  Lightwave-driven terahertz scanning tunnelling microscopy (THz-STM) is
capable of exploring ultrafast dynamics across a wide range of materials with
angstrom resolution. In contrast to scanning near-field optical microscopy,
where photons scattered by the tip apex are analyzed to access the local
dielectric function on the nanoscale, THz-STM uses a strong-field single-cycle
terahertz pulse to drive an ultrafast current across a tunnel junction, thereby
probing the local density of electronic states. Yet, the terahertz field in a
THz-STM junction may also be spectrally modified by the electromagnetic
response of the sample. Here, we demonstrate a reliable and self-consistent
approach for terahertz near-field waveform acquisition in an atomic tunnel
junction that can be generally applied to electrically conductive surfaces. By
combining waveform sampling and tailoring with terahertz scanning tunnelling
spectroscopy (THz-STS), we comprehensively characterize the tunnel junction and
distinguish local sample properties from effects due to terahertz pulse
coupling and field enhancement. Through modelling, we verify the presence of an
isolated unipolar terahertz-induced current pulse, facilitating straightforward
interpretation for differential THz-STS with high spectral resolution. Finally,
we demonstrate the feasibility of atomic-scale terahertz time-domain
spectroscopy via the extremely localized near-fields in the tunnel junction.
