Exciton dynamics is responsible for the photophysical and photochemical functions, such as energy harvesting, light emission, and photosynthesis. In particular, energy conversions, transfers, and dissipations at interfaces and nanostructures govern the dynamics of excitons. Nevertheless, the spatial resolutions of conventional optical methods are limited to several hundred nanometers due to the diffraction limit of light. Luminescence measurements combined with scanning tunneling microscopy (STM) enable to investigate optical properties with sub-nanometer spatial resolution in various systems such as metals^[1], semiconductors^[2], 2D materials^[3], and molecules^[4-5]. In this method, the scanning tunneling luminescence (STL) spectroscopy, photons induced by the tunneling current of STM are detected. Since the tunneling current can be controlled with atomic spatial resolution, it is possible to investigate optical phenomena in localized regions. While STL spectroscopy is a powerful tool for investigating exciton dynamics with atomic-scale spatial resolution, the time resolution of STL spectroscopy is limited because it uses “steady-state” tunneling current for the excitation. Therefore, this method cannot be applicable to track the ultrafast exciton dynamics in real-time.

For overcoming the limitation of time resolution of STM, a single-cycle terahertz (THz) electric field has recently been installed to drive tunneling electrons^[6-9]. By coupling the THz pulse to an atomically sharp metal tip of STM, the electric field is confined to the tip, and it enables to manipulate the tunneling electrons with extremely high spatiotemporal resolution. This method, THz-STM, has been utilized to track the ultrafast carrier dynamics^[6-8] and motion of a molecule^[9] with sub-picosecond time resolution. In this work, we combined a photon detection system with THz-STM to establish THz-STL spectroscopy (Fig. 1a). We measured visible photon emission from the radiative decay of a localized plasmon as the first demonstration of THz-STL spectroscopy^[10]. We anticipate that the THz-STL spectroscopy will open new avenues for “real-time” and “real-space” investigations of exciton dynamics in near future.

In this work, we generated single-cycle THz pulses via optical rectification of laser pulses from a Yb fiber laser in a LiNbO₃ prism using a tilted-pulse-front configuration (Fig. 1b). The generated THz pulses were guided into a low-temperature STM and focused to the STM junction (Fig. 1a). Figure 1c shows a current trace on Ag(111) surface, where the current induced by the THz pulses was measured as 2.65 pA. Based on the simulation of electron tunneling probability, the maximum magnitude of the voltage applied by the THz pulse was estimated to be ~6.5 V. Figure 1d shows STL spectra when the THz pulse was either introduced to the STM (red) or blocked (grey). Whereas no peak is seen in the grey spectrum, a broad peak ranging from 1.3 eV to 2.3 eV appears in the red spectrum, which is originated from the radiative decay of a plasmon localized in the gap.

In this presentation, we would like to discuss the detailed mechanism of plasmon excitation by THz-field-driven electrons as well as the challenges of applying THz-STL to the real-time investigation of exciton dynamics in a molecule.

[1] R. Berndt et al., Phys. Rev. Lett. 67, 3796 (1991). [2] J. K. Gimzewski et al., Z. Phys. B Condensed Matter 72, 497 (1988). [3] Schuler et al., Sci. Adv. 6, eabb5988 (2020). [4] H. Imada et al., Nature 538, 364 (2016). [5] K. Kimura et al., Nature 570, 210 (2019). [6] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [7] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [8] V. Jelic et al., Nature Physics 13, 591 (2017). [9] D. Peller et al., Nature 585, 58 (2020). [10] K. Kimura et al., ACS photon. 8, 982 (2021).