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

Melanie Müller,Martin Wolf,Natalia Martín Sabanés,Tobias Kampfrath

doi:10.1021/acsphotonics.0c00386

Melanie Müller, Martin Wolf + Show 2 more

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https://doi.org/10.1021/acsphotonics.0c00386

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Abstract

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

Highlights

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Ångstrom spatial resolution
Our results demonstrate the suitability of spintronic THz emitters as a broadband source for the application of ultrafast voltage transients in STM, and highlight the importance of the tip antenna response that exhibits significant low-pass filtering and reduction of the THz bandwidth available for T Hz-gated scanning tunneling microscopy (THz-STM) operation
We find that the THz voltage stays approximately constant over the investigated distance range, confirming our experimental observation and being consistent with time-domain simulations reported in previous work.[2]

Summary

This could be achieved by spectral matching of the incident

THz spectrum reciprocal to the STM tip response. we note that additional low-pass filtering is caused by the frequency-dependent orientation of the antenna lobes,[14] leading to less efficient coupling of the high THz frequencies at large incident angles with respect to the tip axis. We find that the THz voltage stays approximately constant over the investigated distance range, confirming our experimental observation and being consistent with time-domain simulations reported in previous work.[2] These results imply that the THz electric field in the junction is quasi-static, that is, the metal surfaces of the tip and sample are equipotential surfaces for the THz field. Easier broadband characterization of the incident THz electric field will be employed by using thinner electro-optic detection crystals.[52] At higher (tens of THz) frequencies, we expect deviations from the quasi-static behavior due to retardation and the frequency-dependent material response In this regard, our work provides a direct route toward the experimental characterization of the phase and amplitude of multi-THz voltage transients applied to an STM junction with fewfemtosecond resolution

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■ ACKNOWLEDGMENTS

Findings

■ REFERENCES