Scanning Tunneling Microscope Based on Strong-Field Terahertz Pulse

Significance The application of modern nanotechnology has been greatly facilitated by the studies of ultrafast dynamics at the nanoscale. Terahertz radiation occupies a unique position in the electromagnetic spectrum, making it a popular choice for the exploration of various physical properties. Through advancement in femtosecond laser technology and terahertz radiation source, the intensity of terahertz radiation has seen a tremendous increase, broadening its range of applications. In recent years, terahertz time-domain spectroscopy (THz-TDS) has achieved remarkable commercial success. Consequently, terahertz radiation has become a key diagnostic tool in the development of new technologies. However, the diffraction limit of hundred microns hinders the use of terahertz radiation at the nano- and sub-nanoscale, thus emphasizing the imperative for the development of terahertz microscopy to further promote terahertz technologies. In 1981, the first scanning tunneling microscope (STM) was conceived in Zurich, enabling the real-time observation of individual atomic arrangements on matter surfaces and elucidating the physical and chemical properties related to the electrons' behavior of the surface. This breakthrough was recognized by the global scientific community as one of the world's top ten scientific and technological achievements of the 1980s. However, the time-resolution of STM is limited to milliseconds, making it impossible to track ultrafast dynamics in materials. To address this challenge, terahertz scanning tunneling microscopy (THz-STM) was developed. By using an atomic-scale tip to enhance and confine terahertz radiation, the spatial resolution of terahertz imaging can be improved by up to six orders of magnitude. This review outlines the historical progression, fundamental principles, system configuration, and potential applications of THz-STM, including ultrafast dynamics studies in semiconductor surfaces and single molecules. Finally, prospects for future advancements in this field are discussed. Progress THz-STM utilizes quantum tunneling effect akin to the conventional STM, but with the difference that it couples terahertz pulses to the STM tip, by using the time-varying terahertz electric field instead of the direct current (DC) bias voltage. Consequently, a comparable spatial resolution to that of STM is attained, along with sub-picosecond temporal resolution by adjusting the relative delay between the pumping and probing pulses. A THz-STM system comprises two primary parts: a strong-field terahertz system and a traditional STM system. Currently, large-area photoconductive antennas (Fig. 2) and optical rectification in lithium niobate crystal (Fig. 3) are utilized to generate strong terahertz field. To implement the terahertz pump-probe technique, the optimal optics guides terahertz pulses to a Michelson interferometer, which splits a terahertz pulse into the pumping and probing pulses. Varied delay is achieved by adjusting the length of one of the arms of the interferometer. THz-STM finds primary applications in semiconductor and molecular research. Ten years ago, when the first THz-STM system was demonstrated in University of Alberta, it was explored to conduct an optical pump-terahertz probe investigation on InAs nanodots on GaAs substrate, successfully observing the ultrafast dynamics of carriers captured by an InAs nanodot. Subsequently, Jelic et al. improved the system by operating the THz-STM system in an ultra-high vacuum chamber, which increased the spatial resolution to the angstrom level along with a temporal resolution on the order of hundreds femtoseconds, allowing for single unit cell reconstruction on the surface of Si(111)-(7x7) (Fig. 4). In 2021, the THz-STS was presented for graphene nanoribbons with atomic precision as well (Fig. 5). In particular, THz-STM offers captivating possibilities for single-molecule investigations. In 2016, Cocker et al. achieved single-electron tunneling on a pentacene molecule with a THz-STM system, which triggers molecular resonance (Fig. 6). Four years later, Peller et al. used THz-STM to achieve ultrafast control of single-molecule switching motion, proposing a more convenient observation method (Fig. 7). Conclusions and Prospects This article reviews the development of THz-STM. The ultrahigh spatiotemporal resolution of THz-STM makes it possible for researchers to observe ultrafast dynamics at the atomic scale, thus promoting the fusion of terahertz technology with other research fields. It is believed that direct observation of the microscopic world and further understanding of some basic physical processes can be achieved through the use of THz-STM in the future. Furthermore, the incorporation of new technologies holds the potential to improve the performance of THz-STM. Promising avenues for further exploration include time-resolved terahertz pulse induced electroluminescence, terahertz tunnel current induced single photon emission, ultrafast atomic force microscopy and multi-probe THz-STM system. In particular, a static STM has been shown to be compatible with the magnetic field. Research can also be conducted to study the nature of metasurfaces and provide solutions to the practical challenges associated with them. In conclusion, the THz-STM reviewed in this paper has immense potential for application and will be a significant presence in microphysical research, thus requiring more researchers to explore the field.