2025 Theses Doctoral

Confined Light-matter Interactions in Van der Waals Heterostructures probed by Scanning Near-field Optical Microscopy

Moore, Samuel

Heterostructures composed of Van der Waals materials host a variety of sub-diffractional optical phenomena, such as moire patterns and emergent collective modes. In this thesis, we explore three separate phenomena in vdW heterostructures: phonons in moire superlattices, Purcell-enhanced spontaneous emission from excitons, and plasmons at ultrahigh doping. In this thesis, we will describe how the Scanning near-field optical microscopy (SNOM) technnique pro-vides sub-diffractional optical insights into excitons, phonons, and plasmons in vdW heterostructures.

First, we use SNOM to map local phonon resonances in a moire superlattices formed by two twisted bilayers of hexagonal boron nitride. Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moiré superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lat- tice response experiments only in their infancy. We therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moiré superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moiré domains of twisted hBN.

Next, we use SNOM to study light emission from excitons into confined waveguide modesof planar vdW waveguides. Atomically layered vdW materials exhibit remarkable properties, including highly-confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit. In this case, we engineered structures that leverage both of these nano-optical functionalities. Specifically, we encased a photoluminescing atomic sheet of MoTe₂ within two bulk crystals of WSe₂, forming a vdW waveguide for the embedded light-emitting monolayer. The modified electromagnetic environment offered by the WSe₂ waveguide alters MoTe₂ spontaneous emission, a phenomenon we directly image with our interferometric nano-photoluminescence technique. We captured spatially-oscillating nanoscale patterns prompted by spontaneous emission from MoTe₂ into waveguide modes of WSe₂ slabs. We quantify the resulting Purcell- enhanced emission rate within the framework of a waveguide quantum electrodynamics (QED) model, relating the MoTe₂ spontaneous emission rate to the measured waveguide dispersion. Our work marks a significant advance in the implementation of all-vdW QED waveguides.

Finally, we investigate phonons and confined plasmonic modes in highly-doped multilayer graphene. Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and 𝛼-RuCl₃. In heterostructures with a single 𝛼-RuCl₃ inter- face, the charge transfer generates displacement fields up to 7 V/nm at the interface between 𝛼- RuCl₃ and the adjacent graphene layer—the highest value achieved through charge transfer meth- ods. As a result of the broken inversion symmetry, we demonstrate enhanced nonlinear optical response and modified phonon selection rules.

Conversely, we find that multilayer graphene sand- wiched between two 𝛼-RuCl₃ flakes causes displacement fields to cancel. There, we achieve carrier densities as high as 8 ×1013 cm−2 in multilayer graphene and restore the phonon selection rules to their unperturbed state. The ultrahigh carrier densities deplete multiple valence bands of multilayer graphene, a previously unexplored regime for plasmon propagation.

Furthermore, the inverted heterostructure sequence—two multilayer graphene sheets encapsulating 𝛼-RuCl3—activates sig- nificant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and 𝛼-RuCl₃ heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.