The terahertz (THz) range of the electromagnetic spectrum hosts material excitations that are particularly important for nanotechnology, such as the collective motion of charges, spins, and ions. These excitations are often studied with terahertz time-domain spectroscopy (THz-TDS), which is one of the central technologies of THz science and industrial applications. However, far-field THz-TDS is limited to macroscopic measurements by diffraction, whereas many open questions in surface science require these properties to be determined at much smaller length scales. For example, THz-TDS of individual atomic sites would allow the role of defects, dopants, and interfaces on charge transport to be studied in remarkable detail. The atomic spatial resolution of THz scanning tunneling microscopy (THz-STM) [1] presents an exciting opportunity for advancing THz-TDS to the smallest possible length scales. We demonstrate an experimental method for atomic-scale THz-TDS in a THz-STM junction and apply our technique to multiple materials, including silicon-doped GaAs [2] and type-II Weyl semimetal candidate WTe₂ [3]. Further, THz-STM allows us to study the THz-induced structural phase transition associated with a shear mode of the top surface layer in WTe₂ with unprecedented spatial resolution [3].

[1]. T. L. Cocker, et al. Nature Photonics, 15, 558-569 (2021).
[2]. V. Jelic, S. Adams, et al. Nature Photonics, 18, 898–904 (2024).
[3]. V. Jelic*, S. Adams*, D. Maldonado-Lopez*, et al. Nature Photonics 19, 1048–1055 (2025).