| dc.description.abstract | This work explores the complex plasmonic phenomena generated by Archimedean nanospirals. Arrays of gold nanospirals are fabricated with varying arm spacings and arm widths, as well as winding numbers ranging from 2 to 4π, to explore the spectral response of a non-symmetric, multiply reciprocating geometry. Scanning electron micrographs of nanospirals within these arrays, as well as graphically generated nanospirals, are then computationally simulated using finite-difference time-domain algorithms to explore the influence of the geometry on the organization of electric near-field enhancements. A spatially and spectrally logical organization arises from the increased control of the enhanced electric near-fields provided by this complex geometry. Unlike simple geometries, whose response is defined by resonant coupling with the driving electromagnetic field and is multipolar in nature, the nanospiral generates numerous resonances with multiple unique configurations of the near-field enhancements ordered by the collective geometry. These patterns can be classified into one of three characteristic configurations: the hourglass, the focusing, and the standing-wave. These features are qualitatively analyzed and quantitative methods are developed to define and analyze individual configurations.
Within these categories, quasi-static organizations of the near-field enhancements, referred to as elements, are preserved as the spectral resonances are shifted by tuning the winding number. The elements constitute a set of tunable tools whose spectral and spatial features can be tailored to a wider range of desired organizations. The effects of circular polarizations are also discussed to further illustrate the complexity of the plasmonic organization and present a new dimension of optical tunability of the near-field structure using polarization modulation. The nanospiral system presents a significant advancement in the control of the near-field organization and spectral response by collecting previously unobserved plasmonic behaviors, logical tunability and unique polarization dependence into a single nanoparticle geometry. | |
