Control of\nSurface Plasmon Propagation and Terahertz\nNear-Field Waveforms in a Scanning Tunneling Microscope

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