Optical Response Of Nanostructures PDF Download

Plasmonic nanostructures are of great interest due to the broad range of applications from biodetection to metamaterial. The desired optical functionality of these nanostructures can only be realized if the designed geometries and constituent material quality are accurately reproduced experimentally. This dissertation research developed new fabrication methods to create planar and freestanding plasmonic nanostructures, including two-dimensional (2D) planar gold (Au) nanoparticle quasicrystals, one-dimensional (1D) Au nanoparticle arrays, and ring-loaded Au nanoparticle dimer nanoantennas. The measured and modeled optical properties of each type of structure were found to be in strong agreement with one another, thereby confirming the effectiveness of the fabrication approaches in reproducing the designed structure. In Chapter 2, planar 2D plasmonic quasicrystal arrays composed of spherical Au nanoparticles were created by Au-enhanced oxidation of lithographically patterned stacks of evaporated amorphous silicon (a-Si) and Au thin films. In contrast to 2D periodic plasmonic structures, which can be accurately simulated for arbitrarily shaped nanoparticles, computationally efficient models for quasicrystals require spherical particle geometries. Using the process developed in this research, broadband Ammann-Beenker and multiband Penrose plasmonic quasicrystals were fabricated and optically characterized. The measured transmission spectra of the fabricated structures agreed well with simulation, thereby enabling an experimental validation of the modeled interaction between the plasmonic and photonic modes of the two structures. In Chapter 3, freestanding 1D Au nanoparticle arrays encapsulated within a silicon dioxide (SiO2) shell were produced by Au-enhanced oxidation of Au-coated, surface modulated Si nanowires. This lithography-free process overcomes the linear relationship between nanoparticle diameter and interparticle spacing imposed by the Rayleigh instability, and provides accurate and reproducible control of both of these parameters over a wide range of particle diameters and spacings. The modeled optical properties of fabricated 1D arrays were confirmed experimentally by extinction measurements of a randomly oriented ensemble of wires as well as by scanning transmission electron microscopy (STEM) electron energy loss spectra (EELS) and energy filtered transmission electron microscopy (EFTEM) analysis of individual wire arrays. In Chapter 4, a nanoring-loaded dimer nanoantenna was designed to give a multiband optical plasmonic response. The center wavelength and bandwidth of the two bands was varied by modifying the nanoring inner diameter. A top-down process was optimized to reproducibly fabricate the ring-loaded nanoantenna with sub-10 nm wide gaps between the three particles and an inner/outer nanoring diameter of 30nm and 55nm, respectively. Electromagnetic modeling showed that the multi-band response originated from differences in coupling between the nanoring and nanoparticle building blocks for the long- and short-wavelength resonances. The optical response was also understood by modeling the electric/magnetic field and charge distribution of the nanoantennas at the two resonant wavelengths.