Atomic-Scale Terahertz Time-Domain Spectroscopy in an Ultrafast Lightwave-Driven Tunnel Junction

Terahertz-pulse-driven scanning tunneling microscopy (THz-STM) offers unprecedented insights into the ultrafast processes in materials, achieving picometer-scale spatial resolution (<100 pm) and sub-picosecond temporal resolution (<0.5 ps). In THz-STM, terahertz pulses propagating in free-space couple to the STM tip, driving ultrafast tunneling currents that track dynamic changes in the sample's surface charge density. Additionally, the temporal profile of the terahertz electric field within the tunnel junction is modified by the sample's local electromagnetic response. We demonstrate atomic-resolution terahertz time-domain spectroscopy by integrating field-resolved waveform sampling with terahertz tunneling spectroscopy to investigate the elusive DX center, a single-atom defect on the gallium arsenide surface. Utilizing a self-consistent approach for acquiring terahertz near-field waveforms, we effectively disentangle intrinsic sample properties from tip-coupling artifacts, providing new avenues for ultrafast spectroscopy of surfaces at the atomic scale.