Inelastic Electron Scattering Spectroscopy: Probing Electromagnetic Surface Mode Resonances of Metallic and Dielectric Interfaces on the Nanoscale

Abstract

Reliable modeling and tailoring of the optical responses of fabricated dielectric nanostructures is critical to the design and implementation of next generation nano-optical devices, in addition to understanding the mechanisms of charge and energy transfer at atomic scales. Perhaps, no two instruments have been as indispensable in the spectroscopic characterization of atomic scale systems as the laser, and the scanning transmission electron microscope (STEM). Concerning the later, recent advances in electron beam monochromation and aberration correction have allowed for accurately probing target responses from thermal to X-ray energies with high spectral and spatial resolution. The majority of this disseration will focus specifically on electron energy loss (EEL) and gain (EEG) spectroscopies, where spectroscopic measurements are generated by a near-field mediated inelastic scattering process between a series of fast moving electrons and the induced field response of an interrogated target. In this dissertation, I will restrict the discussion of target excitation to the spectral window of optical-IR energies, as it is at these relatively low energies that electromagnetic surface mode phenomena such as surface plasmon and surface phonon mode resonances occur. At higher energies, stretching from the ultraviolet to x-ray, the inelastic scattering signal is capable of characterizing electronic interband transitions, core electron excitations, and ionization energies. Intrinsic to this discussion is the role that hybridization plays between ensembles of dielectric particles, and how dielectric nanostructures can couple with a resonant background environment via their induced electromagnetic fields. I then explore how these complex coupling effects are reflected in the EELS/EEGS signals, generated via the inelastic scattering of fast electrons. I attempt to accomplish this by first orienting the reader with classical dielectric theory, and an intuitive characterization of the electromagnetic surface modes intrinsic to dielectric interfaces. I then construct physical models of the spectroscopic observables generated under light excitation and via near-field ion probes. Crucial to all of this, is a robust procedure for mapping the electromagnetic surface modes onto harmonic oscillator coordinates, along with an analysis of the system eigenmodes, and a derivation of their effective masses. This is accomplished by way of the method of Green's functions. This proves to be a powerful and versatile technique for describing the physical and optical properties of the dielectric interfaces which I study herein. My hope is that this intuitive approach to modeling the response of dielectric nanostructures in the presences of light and applied electromagnetic fields, will aid the reader of this dissertation in interpreting spectroscopic measurements in a laboratory setting.