The geometry of a plasmonic nanostructure determines the charge-current distributions of its localized surface plasmon resonances (LSPR), thereby determining the device’s interactions with external electromagnetic fields. To target specific applications, we manipulate the nanostructure geometry to create different electromagnetic multipole moments, from basic electric and magnetic dipoles to more exotic higher order and toroidal multipoles. The nanoscale nature of the resonance phenomena makes electron beam spectromicroscopy techniques uniquely suited to probe LSPRs over a wide spectral range, with nanoscale spatial resolution. We use electron energy loss spectroscopy (EELS) in a monochromated scanning transmission electron microscope and cathodoluminescence spectroscopy (CL) in a scanning electron microscope to probe the near-field and far-field properties of LSPR. Electric dipoles within triangular prisms and apertures in Sierpiński fractals couple as the generation number is advanced, creating predictable spectral bands from hybridized dipole modes of parent generations with hierarchical patterns of high field intensity, as visualized in EELS. A magnetic dipole moment is engineered using a vertical split ring resonator (VSRR), pushing the limits of nanofabrication techniques. On this nanostructure we demonstrate the calculation of spatially resolved Stokes parameters on the emission of the magnetic dipole mode and a series of coupled rim modes. Coupling of the magnetic dipole mode of four VSRRs in a circular array creates an LSPR mode supporting the lesser-known toroidal dipole moment. We further probe the near-field configuration of this 3D array through tilting under the electron beam in EELS, and the far-field emission through CL of higher order rim modes. We also propose further configurations of five and six VSRRs to strengthen the toroidal dipole moment. All of the data presented herein was analyzed using custom Python code, which provides a unique graphical interface to 3D spectromicroscopy datasets, and a parallelized implementation of the Richardson-Lucy deconvolution algorithm. / Thesis / Doctor of Philosophy (PhD) / Certain types of metallic particles are capable of trapping light on a scale far below that which we can see; their light-trapping properties depend on their material and on their geometry. Using these tiny particles, we can manipulate the behaviour of light with greater freedom than is otherwise possible. In this thesis, we study how we can engineer the geometry of these particles to give predictable responses that can then be targeted towards specific applications. We study a fractal structure with predictable self-similar responses useful for high sensitivity detection of disease or hormone biomarkers; a resonating structure emulating a magnetic response which can be used in the design of unique new materials capable of bending light backwards and cloaking objects from sight; and a combination of these resonators in an array to demonstrate exotic electromagnetic behaviour still on the limit of our understanding.

This dissertation is about fabrication and functionalization of metal nanoparticles for use in plasmonic and nonlinear optical (NLO) applications. In the first two chapters, I describe a series of experiments, where I combined silver nanoparticles fabricated by nanosphere lithography with ionic self-assembled multilayer (ISAM) films, tuning the geometry of the particles to make their plasmonic resonances overlap with the frequency of optical excitation. The designed hybrid metallic/organic nanostructures exhibited large enhancements of the efficiency of second harmonic generation (SHG) compared to conventional ISAM films, causing a modified film with just 3 bilayers to be optically equivalent to a conventional 700-1000 bilayer film. SHG responses from Ag nanoparticle-decorated hybrid-covalent ISAM (HCISAM) films were investigated as the next logical step towards high-Ï ²⁺ ISAM films. I found that the plasmonic enhancement primarily stems from interface SHG. Interface effects were characterized by direct comparison of SHG signals from PAH/PCBS ISAM films and PAH/PB HCISAM films. Though interface &chi²⁺ is substantially smaller in PAH/PCBS than in PAH/PB, plasmonically enhanced PAH/PCBS films exhibit stronger NLO response. I propose that the structure of PAH/PB film makes its interface more susceptible to disruptions in the nanoparticle deposition process, which explains our observations. During the fabrication of monolayer crystals for nanosphere lithography, I developed a variation of the technique of convective self-assembly, where the drying meniscus is restricted by a straight-edge located approximately 100 μM above the substrate adjacent to the drying zone. This technique can yield colloidal crystals at roughly twice the growth rate compared to the standard technique. I attribute this to different evaporation rates in the thin wet films in the two cases. I also found that the crystal growth rate depends strongly on the ambient relative humidity. Finally, dithiocarbamate (DTC)-grafted polymers were synthesized and employed to functionalize surfaces of Au nanopartciles. PAH-DTC shows greater stability in different environments than PEI-DTC. I also investigated the stability of PAH-DTC coated particles in suspensions with UV-Vis spectroscopy and autotitration. The covalently bonded PAH-DTC enhances the colloidal stability of the Au nanoparticles and enables subsequent ISAM film deposition onto the particles. / Ph. D.

A study is undertaken of the numerical reconstruction of acoustic and electromagnetic (EM) localized waves (LWs). The latter are carrier-free ultra-wideband pulses characterized by large focusing depths and extended ranges of localization. Special emphasis is placed on finite energy LWs that can be generated by dynamic aperture antennas with independently addressable elements. The reconstruction techniques are based on Huygens and Rayleigh-Sommerfeld integral I and II representations, both in the time and frequency domains. In contradistinction to the Weyl representation,they lend themselves to the physical realization of space-time aperture sources capable of generating localized wave solutions propagating away from the aperture plane. A detailed comparison of the three reconstruction techniques has been carried out in connection with LW solutions to the scalar wave equation, especially with respect to their handling of acausal components incorporated in the aperture excitation fields. In addition, a study is presented of the characteristic properties of LWs propagating through dispersive media modeled by the Klein-Gordon equation. It is demonstrated that contrary to expectation, the depletion of the spectral components of the LW Klein-Gordon field may be slower than that associated with the free space scalar field. Previous work by Power et al. [73] is extended by studying the acoustic bistatic scattering of a modified power spectrum (MPS) pulse from rigid and compressible spheres. The analysis allows the extraction of the radius of a sphere from the backscattered data. Finally, a special class of electromagnetic (EM) LWs, referred to as azimuthally polarized X waves (APXWs), is derived and their reconstruction is addressed, both in the time and frequency domains. / Ph. D.

Optical studies of heavily-doped GaAs:C grown at low temperature by molecular beam epitaxy were performed using room-temperature photoluminescence, infrared transmission, and Raman scattering measurements. The photoluminescence experiments show that in LT-GaAs:C films grown at temperatures below 400 °C, nonradiative recombination processes dominate and photoluminescence is quenched. When the growth temperature exceeds 400 °C, band-to-band photoluminescence emission appears. We conclude that the films change in character from LT-GaAs:C to normal GaAs:C once the growth temperature reaches 400 °C. Annealing, however, shows a different behavior. Once grown as LT-GaAs:C, this material retains its nonconducting nonluminescing LT characteristics even when annealed at 600 °C. The Raman-scattering measurements showed that the growth temperature and the doping concentration influence the position, broadening, and asymmetry of the longitudinal-optical phonon Raman line. We attribute these effects to changes in the concentration of interstitial carbon in the films. Also, the shift of the Raman line was used to estimate the concentration of arsenic-antisite defects in undoped LT-GaAs. The infrared transmission measurements on the carbon-doped material showed that only a fraction of the carbon atoms occupy arsenic sites, that this fraction increases as the growth temperature increases, and that it reaches about 100% once the growth temperature reaches 400 °C. The details of all these measurements are discussed. Infrared transmission and photoluminescence measurements were also carried out on heavily-doped GaAs:C films grown by molecular beam epitaxy at the standard 600 C temperature. The infrared results reveal, for dopings under 5 x 10⁹ cm⁻³, a linear relation between doping concentration and the integrated optical absorption of the carbon localized-vibrational-mode band. At higher dopings, the LVM integrated absorption saturates. Formation of C_As-C_As clusters is proposed as the mechanism of the saturation. The photoluminescence spectra were successfully analyzed with a simple model assuming thermalization of photoelectrons to the bottom of the conduction band and indirect-transition recombination with holes populating the degenerately doped valence band. The analysis yields the bandgap reduction and the Fermi-level-depth increase at high doping. / Ph. D.

Work-related musculoskeletal disorders (WMSDs) are prevalent in the workplace, and epidemiology studies show that these problems do not tend to diminish. While the use of new and advanced technology has substantially reduced the amount of physical workload, repetitive manual activities are still typically observed in various work settings. Despite their fairly low workload intensity, prolonged repetitive tasks have been associated with the development of musculoskeletal complaints and problems. Research on localized muscle fatigue (LMF) has been viewed as a viable endeavor toward understanding the processes and mechanisms associated with WMSDs. A mounting of evidence on local fatigue during sustained static work has been presented, but much less is known with respect to muscle fatigue during more complex activities. A study was conducted with the primary objectives of determining the repeatability of several commonly used fatigue measures, and to evaluate the presence of long-lasting effects of fatigue from different recovery periods. Based on low-level intermittent arm abductions, findings from this study demonstrated that the use of perceptions of muscular discomfort and muscle strength as fatigue measures was satisfactory. In contrast, electromyography (EMG)-based measures were characterized by a fairly low repeatability. The study also suggested that, whenever practical, two days of recovery should be allotted in studies involving multiple exposures to fatiguing protocols. Long lasting effects of fatigue could be present when shorter amounts of recovery period were assigned. A second study was also carried out to investigate the effects of work parameters (force-level, work-rest ratio, and work cycle) on muscular fatigue during intermittent static efforts. It was suggested that work conditions with muscular contraction level less than 12% MVE was non-fatiguing, irrespective of the values of the work parameters selected. Intermittent work with higher levels of muscle contraction might be acceptable, but it was dependent upon interactions of the other two parameters. The effects of dynamic work conditions on muscle fatigue were investigated in another study. Findings from this third study suggested that muscles responded differently under dynamic conditions and the use of typical EMG measures (dynamic EMG) could be less sensitive. This study further demonstrated that fatigue evaluations during such conditions were difficult, and only a limited number of EMG-based measures could be potentially employed. / Ph. D.

This study is a subset of a larger body of research that examined shoulder time to fatigue during overhead work in an attempt to reduce the prevalence and impact of work-related musculoskeletal problems in the shoulder associated with overhead work, particularly during automobile assembly. Existing evidence suggests that shoulder injuries are diverse in terms of tissues affected and symptoms presented. Furthermore, the cause of these injuries is multifactorial. The work presented here assumes that musculoskeletal injuries of the shoulder mechanism are at least related to, if not caused by, fatigue localized to the shoulder musculature. While the exact relationship between fatigue and injury has not been clearly established, there is consensus among researchers that fatigue plays and important role. Muscular fatigue, therefore, is viewed as a surrogate measure of risk, and task design to avoid fatigue is seen as a rational method to minimize this risk. An experiment to determine the effects of tool weight on shoulder fatigue and performance during overhead work with work/rest cycles was performed. Times to fatigue were derived based on dependent measures including total task duration, controlled maximum muscle contractions, subjective ratings based on Borg's CR-10 RPE scale, electromyogram behavior (MdPF), and hand force performance measures. Experimental findings indicated that duty cycle (percentage of total task cycle time spent working) significantly affected task duration (p<0.0001), changes in maximum voluntary contraction values for the infraspinatus (p<0.05), and the minimum time for any shoulder muscle to fatigue as determined by changes in the EMG power spectrum (p<0.05). Time to fatigue for the mid deltoid as determined by changes in the median frequency of the EMG power spectrum was shown to change significantly (p<0.05) with change in tool weight. Large intersubject variation was observed for the dependent measures, which showed subjects experiencing different levels of fatigue while performing the same task. Limitations of the study and recommendations for future direction are also discussed. / Master of Science

Nanoscale materials have great applications in many areas. One of these applications is for manufacturing ultra-compact and efficient sensors for chemical and biological molecule detection. Noble metals, such as gold (Au) and silver (Ag), because of their distinguished optical property"localized surface plasmon resonances (LSPRs) that exhibit low loss, are ideal materials to fabricate these nanoscale plasmonic particles or structures. This work addresses the synthesis, characterization, and sensing applications of Au and Ag nanoparticles (NPs). The progress on certain subjects related to our work"NP synthesis, surface functionalization, Au sphere-film structure and two-photon fluorescence"are reviewed in Chapter 1. We also show the calculation results of LSPRs of Au nanosphere suspensions using Mie theory. The measured extinction spectra of Au nanosphere suspensions agree with the calculated results very well. Chapter 2 is a chapter describing the chemical synthesis of a variety of NPs, such as Ag prisms and cubes, Au spheres, rods, and bipyramids. These experiments involved different synthetic mechanisms and methods which enabled us to prepare NPs with desired shapes and optical properties. To put these NPs into application, it is desirable and sometimes necessary to functionalize their surfaces. In Chapter 3, we present the functionalization of Ag cubes with poly(allylamine hydrochloride) (PAH) and poly(allylamine hydrochloride)-dithiocarbamate (PAH-DTC), which follows our previous work on Au NPs. The purpose of studying Ag instead of Au is to use the stronger plasmonic enhancement in Ag when applied to two-photon imaging applications. However, we found that PAH-DTC shrank the Ag cubes. We also functionalized the cationic hexadecyltrimethylammonium bromide (CTAB)-stabilized Au NRs with anionic poly(sodium 4-styrenesulfonate) (PSS). Coated with the strong polyelectrolyte PSS, the NRs become more manageable and can be stable for over six months and are easily immobilized onto positively charged substrate. We put PSS-functionalized Au NPs into use and studied their adsorption process onto PAH-coated optical fiber tapers by monitoring the transmission light through the fiber. When the diameter of the fiber taper gets smaller, stronger coupling occurred between transmitted light inside the taper and the Au NPs on the taper surface (cylinder). This coupling resulted in a loss of the guided light at the plasmon resonance wavelength of the NPs. By monitoring this loss, we can study the adsorption rate of Au NPs onto the fiber. In Chapter 4, we used Au nanospheres to study the adsorption rate on substrates with different curvatures. We also established a theoretical model to explain this phenomenon for cylindrical surface as well as planar and spherical surfaces. Our results fit well with the theory, which predicts that particle adsorption rates depend strongly on surface geometry, and can exceed the planar surface deposition rate by over two orders of magnitude when the diffusion length of the particle is large compared to the surface curvature. In Chapter 5, we studied the optical properties of Au nanospheres separated from a thick Au film by a polyelectrolyte multilayer (PEM) film assembled from PAH and PSS under specific pH condition. The PEM film undergoes swelling and shrinking when the environmental pH is changed as a result of charging and discharging of the polyelectrolytes. Therefore, the PEM film provides an efficient means to tune the distance between Au spheres and Au film. The extinction peak blue-shifted as much as 100 nm when the pH of the water changed from pH 10 to pH 3 for 100 nm diameter Au spheres on a PEM film assembled at pH 9.5. Our preliminary estimates that the gap between sphere and surface can be as small as a few nm even though the film itself is tens of nm thick when it is not constrained by Au spheres. We studied two-photon excitation fluorescence (TPEF) from Ag triangles in Chapter 6. The triangles were fabricated by nanosphere lithography, which used convective self-assembly to make the nanosphere mask. The LSPRs of the nanotriangles were tuned to be in the 800--900 nm range to match with the Ti:Sapphire pulse laser at 880 nm. We found that certain spots on the fluorescence images gave rise to larger fluorescence intensity than rest of the area. SEM imaging reveals that the unusually bright spots seen on the surface were related to regions where the triangles transformed to spherical particles. The larger intensity is tentatively ascribed to the plasmon resonance of those spherical particles in ~400 nm range. / Ph. D.

A number of issues associated with the generation and applications of localized waves are addressed in this thesis. First, the salient characteristic features of two canonical localized wave solutions to the scalar wave equation are discussed. Second, novel azimuthally polarized focus wave mode-type and X wave-type localized electromagnetic fields are derived using a vector-valued spectral approach. Third, all reported experiments dealing with the generation of localized waves are discussed and a concise report on field depth measurements, together with practical implications, is presented. Fourth, new methods for generating X waves in the microwave frequency regime are proposed. Emphasis is placed on increasing the field depth. The proposed new feed scheme increases the field depth as much as 10 times compared to the experimental results reported by Mugnai et al. [2000]. Two modified reflector systems are introduced for the generation of X waves. The first uses an offset launcher reflector configuration. The second uses a Cassagrain reflector system with an integrated circular slit. Finally, future work on electromagnetic X wave generation by means of independently addressable array elements is discussed. / Master of Science

Human postural control is a complex process, but that is critical to understand in order to reduce the prevalence of occupational falls. Localized muscle fatigue (LMF), altered sensory input, and inter-individual differences (e.g. age and gender) have been shown to influence postural control, and numerous methods have been developed in order to quantify such effects. Recently, methods based on statistical mechanics have become popular, and when applied to center of pressure (COP) data, appear to provide new information regarding the postural control system. This study addresses in particular the stabilogram diffusion and Hurst exponent methods. An existing dataset was employed, in which sway during quiet stance was measured under different visual and surface compliance conditions, among both genders and different age groups, as well as before and after induction of localized muscle fatigue at the ankle, knee, torso, and shoulder. The stabilogram diffusion method determines both short-term and long-term diffusion coefficients, which correspond to open- and closed-loop control of posture, respectively. To do so, a "critical point" (or critical time interval) needs to be determined to distinguish between the two diffusion regions. Several limitations are inherent in existing methods to determine this critical point. To address this, a new algorithm was developed, based on a wavelet transform of COP data. The new algorithm is able to detect local maxima over specified frequency bands within COP data; therefore it can identify postural control mechanisms correspondent to those frequency bands. Results showed that older adults had smaller critical time intervals, and indicating that sway control of older adults was essentially different from young adults. Diffusion coefficients show that among young adults, torso LMF significantly compromised sway stability. In contrast, older adults appeared more resistance to LMF. Similar to earlier work, vision was found to play a crucial role in maintaining sway stability, and that stability was worse under eyes-closed (EC) than eyes-opened (EO) conditions. It was also found that the short-term Hurst exponent was not successful at detecting the effects of LMF on sway stability, likely because of a small sample size. The new critical point identification algorithm was verified to have better sensitivity and reliability than the traditional approach. The new algorithm can be used in future work to aid in the assessment of postural control and the mechanisms underlying this control. / Master of Science

Anisotropic plasmonic noble metallic nanoparticles (APMNs) have received enormous attention due to their distinct geometric features and fascinating physicochemical properties. Owing in large part to their tailored localized surface plasmon resonance (LSPR) and the intensive electromagnetic field at the sharp corners and edges, APMNs are exceptionally well suited for biomedical applications such as biosensing, bioimaging, diagnostics and therapeutics. Although a rich variety of surfactant-assisted colloidal routes have been developed to prepare well-defined APMNs, biomedical applications necessitate tedious and rigorous purification processes for the complete removal of toxic surfactants. In this dissertation, we aim to develop generic bioenabled green synthetic methodologies towards APMNs. By applying a series of thermodynamic, kinetic and seed quality control, a series of APMNs with varied morphologies such as branched nanostars and triangular nanoprisms have been successfully prepared. We first presented the preparation of gold nanostars (Au NSTs) through a two-step approach utilizing a common Good's buffer, HEPES, as a weak reducing agent. Single crystalline Au NSTs with tunable branches up to 30 nm in length were produced and the halide ions rather than the ionic strength played a significant roles on the length of the branches of Au NSTs. Then consensus sequence tetratricopetide repeat (CTPR) proteins with increasing number of repeats were used as model proteins to probe the effects of concentration as well as the protein shape on the morphology and resulting physicochemical properties of plasmonic gold nanoparticles. Since the underlying growth mechanism for the biomimetic synthesis of APMNs remains elusive and controversial, the other objective is to elucidate the molecular interactions between inorganic species and biopolymers during the course of NP evolution. Fluorescent quenching and 2D NMR experiments have confirmed the moderate binding affinity of CTPR to the Au(0) and Au(III). We observed that the initial complexation step between gold ions and CTPR3 is ionic strength dependent. Furthermore, we also found that NPs preferentially interact with the negatively charged face of CTPR3 as observed in 2D NMR. Knowledge of binding behavior between biospecies and metal ions/NPs will facilitate rational deign of proteins for biomimetic synthesis of metallic NPs. A modified seed-mediated synthetic strategy was also developed for the growth of silver nanoprisms with low shape polydispersity, narrow size distribution and tailored plasmonic absorbance. During the seed nucleation step, CTPR proteins are utilized as potent stabilizers to facilitate the formation of planar-twinned Ag seeds. Ag nanoprisms were produced in high yield in a growth solution containing ascorbic acid and CTPR-stabilized Ag seeds. From the time-course UV-Vis and transmission electron microscopy (TEM) studies, we postulate that the growth mechanism is the combination of facet selective lateral growth and thermodynamically driven Ostwald ripening. By incorporation of seeded growth and biomimetic synthesis, gold nanotriangles (Au NTs) with tunable edge length were synthesized via a green chemical route in the presence of the designed CTPR protein, halide anions (Br⁻) and CTPR-stabilized Ag seeds. The well-defined morphologies, tailored plasmonic absorbance from visible-light to the near infrared (NIR) region, colloidal stability and biocompatibility are attributed to the synergistic action of CTPR, halide ions, and CTPR-stabilized Ag seeds. We also ascertained that a vast array of biosustainable materials including negatively charged lignin and cellulose derivatives can serve as both a potent stabilizers and an efficient nanocrystal modifiers to regulate the growth of well-defined Ag nanoprisms using a one-pot or seeded growth strategy. The influential effects of reactants and additives including the concentration of sodium lignosulfonate, H2O2 and NaBH4 were studied in great detail. It implies that appropriate physicochemical properties rather than the specific binding sequence of biomaterials are critical for the shaped-controlled growth of Ag NTs and new synthetic paradigms could be proposed based on these findings. Last but not the least, we have demonstrated the resulting APMNs, particularly, Au NSTs and Ag NTs exhibit remarkable colloidal stability, enhanced SERS performance, making them promising materials for biosensing and photothermal therapy. Since the Ag nanoprisms are susceptible to morphological deformation in the presence of strong oxidant, they also hold great potential for the colorimetric sensing of oxidative metal cation species such as Fe3+, Cr3+, etc. / Ph. D. / When a beam of light impinges on the surface of noble metallic nanoparticle (NP), particularly gold (Au) and silver (Ag), the conduction electrons are excited which induces a collective oscillatory motion, resulting in an intense localized surface plasmon resonance (LSPR) absorbance as well as the amplified localized electromagnetic filed. Owing in large part to the tailored LSPR and the intensive electromagnetic field at the sharp corners and edges, anisotropic plasmonic noble metallic nanoparticles (APMNs) can be utilized to span an array of applications such as biosensing, bioimaging, diagnostics and therapeutics. Although great advancement has been made to prepare well-defined APMNs through versatile surfactant-assisted colloidal methodologies, biomedical applications necessitate tedious and rigorous purification processes for the complete removal of toxic surfactants. To address this ubiquitous challenge, biomimetic and bioinspired green synthesis have been extensively explored to fabricate APMNs under mild and ambient conditions. In this dissertation, we aim to develop generic bioenabled synthetic strategies towards APMNs, particularly, Au nanostars and Au/Ag nanoprisms. Herein, protein mediated shape-selective synthesis of APMNs were presented, in which consensus sequence tetratricopetide repeat (CTPR) proteins and biological Good’s buffers were employed as nanocrystal growth modifiers and mild reducing agents, respectively. The dramatic implications of repeat proteins on the morphological and optical properties of the Au NPs were explicitly discussed. The other objective of this dissertation is to elucidate the molecular interactions between inorganic species and biopolymers to further unravel the underlying growth mechanism during the course of APMNs evolution. By incorporation of seeded growth and biomimetic synthesis, Ag/Au nanotriangles (Au NTs) with tunable edge length were synthesized in the presence of the designed CTPR protein, halide anions (Br⁻) and CTPR-stabilized Ag seeds. The well-defined morphologies, tailored plasmonic absorbance from visible-light to the near infrared (NIR) region, colloidal stability and biocompatibility are attributed to the synergistic action of each components in the synthetic system. Last but not the least, we have demonstrated the resulting NPs exhibit remarkable colloidal stability, mitigated cytotoxicity and surface enhanced Raman spectroscopy (SERS) performance, making them good candidates for biosensing and photothermal therapy. This work might shed light on the roles biomolecules play in green synthesis of APMNs, along with rationalizing the design of biomimetic systems to bridge the gap between the bioenabled technique and traditional colloidal synthesis.