Terahertz Near-Field Coupling in Scanning Tunneling Microscopy
Creators: Nguyen, Peter H
Publisher: name: University of Alberta Library
Year: 2024

<p>THz optics and scanning probe microscopy (SPM) techniques have gone through several decades of development, enabling numerous research studies and applications related to ultrafast phenomena at the nanoscale. Advancements in THz technology enable physical processes in materials to be identified at precise timescales from nanoseconds down to femtoseconds. Modern scanning tunneling microscopy (STM) systems readily scan surfaces to reveal molecular and atomic structure. Physical phenomena on individual atoms or molecules can be studied at ultrashort timescales simultaneously by combining THz optics and STM to develop a terahertz scanning tunneling microscope (THz-STM), an atomic imaging microscope coupled to ultrafast light pulses.  In THz-STM, THz pulses generated by a femtosecond laser source propagate to the STM probe tip, then become focused down to the microscopic volume around the STM tip apex, which drives the tunneling in the sub-nanometer regime.  Amidst recent advances in THz-STM techniques where simultaneous atomic spatial resolution and sub-picosecond time resolution has been demonstrated, the nature of the transient bias resulting from the coupling of THz radiation to the STM junction is not completely understood. In order realize the full potential of THz-STM, there is a need to understand the nature of electromagnetic radiation coupling with the STM system and the sample material being probed.     In this thesis, a detailed investigation is carried out using finite-element method (FEM) simulations with COMSOL Multiphysics, as well as analytical approaches and numerical modeling on the THz-STM. Full scale simulations and modeling are conducted in the far-field regime, the region away from the STM probe at about a few THz wavelengths and beyond, then focuses on the near-field regime, the gap region between the tip and sample of the STM. The results show several interesting aspects about the tip and sample responding to coupled THz pulses in the near-field regime, which lead to insights for understanding the physical mechanisms in THz-STM operation.    In the THz-STM experiment, the COMSOL simulations visualize the propagation and focusing of a THz pulse to the STM junction and computes a near-field THz electric field existing in the nm-sized gap that is largely enhanced by factors of 1E4 to 1E5 compared to the incident THz electric field. Furthermore, the simulated near-field transients in the STM gap show spectroscopic dependence that resembles THz antenna-wire coupling.  Alternatively, the THz near-field is calculated with electromagnetic models used in scanning near-field optical microscopy (SNOM). The THz field generation, propagation and interaction with the STM geometry is simulated with a discrete dipoles configuration, reproducing similar results as the FEM simulations from COMSOL. A lumped element circuit model that parameterizes the STM junction as RLC circuit elements is also used to calculate the THz near-field voltage, which acts as the bias voltage for the STM junction. The circuit model is linked to a transmission line to simulate pulse propagation and pulse reflections along the tip shaft. Expanded circuit models are also developed for metal-to-metal and the metal-to-semiconductor junctions. Experiments and simulations reveal that subsurface transport of charge carriers driven by penetrating THz near-fields inside semiconductor samples is needed to account for extreme transient current densities and to also obtain sampled near-field waveforms observed in THz-STM.</p>