Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction

• DOI: 10.1021/acsphotonics.0c00386
• Publication Date: 2020-07-08T16:17:53Z
• Authors: Melanie Müller; Natalia Martín Sa...; Tobias Kampfrath; Martin Wolf
• Keywords: THz voltage sampling; Condensed Matter - Mesoscale and Nanoscale Physics; 500 Naturwissenschaften und Mathematik::530 Physik::530 Physik; spintronic THz emitter; FOS: Physical sciences; Physics - Applied Physics; Applied Physics (physics.app-ph); 02 engineering and technology; 530; 01 natural sciences; broadband THz pulses; ultrafast photocurrents; 0103 physical sciences; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); scanning tunneling microscopy; 0210 nano-technology; tip antenna response

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Angstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the subcyde 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 ultra-broadband (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.