Probing Phonon Coupling in Infrared Nanophotonic Materials with Electrons

Abstract

Phonon polaritons—quasiparticles arising from the coupling of photons and optical phonons—enable sub-diffractional control of infrared light in polar dielectrics. This thesis presents a study of the physics of phonon coupling in two interrelated platforms for tuning phononic and polaritonic behavior: twisted bilayer α-MoO₃ heterostructures and nanoparticle–substrate systems. The first focus addresses the fabrication and characterization of twisted α-MoO₃ bilayers, a naturally biaxial van der Waals material known for its hyperbolic dispersion in the mid-to-far infrared range. A multi-step protocol was developed to create suspended, twisted bilayers with angular precision better than ±5°, employing hydration-assisted softening and micromanipulation. Using aloof-mode electron energy loss spectroscopy (EELS), we observe twist-angle–dependent spectral changes in hyperbolic phonon polariton (HPhP) spatial EELS scattering intensities. The results provide physical insights into the spatial distribution of electromagnetic density of states of the twisted platform. Finite-element simulations corroborate experimental results, confirming tunability via twisted angle induced symmetry breaking. Furthermore, we showed that the twisted system generates local regions where the propagation of hyperbolic polaritons can be stirred. The second component investigates localized surface phonon modes in isotropic dielectric nanoparticles near thin-film substrates. Vibrational EELS and boundary element modeling reveal multipolar mode mixing driven by image-charge interactions and Fuchs–Kliewer mode hybridization. We identify spectral fingerprints of phonon coupling and delineate conditions under which substrate permittivity and thickness significantly alter phononic response. These findings unveil the channels in which a fast electron can deposit energy into a supported nanostructure during an inelastic scattering event. Also, shed lights into the available mechanism for infrared light to couple with a nanoscale object. Together, these studies demonstrate novel mechanisms for steering nanoscale infrared light and expand experimental access to tuneable phonon-polariton physics. In particular, the observation of substrate-induced hybridization and image-charge-driven multipolar mode mixing in nanoparticle–film geometries reveals an additional design degree of freedom for tailoring near-field vibrational responses. These insights, combined with the demonstrated twist-angle–dependent control in layered van der Waals systems, can be potentially considered for future developments in reconfigurable nanophotonics and advanced infrared sensing platforms at the nanoscale.