  Coupling phase-stable single-cycle terahertz (THz) pulses to scanning
tunneling microscope (STM) junctions enables spatio-temporal imaging with
femtosecond temporal and \r{A}ngstrom spatial resolution. The time resolution
achieved in such THz-gated STM is ultimately limited by the sub-cycle temporal
variation of the tip-enhanced THz field acting as an ultrafast voltage pulse,
and hence by the ability to feed high-frequency, broadband THz pulses into the
junction. Here, we report on the coupling of ultrabroadband (1-30 THz)
single-cycle THz pulses from a spintronic THz emitter(STE) into a metallic STM
junction. We demonstrate broadband phase-resolved detection of the THz voltage
transient directly in the STM junction via THz-field-induced modulation of
ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform
reveals the antenna response of the STM tip. Despite tip-induced low-pass
filtering, frequencies up to 15 THz can be detected in the tip-enhanced
near-field, resulting in THz transients with a half-cycle period of 115 fs. We
further demonstrate simple polarity control of the THz bias via the STE
magnetization, and show that up to 2 V THz bias at 1 MHz repetition rate can be
achieved in the current setup. Finally, we find a nearly constant THz voltage
and waveform over a wide range of tip-sample distances, which by comparison to
numerical simulations confirms the quasi-static nature of the THz pulses. Our
results demonstrate the suitability of spintronic THz emitters for ultrafast
THz-STM with unprecedented bandwidth of the THz bias, and provide insight into
the femtosecond response of defined nanoscale junctions.
