Nanoscale terahertz STM imaging of a metal surface

Terahertz scanning tunneling microscopy (THz-STM) has enabled studies of ultrafast dynamics in materials down to the atomic scale. However, despite recent advances, more work is needed to better understand and quantify the subpicosecond THz pulse-induced tunnel currents and corresponding THz-STM images of nanoscale features on surfaces. Here, we perform THz-STM on a metal surface and fully characterize the observed THz pulse-induced tunnel current and nanoscale imaging at atomic steps and defects using a Bardeen tunneling model in a three-dimensional (3D) tip geometry. We show that the measured steady-state STM current-voltage curves can be used in our model to accurately map the observed ultrafast THz-induced tunnel currents and calibrate the magnitude of the near-field peak transient THz voltage bias in the tunnel junction. Peak THz voltage bias transients greater than 10 V across the STM junction are achieved leading to field emission of subpicosecond tunnel currents with current densities exceeding 10⁹0.28em0exA/cm² in THz-STM imaging of a Cu(111) surface. Our results establish an important benchmark for future studies in THz-STM by quantifying the ultrafast THz-induced currents and bias voltages in the tunnel junction and providing a 3D tunneling model for understanding and accurately simulating THz-STM images of nanoscale features on metal surfaces.