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.

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

[1] T. L. Cocker, V. Jelic, M. Gupta, S. J. Molesky, J. A. J. Burgess, G. De Los Reyes, L. V. Titova, Y. Y. Tsui, M. R. Freeman, and F. A. Hegmann, “An ultrafast terahertz scanning tunnelling microscope,” Nat. Photon. 7, 620 (2013)
[2] V. Jelic, et al., “Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surface,” Nat. Phys. 13, 591 (2017)
[3] T. L. Cocker, et al., “Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging,” Nature 539, 263 (2016)
[4] Y. Luo, V. Jelic, et al., “Nanoscale terahertz STM imaging of a metal surface,” Phys. Rev. B 102, 205417 (2020)
[5] S. E. Ammerman, et al., “Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons,” Nat. Commun. 12, 6794 (2021)
[6] S. Yoshida, et al., “Subcycle Transient Scanning Tunneling Spectroscopy with Visualization of Enhanced Terahertz Near Field,” ACS Photonics 6, 1356 (2019)
[7] S. Yoshida, et al., “Subcycle Mid-Infrared Electric-Field-Driven Scanning Tunneling Microscopy with a Time Resolution Higher Than 30 fs,” ACS Photonics 8, 315 (2021)
[8] D. Peller, et al., “Sub-cycle atomic-scale forces coherently control a single-molecule switch,” Nature 585, 58 (2020)
[9] K. Kimura, et al., “Terahertz-Field-Driven Scanning Tunneling Luminescence Spectroscopy,” ACS Photonics 8, 982 (2021)
[10] C. Roelcke, et al., “Ultrafast atomic-scale scanning tunnelling spectroscopy of a single vacancy in a monolayer crystal,” Nat. Photon. (2024);
[11] S. Chen, et al., “Single-Molecule Continuous-Wave Terahertz Rectification Spectroscopy and Microscopy,” Nano Lett. 23, 2915 (2023)
[12] Y. Xia, et al., “Mechanisms Underlying a Quantum Superposition Microscope Based on THz-Driven Coherent Oscillations in a Two-Level Molecular Sensor,” Phys. Rev. Lett. 132, 076903 (2024)
[13] P. H. Nguyen, et al., “Coupling THz Pulses to a Scanning Tunneling Microscope,” Phys. Can. 71, 157 (2015)
[14] D. Peller, et al., “Quantitative sampling of atomic-scale electromagnetic waveforms,” Nat. Photon. 15, 143 (2021)
[15] J. Takeda and I. Katayama, “Waveform sampling on an atomic scale,” Nat. Photon. 15, 70 (2021)
[16] K. Yoshioka, et al., “Real-space coherent manipulation of electrons in a single tunnel junction by single-cycle terahertz electric fields,” Nat. Photon. 10, 762 (2016)
[17] K. Yoshioka, et al., “Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields,” Nano Lett. 18, 5198 (2018)
[18] M. Müller, et al., “Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction,” ACS Photonics 7, 2046 (2020)
[19] S. E. Ammerman, et al., “Algorithm for subcycle terahertz scanning tunneling spectroscopy,” Phys. Rev. B 105, 115427 (2022)
[20] H. Li, et al., “Real-Space Sampling of Terahertz Waveforms Under Scanning Tunneling Microscope,” ACS Photonics 11, 1428 (2024)
[21] V. Jelic, et al., “Atomic-Scale Terahertz Near Fields for Ultrafast Tunnelling Spectroscopy,” arXiv:2310.14335 [cond-mat.mes-hall]
[22] J. Allerback, et al., “Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy,” ACS Photon. 10, 3888 (2023)
[23] M. Abdo, et al., “Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy,” ACS Photonics 8, 702 (2021)
[24] H. Azazoglu, et al., “Variable-temperature lightwave-driven scanning tunneling microscope with a compact, turn-key terahertz source,” Rev. Sci. Instrum. 95, 023703 (2024)
[25] N. M. Sabanés, et al., “Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction,” ACS Nano 16, 14479 (2022)
[26] H. Azazoglu, et al., “Thermal expansion in photo-assisted tunneling: Visible light versus free-space terahertz pulses,” Surf. Sci. 743, 122465 (2024)
[27] M. Garg and K. Kern, “Attosecond coherent manipulation of electrons in tunneling microscopy,” Science 367, 411 (2020)
[28] M. Garg, et al., “Real-space subfemtosecond imaging of quantum electronic coherences in molecules,” Nat. Photon. 16, 196 (2022)
[29] T. L. Cocker, V. Jelic, R. Hillenbrand, and F. A. Hegmann, “Nanoscale terahertz scanning probe microscopy,” Nat. Photon. 15, 558 (2021)
[30] T. L. Cocker and F. A Hegmann, “Ultrafast lightwave-driven scanning tunnelling microscopy”, in J Lloyd-Hughes, et al., “The 2021 ultrafast spectroscopic probes of condensed matter roadmap,” Journal of Physics: Condensed Matter 33, 353001 (2021)
[31] T. Tachizaki, et al., “On the progress of ultrafast time-resolved THz scanning tunneling microscopy,” APL Mater. 9, 060903 (2021)
[32] R. Gutzler, et al., “Light–matter interaction at atomic scales,” Nat. Rev. Phys. 3, 441 (2021)
[33] M. Peplow, “The next big hit in molecule Hollywood,” Nature 544, 408 (2017)
[34] Y. Tian, et al., “Controlling photocurrent channels in scanning tunneling microscopy,” Surf. Rev. Lett. 25, 1841003 (2018)
[35] M. Müller, “Imaging surfaces at the space–time limit: New perspectives of time-resolved scanning tunneling microscopy for ultrafast surface science,” Prog. Surf. Sci. 99, 100727 (2024)
[36] I. Katayama, et al., “Investigation of ultrafast excited-state dynamics at the nanoscale with terahertz field-induced electron tunneling and photon emission,” J. Appl. Phys. 133, 110903 (2023)

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