Abstract
Herein we numerically study the excitation angle-dependant far-field and near-field optical properties of vertical plasmonic nanowires arranged in an unconventional linear geometry: Fibonacci number chain. The first five numbers in the Fibonacci series (1, 1, 2, 3, 5) were mapped to the size of gold nanowires, and arranged in a linear chain to study their optical interactions, and compared them to conventional chain of vertical gold nanowires. By harnessing the radiative and evanescent coupling regimes in the geometry, we found a systematic variation in the far-field extinction and near-field confinement in the geometries. Our simulation studies revealed enhanced backscattered intensity in the far-field radiation pattern at excitation angles along the chain-length of Fibonacci geometry, which was otherwise absent for conventional chain of plasmonic nanowires. Such angular reconfiguration of optical fields in unconventional linear geometries can be harnessed for tunable on-chip plasmonics.
Highlights
We asked the following questions: a) whether excitation angle can be used as a variable parameter to tune the far-field extinction spectra of the chosen Fibonacci number chain (FC) and conventional chain (CC) geometries and; b) what will be the far-field radiation pattern as function of excitation angle
We have studied excitation-angle-dependent far-field and near-field optical properties of gold nanowires arranged according to Fibonacci number chain (FC) and compared them to nanowires arranged in conventional chain (CC)
We revealed an enhanced backscattering efficiency for the FC geometry at excitation angles 0 and 180 degrees, which was otherwise absent in the case of CC geometry
Summary
Propagation and confinement of light at subwavelength scale is an important issue in nanophotonics.[1,2,3] Plasmonic nanomaterials of various shapes and sizes made of gold, silver and other noble metals have shown capabilities to facilitate surface plasmon polaritons and localized surface plasmons.[4,5,6] In addition to individual shape and size of plasmonic nanostructure, their geometrical arrangement can give rise to collective behaviour that can be further harnessed for propagation and localization of light.[7,8,9,10,11] One such geometrical arrangement which has captured attention in recent times is the linear chain of plasmon nanostructures.[11,12,13,14] The metallic nanoparticle chains made of gold or silver can sustain plasmon modes that can laterally squeeze the associated near-fields due to strong dipole-dipole interaction between the nanoparticles.[7,8,12,14,15,16] In addition to this, far-field interference effects[8] can be tuned by choosing appropriate size and gap between the periodic nanostructures. The electromagnetic transport and coupling capabilities of linear chains of metallic nanostructures have been extensive studied.[7,8,10,12,13,14,15,17,18,19,20,21,22] It has been shown that the far-field and near-field optical effects in such geometries can be harnessed as tunable photonic elements at sub-wavelength scales. We have shown[30,31] that certain principles of deterministic inhomogeneity in the size and arrangement of plasmonic nanostructures can be harnessed to obtain unconventional array of nanophotonic elements. The key idea is to use a mathematical principle that can be adapted to the geometrical arrangement of plasmonic nanostructures and further utilize their unconventional optical properties
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