Advances in ultrafast terahertz scanning tunneling microscopy

Frank A. Hegmann

Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada

The ability to directly probe ultrafast phenomena on the nanoscale is essential to our understanding of excitation dynamics in materials and in the development of new device technologies. However, achieving this capability has been challenging and is the focus of research in many labs around the world. Terahertz scanning tunneling microscopy, or THz-STM, is a powerful new technique that enables direct imaging of sub-picosecond dynamics in materials down to the atomic scale. In this technique, picosecond-duration THz pulses are antenna-coupled to the sharp metal tip of an STM, and the resulting field enhancement at the junction produces sub-picosecond transient tunnel currents that can be used to probe ultrafast dynamics on the nanoscale [1-36].

THz-STM was first demonstrated by Cocker et al. [1], showing the photoexcitation dynamics in a single InAs nanodot with simultaneous 0.5 ps time resolution and 2 nm spatial resolution under ambient conditions. Operation in ultrahigh vacuum allows for THz-pulse-induced tunnel currents confined to single atoms, as reported for THz-STM of silicon surfaces [2] and single pentacene molecules [3]. THz-STM has recently been used to study metal surfaces [4], graphene nanoribbons [5], 2H-MoTe₂ and Bi₂Se₃ [6], C₆₀ films [7], applying localized forces to single molecules [8], inducing luminescence in materials [9], probing the dynamics of single vacancies in WSe₂ [10], and THz wave rectification and molecular coherent oscillations [11,12]. Much work has focused on characterizing the near-field waveform [13-21], coherent control of tunnel currents [2,6,16,17,22], modeling the THz-STM signal [2,4,19], increasing bandwidth [18] and efficiency [23], increasing the THz pulse source repetition rate [23,24], studying thermal and nonthermal tunneling effects [25,26], and achieving attosecond time resolution [27,28]. There have also been several THz-STM review articles [29-36].

This talk will discuss how THz-STM works and how it can provide new insight into ultrafast nanoscale dynamics of materials and devices. Recent advances, current challenges, and future directions will also be discussed.

Acknowledgments
Funding support from NSERC, CFI, Alberta Innovates, and ATUMS is acknowledged.

References
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