Exploring Light-Matter Interactions at the Nanoscale with Synchrotron Infrared Nanospectroscopy and Spatial Modulation Spectroscopy

My thesis focuses on the optical investigation of materials, specifically in the near to mid-infrared regime. Whether using a synchrotron or an optical parametric oscillator as the source of electromagnetic radiation, the goals of my studies remain syncopated: to gain a better understanding of how nanomaterials and molecules of interest interact with light.

This thesis begins with the work I performed at the Advanced Light Source in Berkeley, California. As part of Lawrence Berkeley National Laboratory, the Advanced Light Source is host to a synchrotron with over 40 beamlines. Over my graduate career at Notre Dame, our beamline proposals have granted us with time on beamlines 2.4 and 5.4 over 8 visits. These specific beamlines are catered to experiments at mid-infrared energies (750 - 5000 cm^{-1}). Specifically, we performed synchrotron infrared nanospectroscopy (SINS) to primarily explore plasmonic/phononic systems. Our results indicate strong coupling between the plasmonic resonances of gold rods with the phononic response of a polar dielectric substrate. Secondary experiments were performed on thin molecular films and monolayers. Results from a limit of detection study suggest that more than a single layer of molecules is needed for adequate detection in SINS.

After, this work switches focus to the conceptualization, construction, and characterization of a home-built spatial modulation spectroscopy (SMS) system. This technique utilizes oscillations of a nanoparticle in and out of the focal plane of a tightly focused laser to measure a signal that is directly proportional to the particles' optical extinction cross section. As nanoparticles decrease in size, the primary component of their extinction becomes increasingly dominated by absorption. Knowledge of the laser's focal parameters (such as its position and beam waist) are crucial to extracting extinction parameters. To measure the beam position and beam waist of our system, a characterization experiment, called the knife edge, was performed. In the knife edge, a double-edged razor blade is placed in the focal plane of the laser to variably obstruct the beam based on its position in the plane. Transmitted laser intensity is measured versus blade position resulting in a sigmoidal response whose fit parameters yield the beam position and beam waist. Results from an initial characterization or our system suggest we are able to focus a helium neon laser close to its theoretical limitation given our currently installed optics. When switching to our optical parametric oscillator source, knife edge results suggest that further work is needed to tighten up the focal spot.

Last, a cumulative look at the overarching outcomes of my projects and my time as a whole at Notre Dame will be explored. My work at the Advanced Light Source shows that SINS has clear utility in the field of plasmonics and phononics, whereas it has a more limited ability to measure thin molecular films. Our limit of detection study further demonstrates that the sensitivity of SINS is highly dependent on the molecule, its functional groups, and its orientation. In an effort to further understand the optical properties of individual nanoparticles, I constructed a SMS system that has undergone numerous characterization experiments. As a whole, these projects all explore light-matter interactions at the nanoscale.