Mie and Finite-Element Simulations of the Optical and Plasmonic Properties of Micro- and Nanostructures
Hu, Ying Samuel
Drezek, Rebekah A.
Doctor of Philosophy
A Mie-based code is developed for multilayer concentric spheres. The code is used in conjunction with a finite-element package to investigate the plasmonic and optical properties of micro- and nanostructures. For plasmonic nanostructures, gold-silica-gold multilayer nanoshells are computationally investigated. A plasmon hybridization theory is used to interpret the optical tunability. The interaction between the plasmon modes on the inner core and the outer shell results in dual resonances. The low-energy dipole mode is red-shifted by reducing the spacing ( i.e. , the intermediate silica layer) between the core and the shell. This extra tunability allows the plasmon resonance of a multilayer nanoshell to be tuned to the near-infrared region from a visible silica-gold nanoshell whose gold shell cannot be further reduced in thickness. For multilayer nanoshells with reduced geometrical symmetry ( i.e. , the inner core is offset from the center), modes of different orders interact. The mixed interaction introduces the dipolar (bright) characteristic into the higher-order (dark) modes and improves their coupling efficiency to the excitation light. The excitation of the dark modes attenuates and red-shifts the dipole mode and gives it higher-order characteristics. For non-plasmonic structures, simulations have demonstrated that multilayered structures can either reduce or enhance the scattering of light. By adding an anti-reflection layer to as microsphere made of a high-index material, the scattering force can be dramatically reduced. The reduced scattering allows optical trapping of high-index particles. Additionally, the improved trapping is not largely sensitive to the refractive index or the thickness of the coating. The technique has the practical potential to lower the requirement on the numerical aperture of the microscope objectives, making possible the integration of the imaging and optical trapping systems. While the anti-reflection coating reduces scattering, the photothermal bubble (PTB) generated by gold nanoparticles by and large enhances the scattering of light. Transient PTBs are generated by super-heating gold nanoparticles with short laser pulses. Mie-based simulations predict that the scattering of PTBs strongly depends on the transient environment immediately surrounding the nanoparticles. A scattering enhancement of two-to-four orders of magnitude from PBT is demonstrated from both calculations and experiments. Lastly, the near-field coupling between different plasmonie structures for surface-enhanced Raman scattering is investigated. A gold-coated silicon-germanium nanocone substrate has been fabricated and characterized. Finite-element simulations reveal that individual nanocones generate strong tip enhancement with axially polarized light ( i.e. , light polarized along the vertical axis of the nanocone) while the enhancement from transversely polarized light ( i.e. , light polarized in the plane of the substrate) is relatively weak. By simply filling the valleys between nanocones with plasmonic gold nanoparticles, the performance of the substrate is improved with in-plane excitation. Simulations reveal strong coupling between nanoparticles and adjacent nanocones with transverse exactions. An over one order-of-magnitude improvement has been experimentally observed.