• Sabanés, Natalia Martín

Abstract

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatio-temporal imaging with femtosecond temporal and {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 tip-enhanced THz waveform via THz-field-induced modulation of ultrafast photocurrents across the junction. 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 enhanced near-field, resulting in THz transients with a ha lf-cycle period of 115 fs. We further demonstrate versatile phase and polarity control of the THz waveform depending on the STE excitation conditions and magnetization, and show that up to 2 Volts 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 and provide insight into the femtosecond response of defined nanoscale junctions.

Topics

• impedance spectroscopy
• phase
• simulation
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• magnetization
• scanning tunneling microscopy