Speaker
Description
Tracing ultrafast dynamics of photoexcited charge carriers after femtosecond optical excitation provides insights into microscopic coupling mechanisms and energy transfer processes. While various pump–probe techniques are well established to trace electron dynamics in real time, real-space observation at the atomic scale remains a challenge. THz lightwave-driven scanning tunneling microscopy (STM) has emerged as a promising time-resolved microscopy tool that provides simultaneous femtosecond temporal and atomic spatial resolution.
Here we report on the development of an ultra-broadband THz-gated STM (THz-STM) suitable for photoexcitation over the UV to THz spectral range, aiming at atomic-scale imaging and analysis of nonequilibrium electronic states of photoexcited surfaces. As a key technical challenge, we report on the successful operation of spintronic THz emitters (STE) at 2 MHz and several Watts average pump powers to meet the require¬ments for ultra-broadband THz-STM operation. As a prerequisite to trace ultrafast photocarrier dynamics on metal surfaces with THz-STM and for future THz scanning tunneling spectroscopy (THz-STS), we demonstrate that the quasi-static femtosecond THz bias induced by the tip-enhanced THz field can be measured inside the STM by THz-field-induced modulation of ultrafast photocurrents [1]. This approach allows for easy, quantitative and phase-resolved access to the THz bias and for characterization of the junctions THz antenna response.
In a next step we applied our phase-resolved sampling approach to study ultrafast hot electron dynamics in a photoexcited metal using THz-STM [2]. Because in metals photo¬excited hot carriers decay very fast, typically on sub-picosecond time scales, a time resolution faster than a single THz near-field cycle in the STM is required. However, the gap-less structure and continuous density of states of metals prohibits application of ‘state-selective’ tunneling, as used previously in THz-STM on molecules, to attain THz-sub-cycle time resolution. We demonstrate that hot electron dynamics in a metal can be measured using ‘phase-resolved’ THz-STM and investigate the role of ultrafast thermionic currents for photoinduced tunneling after ultrafast laser excitation in STM [2]. Furthermore, we proof experimentally that hot electron tunneling from a photoexcited STM tip is dominated by nonthermal electron distributions.
References
[1] M. Müller, N. Martín Sabanés, T. Kampfrath, M. Wolf, ACS Photonics 7, 2046 (2020).
[2] N. Martín Sabanés, F. Krecinic, T. Kumagai, F. Schulz, M. Wolf, M. Müller, ACS Nano (accepted).
| Abstract Number (department-wise) | PC 12 |
|---|---|
| Department | PC (Wolf) |
