Manipulate polaritonic nano-light with environmental permittivity

Abstract

Polaritons, hybrid quasiparticles formed by coupling electromagnetic waves with dipolar excitations in materials, offer deep subwavelength confinement of light. This unique feature allows access to nanoscale optical fields and energy transfer, far beyond the diffraction limit of conventional optics. To visualize and analyze such nanoscale light-matter interactions, we employ scattering-type scanning near-field optical microscopy (s-SNOM), which achieves spatial resolution down to ~10 nm. This dissertation investigates phonon and exciton polaritons in van der Waals (vdW) materials and heterostructures, focusing on how environmental and structural modifications enable control over polaritonic behavior. Chapters 2, 3, and 5 explore mid-infrared phonon polaritons in anisotropic materials such as α-MoO3 and hexagonal boron nitride (hBN). First, we study suspended α-MoO3 flakes and demonstrate that removing the substrate results in a ~100% increase in polariton wavelength due to altered dielectric screening and negative dispersion. Simulations confirm strong anisotropic and mode-specific elongation effects. Next, we investigate α-MoO3/graphene heterostructures and reveal a new class of charge transfer–induced hyperbolic polaritons. Direct charge transfer increases the polariton wavelength by nearly 100% and induces a Fermi level shift in graphene to ~0.6 eV. Lastly, we explore hBN/graphene stacks and demonstrate tunable hybrid plasmon-phonon polaritons. Using electrostatic gating, we achieve dynamic and reversible control over polariton reflectivity and propagation length at the graphene edge. In Chapter 4, we turn to exciton-polariton waveguides in transition metal dichalcogenide (TMDC) heterostructures. We visualize guided optical modes in MoSe₂/MoS₂ bilayers using s-SNOM and observe stacking-induced wavelength modulation. Fourier analysis and geometric corrections reveal energy-dependent multimodal propagation. Dispersion modeling and finite element simulations confirm strong exciton-polariton coupling and validate our experimental findings. Chapter 6 reviews recent advances in scanning probe IR imaging and spectroscopy for natural and biological materials, highlighting the broader relevance of these techniques. Overall, this work provides fundamental insights into the manipulation of polaritons through environmental and structural engineering of vdW materials. The findings open up new opportunities for designing tunable nanophotonic devices, including reconfigurable optical circuits, mid-IR sources, quantum sensors, and nanoscale transistors based on light-matter interactions at the atomic scale.