Atomic-scale terahertz time-domain spectroscopy

Lightwave-driven terahertz scanning tunnelling microscopy (THz-STM) is capable of exploring ultrafast dynamics across a wide range of materials with ångström resolution (10⁻¹⁰ m). In contrast to scanning near-field optical microscopy, where photons scattered by the tip apex are analysed 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 local electromagnetic response of the sample. Here we demonstrate atomic-scale terahertz time-domain spectroscopy by combining waveform sampling with terahertz scanning tunnelling spectroscopy to study a single gallium arsenide surface defect, which exhibits a strongly localized terahertz resonance and resembles the elusive DX centre. These results are based on a generally applicable and self-consistent approach for terahertz near-field waveform acquisition in a tunnel junction that can distinguish local sample properties from effects due to terahertz pulse coupling, enabling comprehensive near-field microscopy on the atomic scale.