The Faraday rotation spectrum of composites containing magnetite nanoparticles is found to be dependent on the interparticle spacing of the constituent nanoparticles. The composite materials are prepared by combining chemicallysynthesized Fe3O4 (magnetite) nanoparticles (8 nm diameter) and poly(methylmethacrylate) (PMMA). Composites are made containing a range of nanoparticle concentrations. The peak of the main spectral feature depends on nanoparticle concentration; this peak is observed to shift from approximately 470 nm for (dilute composites) to 560 nm (concentrated). A theory is presented based on the dipole approximation which accounts for optical coupling between magnetite particles. Qualitative correlations between theoretical calculations and experimental data suggest the shifts in spectral peak position depend on both interparticle distance and geometrical configuration.

Noble metal nanoparticles are of great interest due to their tunable optical and radiative properties. The specific wavelength of light at which the localized surface plasmon resonance occurs is dependent upon the shape, size and composition of the particle as well as the dielectric constant of the host medium. Thus, the optical properties of noble metal nanoparticles can be systematically tuned by altering these specific parameters. The purpose of this thesis is to investigate some of these properties related to metallic nanoparticles. The first several chapters focus on theoretical modeling to predict and explain various plasmonic properties of gold and silver nanoparticles while the later chapters focus on more accurately combining experimental and theoretical methods to explain the plasmonic properties of hollow gold nanoparticles of various shapes. The appendix contains a detailed description of the theoretical methods used throughout the thesis. It is intended to serve as a guide such that a user could carry out the various types of calculations discussed in this thesis simply by reading this appendix.

Doctor of Philosophy / Department of Physics / Christopher M. Sorensen / This work presents a microphysical model of the classical interaction of electromagnetic waves with arbitrary single and multiple particles. The model is based on the volume integral equation solution to the macroscopic time-harmonic Maxwell equations. The integral is discretized over a particle's volume. The near and far-field scattered wave is then described by the secondary radiation from the discretized elements. The physical origin of the angular structure of the scattered wave is characterized by the superposition of these secondary waves. A graphical technique is developed to visualize how this superposition relates to the physical features of a particle, e.g., its size, shape, and refractive index. Numerical and analytical implementations of the model are presented for spherical and spheroidal particles and fractal-like spherical-particle aggregates. The connection between the reflection symmetry of a particle and the polarization state of its far-field scattered wave is illustrated. The model is used to explain the cause of the angular power-law patterns in a particle's scattered intensity. An analysis of the internal field distribution in fractal-like aggregates is performed and the results are compared to the Rayleigh-Debye-Gans theory. Extinction and the optical theorem are examined within the context of the model, resulting in a new understanding for the physical mechanism causing extinction and implications regarding its measurement. The culmination of this work is the unification of multiple scattering-concepts, often regarded as distinct, and the resulting insight afforded by the unified microphysical picture. This unified view is shown to reveal a new and simple explanation for the famous extinction paradox.

Plasmonic nanoparticles support surface plasmon resonances that are sensitive to the environment. Factors contributing to the refractive index sensitivity are explored systematically through simulation, theory, and experiment. Particles small with respect to the wavelength of light and with size parameters much less than 1 have optical properties accurately predicted by quasi-electrostatic theory while particles with larger size parameters necessitate electrodynamics. A theory is developed that captures the effects of geometry on the refractive index sensitivity with a single factor, plasmon band location, and, although based on electrostatic theory, well predicts the sensitivity of particles whose properties are beyond the electrostatic limit. This theory is validated by high quality simulations for compact particles with shape parameters approaching 1 and, therefore, electrodynamic in nature, as well as higher aspect ratio particles that are electrostatic. Experimentally observed optical spectra for nanorods immobilized on glass and subjected to changes in n of the medium are used to calculate the sensitivity of the particles, found to be well matched by a variation on the homogeneous plasmon band theory. The separate electrostatic and electrodynamic components of plasmon band width, are explored and the overall width is found to affect the observability of the aforementioned sensitivity similarly within each particle class. The extent of the sensing volume around a spherical particle is explored and found to vary with particle size for small particles. Through simulation of oriented dielectric layers, it is shown particles are most sensitive to material located in regions of highest field enhancement. Variations on seed-mediated growth of gold nanorods results in spectra exhibiting a middle peak, intermediate to the generally accepted longitudinal and transverse modes. Simulated optical properties and calculated field enhancement illustrates the correlation between geometry and optical properties and allows for identification of the middle peak. / Dissertation

In this study, optical behavior of metal nanoparticles having dimensions smaller than the wavelength of visible light is studied experimentally and numerically. Gold (Au) and silver (Ag) nanoparticles are studied due to their superior optical properties when compared to other metals. A compact code based on Discrete Dipole Approximation (DDA) is developed to compute extinction efficiencies of nanoparticles with various different properties such as material, dimension and geometry. To obtain self consistent nanoparticle arrays with well defined geometries and dimensions, Electron Beam Lithography (EBL) technique is mainly used as the manufacturing method. Dose parameters required to produce nanoparticles with dimensions down to 50 nm over substrates with different electrical conductivities are determined. Beam current is found to affect the doseV size relation. The use of thin Au films as antistatic layer for e-beam patterning over insulating substrates is considered and production steps, involving instabilities due to contaminants introduced to the system during additional removal steps, are clarified. 4 nm thick Au layer is found to provide sufficient conductivity for e-beam patterning over insulating substrates. An optical setup capable of performing transmittance and reflectance measurements of samples having small areas patterned with EBL is designed. Sizes of the metal nanoparticles are determined by scanning electron microscope (SEM) and spectral data obtained using the optical setup is analyzed to find out the parameters affecting the localized surface plasmon resonances (LSPR). Arrays of particles with diameters between 50 &ndash / 200 nm are produced and optically analyzed. Size and shape of the nanoparticles are found to affect the resonance behavior. Furthermore, lattice constants of the particle arrays and surrounding medium are also shown to influence the reflectance spectra. Axes with different lengths in ellipsoidal nanoparticles are observed to cause distinguishable resonance peaks when illuminated with polarized light. Peak intensities obtained from both polarizations are observed to decrease under unpolarized illumination. Binary systems consisting of nanosized particles and holes provided better contrast for transmitted light.

Optically active helical microparticles are studied in the forms of thin films, suspensions and powders. From fabricated helical porous thin films, microparticle suspensions are obtained by removing the microparticles (film columns) from their substrates and dispersing them into water. For removing microparticles, four methods are explored and compared: sacrificial NaCl layer, gold (Au) layer, buffered oxide etching, and direct ultrasonic agitation. The primary film material studied in this work is amorphous silicon (Si). Physical morphology of the microparticles is examined with scanning electron microscopy (SEM). Methods employed to characterize optical activity of the microparticles include: polarimetry, spectrophotometry, and spectroscopic ellipsometry (SE). The produced chiral microparticles exhibit optical activity: optical rotation (OR) and circular dichroism (CD - in the form of differential circular transmission (DCT)). Significant findings include: (a) we observe the largest optical rotatory power ever reported in scientific literature, 11◦/μm at 610 nm wavelength for a Si film; (b) for the helical thin films, there is one dominant DCT band in the measured wavelength range; however for microparticle suspensions and powders, there exist two DCT bands: one broad band at long wavelengths, and one narrow band in the short wavelength range; compared to their thin film forms, microparticle suspensions and powders have inverted sign for the broad DCT band. A discrete dipole approximation (DDA) model is employed to calculate optical response (e.g. extinction, scattering, and absorption cross-sections) of the microparticles, so as to enable us to understand the effects of different structural parameters of the microparticles on their optical response. Calculation confirms that optical activity of chiral microparticles is due to coherent light scatterings with the chiral structures of the particles. The inversion in sign of the broad DCT bands of microparticle suspensions and powders is likely due to the averaging effect from random orientation of the helical microparticles, as is indicated both from experimental results and from calculation. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-07-06 09:59:20.751

Metallic films, in conjunction with biochemical-targeted probes, are expected to provide early diagnosis, targeted therapy and non-invasive monitoring for epidemiology applications [1-4]. The resonance wavelength peaks, both plasmonic and Wood-Rayleigh Anomalies (WRAs), in the scattering spectra are affected by the metallic architecture. As of today, much research has been devoted to extinction efficiency in the plasmonic region. However, Wood Rayleigh Anomalies (WRAs) typically occur at wavelengths associated with the periodic distance of the structures. A significant number of papers have already focused on the plasmonic region of the visible spectrum, but a less explored area of research was presented here; the desired resonance wavelength region was 400-500nm, corresponding to the WRA for the silver film with perforated hole with a periodic distance of 400nm. Simulations obtained from the discrete dipole approximation (DDA) method, show sharp spectral bands (either high or low scattering efficiencies) in both wavelength regions of the visible spectrum simulated from Ag film with cylindrical hole arrays. In addition, surprising results were obtained in the parallel scattering spectra, where the electric field is contained in the XY plane, when the angle between the metallic surface and the incident light was adjusted to 14 degrees; a bathochromic shift was observed for the WRA peak suggesting a hybrid resonance mode. Metallic films have the potential to be used in instrumental techniques for use as sensors, i.e. surface plasmon resonance affinity biosensors, but are not limited to such instrumental techniques. Although the research here was aimed towards affinity biosensors, other sensory designs can benefit from the optimized Ag film motifs. The intent of the study was to elucidate metal film motifs, when incorporated into instrumental analysis, allowing the quantification of genetic material in the visible region. Any research group that routinely benefits from quantification of various analytes in solution matrices will also benefit from this study, as there are a bewildering number of instrumental sensory methods and setups available.

Ce travail concerne la modélisation, à l’échelle moléculaire, de l’interaction entre des nanoparticules carbonées et le rayonnement électromagnétique. Le but est d’aider à la compréhension des propriétés optiques des particules de suie afin de mieux quantifier l’influence des suies sur l’atmosphère et le climat. L’étude de l’interaction rayonnement/particules de suie fraîche a été effectuée par la méthode PDI ; il a été montré que : i) le coefficient d’absorption massique (MAC) des particules de suie dépend de la répartition des atomes dans la particule et de leurs liaisons, en particulier entre 200 et 350 nm ; ii) le MAC diffère selon que le cœur de la particule carbonée est occupé ou non par des plans graphitiques ; iii) un modèle analytique n’est pas adapté pour calculer le MAC d’une nanoparticule carbonée présentant des défauts structuraux. De plus, des méthodes de chimie quantique ont été utilisées pour caractériser le vieillissement des suies. Les résultats montrent que : i) NO, Cl, et HCl sont physisorbées sur une surface carbonée parfaite alors que sur une surface défective, ces espèces sont chimisorbées et conduisent à une modification de la surface ; ii) la présence de Cl conduit à un piégeage fort des molécules d’eau supérieur à celui obtenu lorsqu’un site oxygéné est présent sur la surface carbonée, expliquant ainsi le caractère hydrophile des suies émises lors d’incendies dans des milieux industriels. Enfin, la méthode PDI a été appliquée au calcul de la polarisabilité de HAP afin d’interpréter des spectres d’absorption des grains carbonés du milieu interstellaire, en incluant des molécules pour lesquelles aucune donnée n’était actuellement disponible. / This work concerns the modeling, at the molecular level, of the interaction between carbonaceous particles of nanometric size and the electromagnetic radiation. The goal is to improve our understanding of the optical properties of soot particles, to better quantify the influence of soot on the atmosphere and on climate change. The study of the interaction between radiation and fresh soot particles was carried out using the point dipole interaction method; it has been shown that: i) the mass absorption coefficient (MAC) of these soot nanoparticles may significantly depend on their atomistic details, especially between 200 and 350 nm; ii) the MAC depends on whether the heart of the carbonaceous particle is occupied or not by graphite planes; iii) an analytical model is not suitable for calculating the MAC of carbonaceous nanoparticles having structural defects. In addition, quantum chemical methods have been used to characterize the ageing of soot. The results obtained are i) NO, Cl, and HCl are physisorbed on a perfect carbonaceous surface whereas on a defective surface, these species are chemisorbed and lead to a modification of the surface; ii) on a carbonaceous surface, the presence of adsorbed Cl atoms leads to a strong trapping of the surrounding water molecules. This may be related to the highly hydrophilic nature of soot emitted during fires in industrial environments. Finally, the PDI method was applied to calculate the polarizability of PAHs to help at interpreting the absorption spectra of carbonaceous grains in the interstellar medium, including molecules for which no data was currently available.