Tuning far-field light-matter interactions using three dimensional plasmonic meta-structures
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Tuning far-field light-matter interactions using three dimensional plasmonic meta-structures

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Plasmonic meta-structure paves the way to study and manipulate light both in far and near filed. Achieving invisibility (cloaking) by suppressing scattering from an object using a nanoassembled 3D plasmonic meta-structure is the principal study of this dissertation. The concept of "cloaking" an object is a very attractive one, especially in the visible (VIS) and near infra-red (NIR) regions of the electromagnetic spectrum, as that would reduce the visibility of an object to the eye. One possible route to achieving this goal is by leveraging the plasmonic property of metallic nanoparticles (NPs). In this dissertation a model was developed to simulate light in the VIS and NIR scattered by a core of a homogeneous medium, covered by plasmonic cloak that is a spherical shell composed of gold nanoparticles (AuNPs). To consider realistic, scalable, and robust plasmonic cloaks that are comparable, or larger, in size to the wavelength, a multiscale simulation platform was introduced. This model uses the multiple scattering theory of Foldy and Lax to model interactions of light with AuNPs combined with the method of fundamental solutions to model interactions with the core. Numerical results of the simulations for the scattering cross-sections of core-shell composite indicate significant scattering suppression of up to 50% over a substantial portion of the desired spectral range (400 - 600 nm) for cores as large as 900 nm in diameter by a suitable combination of AuNP sizes and filling fractions of AuNPs in the shell. Suppressing total scattering cross-section by a plasmonic meta-structure effects the angular distribution of the scattered energy both spectrally and spatially. The second project of this dissertation studies the engineering of spatial and spectral profiles applying the plasmonic meta-structures. The possibility of engineering spectral scattering was explored by three-dimensional mesoscale dielectric targets coated with gold nanoparticles (AuNPs) on the surface. By varying AuNP sizes (5-20 nm) and filling fractions of the AuNP coatings (0.1 - 0.3), simulations reveals that under optimal combination of these two parameters, a meta-structure demonstrates reduced or enhanced scattering efficiency compared to the bare core. Furthermore, analysis of the differential scattering cross-section shows that the presence of the AuNP coating alters the angular distribution of scattering by suppressing the angular sidelobes, thereby guiding the scattered power preferentially in the forward direction. The simulated results highlight that with the ability to tune both the spatial and spectral aspects of the scattering profile, these coated structures may serve as a platform for a variety of applications, including passive cloaking and high-resolution imaging. The final part of this dissertation is the experimental realization of nano assembled 3D plasmonic meta-structures following the demonstration of plasmonic cloaking by these structures. These meta-structures were designed based on the simulated results, they are comprised of a dielectric (silica) core coated with randomly distributed AuNPs. Silica surface modified by the suitable amine ligand enabled adsorption of the AuNPs, and electrostatic interactions between AuNPs promoted nanoscale self-assembly, resulted in robust core-shell structures. Furthermore, the meta-structure fabrication process was optimized to achieve the desired surface coverage (> 20%) of AuNPs for varied meta-structure sizes (500 nm, 700 nm). Measured scattering cross-section of bare silica and AuNP coated silica sphere revealed broadband scattering suppression by the plasmonic meta-structures up to 570 nm in the visible spectrum. Simulated and the measured scattering cross-sections of the bare cores and core-shell structures showed a very good agreement confirming the applicability of the multiscale simulation platform to real-world systems.

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