of local electronic structure following resonant interband excitation. 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. Supported by modeling, we experimentally resolve the time-dependent band bending produced by photoinduced charge carriers across the atomic-scale landscape of the sample surface. Crucially, we employ terahertz time-domain spectroscopy in the tip near-field to disentangle the coherent sub-cycle dynamics induced by the terahertz driving field from the intrinsic sample response. We establish a new regime of ultrafast tunneling 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.