Control of Surface Plasmon Propagation and Terahertz Near-Field Waveforms in a Scanning Tunneling Microscope

Coupling subcycle THz pulses to a scanning tunneling microscope (STM) enables ultrafast spectroscopy at the atomic scale. This technique critically depends on the shape of the THz near-field waveform in the tunnel junction. We characterize the THz electric field waveform in the STM junction by electro-optic sampling of tip-scattered THz light (<i>s</i>-EOS) and pulse correlation using the THz-induced current. Combined with full-wave simulations, we identify THz spectral distortions and reflections arising from THz surface plasmon propagation along the tip wire and cavity modes at the tip apex. By optimizing the tip shape, tip holder geometry and materials, we achieve a drastically flattened THz near-field waveform. This optimization ensures point-like coupling to the far-field and, thus, allows precise Gouy phase control at the STM tip. The improved THz waveforms facilitate atomically-resolved THz time-domain spectroscopy in the STM with high dynamic range for investigating local electron and phonon dynamics on surfaces.