Nonlinearities play critical roles in regulating natural processes. Effects like nuclear fission, stimulated emission in lasers, and chaos theory all derive from nonlinearities that govern their natural systems, giving humans perspectives on their origins and impacts of their nonlinearities. Similarly, nonlinear optics can provide interesting insights into investigating and characterizing new materials. In the materials sciences, nonlinearities can be broken up into two categories: incoherent and coherent nonlinearities. I explore how incoherent and coherent nonlinear optical processes reveal the multifaceted behavior of materials at the nanoscale. Furthermore, nonlinearities are strongly tied to the increasing imaging resolutions at the nanoscale. Yet, here, I show that nonlinearities improve resolutions for imaging purposes and provide insights into the local material properties that cannot be visualized with linear optics.
For the first part of the thesis, Chapter 3 explores the uniquely dynamic optical sensitivities of avalanching nanoparticles (ANPs). ANPs recently emerged as promising nanomaterial candidates for local sensing of their environments. However, extreme incoherent nonlinearities of this sort are poorly understood, necessitating a comprehensive study into the origins of nonlinearities and, particularly, heterogeneous nonlinear emissions seen in ANPs. Through systematic investigations and computational fittings of over 300+ single particle studies, ANPs demonstrated extraordinary sensitivities to their local environment, including shell thicknesses, substrate, and ligands. Nanoscale effects perturb the extremely sensitive photon avalanching effect to the point that inhomogeneities during chemical synthesis also manifest into large optical heterogeneities, with avalanching thresholds varying from 4 kW cm⁻² to 12 kW cm⁻². Shell thicknesses of 5.6 nm and greater were found to greatly passivate the ANP from environmental effects, which aligns with what other researchers have demonstrated.
For the latter part of the thesis, Chapters 4 and 5 utilize parametric nonlinearities to investigate material properties of emerging layered nanosheet materials. In Chapter 4, Bismuth tellurohalides, BiTeX, were shown to contain enormous nonlinearities that increase as the fundamental field moves towards the infrared. Quantitatively, BiTeI exhibits effective second-order nonlinearities that rival other nonlinear materials (𝛸⁽²⁾_𝑒𝑓𝑓 ∼ 2 (nm/V)). Furthermore, the second-order nonlinearities measured within BiTeI play outsize roles in allowing the visualization of permanent polar domains that have only been seen with scanning probe techniques. Through rigorous rotational second harmonic generation studies and second harmonic generation microscopy, BiTeI contained rather interesting domain behavior consistent with what experts predict in materials with Ising-type domains.
Chapter 5 investigates Graphullerene, an emerging layered carbon allotrope, for its nonlinear optical properties. The types of disorders that are especially interesting are the second harmonic and third harmonic generation studies revealed in Graphullerene. Although much remains unknown about this new class of carbon materials, second-order nonlinearities confirm the type of strain theoretically predicted in Graphullerene. Furthermore, combined with linear temperature studies of photoluminescence in Graphullerene, vibronics correlate with the vibronics seen in C60 molecules. Nonlinear excitation spectroscopy in the second and third order reveals electronic resonances that correlate with the shoulders and peaks in Graphullerene.
Lastly, the appendices consist of two projects that remain works in progress. Tapping near-field scanning optical microscopy (Tapping NSOM) and magneto-photoluminescence (MPL) represent two cutting-edge techniques I was thankfully involved in developing during my graduate studies. Tapping-NSOM represents a flavor of work where time-correlated NSOM can reveal the temporal dynamics of the probe itself, in addition to the excited state dynamics of a nanomaterial. The MPL spectroscopy was carried out at Los Alamos National Laboratory to investigate ANPs as optical magnetometers. Though much work remains to be completed before I can take these emerging spectroscopic techniques to the next application level, they were formative projects that shaped my experience as a graduate researcher in the spectroscopic sciences.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/bera-ea57 |
| Date | January 2024 |
| Creators | Kwock, Kevin Wen Chi |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
Page generated in 0.0022 seconds