Probing Quantum States in Low-Dimensional Materials via Laser-Assisted Scanning Tunneling Microscopy and Structured Light

Citation

Park, Akiyoshi (2026) Probing Quantum States in Low-Dimensional Materials via Laser-Assisted Scanning Tunneling Microscopy and Structured Light. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p17k-3619. https://resolver.caltech.edu/CaltechTHESIS:07312025-212004631

Abstract

This dissertation presents the development and application of a laser-assisted scanning tunneling microscope (STM) to probe and control emergent quantum phenomena in two-dimensional (2D) and topological materials. A custom-built STM system was designed to operate under ultra-high vacuum and cryogenic conditions, integrating electronic, mechanical, and optical subsystems to enable high-resolution tunneling spectroscopy with in situ optical excitation. This platform allows for the detection of light-induced tunneling photocurrents with polarization control, facilitating direct investigations of light–matter interactions at the atomic scale.

Using this instrument, we first investigate the dynamics of photoexcited quasiparticles in strained monolayer (ML) transition metal dichalcogenides (TMDs), specifically ML-MoS$_2$ grown on corrugated Au(111). Nanoscale strain induces local bandstructure modulation, which acts as a trapping potential for excitons, trions, and electron-hole plasmas. Optical excitation in these strained regions leads to strongly enhanced band renormalization effects, which we interpret through a tight-binding model that incorporates non-uniform strain and many-body interactions. These results establish a framework for manipulating quasiparticle interactions and optoelectronic properties in 2D materials via strain and light.

Moreover, we utilize STM/STS to explore magnetically doped topological insulators exhibiting the quantum anomalous Hall (QAH) effect. Scanning tunneling spectroscopy (STS) measurements on six-quintuple-layer films reveal a topologically non-trivial energy gap at low temperature. However, spatially inhomogeneous band alignment driven by many-body interactions, including electron-defect and electron-phonon coupling results in local band overlap and a breakdown of the insulating QAH state into a metallic phase, even below the Curie temperature. These findings highlight the role of microscopic disorder and finite-temperature renormalization in destabilizing topological phases.

Finally, we demonstrate angular momentum transfer from structured light to Rydberg excitons in monolayer MoSe₂. Optical vortex beams carrying orbital angular momentum (OAM) selectively couple to exciton degrees of freedom. At low light intensity, photonic OAM modifies the center-of-mass motion of excitons, while at higher light intensities, exchange-mediated processes transfer angular momentum of light to the internal excitonic orbitals. Photoluminescence measurements under Laguerre-Gaussian beam excitation reveal fingerprints of higher orbital Rydberg exciton states, providing a new pathway for controlling exciton orbital structure in solid-state systems.

Together, these studies showcase how laser-assisted STM and structured light can be harnessed to probe, engineer, and control quantum states in low-dimensional materials, with implications for topological electronics, nanoscale optoelectronics, and quantum excitonic devices.