Plasmonic Toroidal Response at Optical Frequencies
Date Issued
2012
Date
2012
Author(s)
Huang, Yao-Wei
Abstract
Toroidal shapes are often found in bio-molecules, viruses and proteins, but only recently it was proved experimentally that toroidal structures can support exotic high-frequency electromagnetic excitations that are neither electric or magnetic multipoles. Such excitations, known as toroidal moments, could be playing an important role in inter-molecular interaction and directive energy transfer on the molecular level, but are weekly coupled to free space and are difficult to observe. Whether or not the toroidal dipoles will resonate at higher frequencies remains to be answered, since Joule loss of metals is higher at optical frequencies.
In this paper, we present two new related classes of plasmonic metamolecules composed of purposely arranged of four U-shaped split-ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. First, the toroidal and magnetic responses were investigated by the finite-element simulations. A phenomenon of reversed toroidal responses at higher and lower resonant energy has also been reported between this two related metamaterials which results from the electric and magnetic dipoles interaction. We propose a physical model based on coupled LC circuits to quantitatively analyze the inter-molecular interaction system of the plasmonic toroidal metamaterials. Using a model metamaterial system we show that coupling optical gain medium with toroidal molecules can enhance the single pass amplification on up to 65 dB of the toroidal resonance frequency. This offers an opportunity of creating the “lasing spaser” a source of coherent optical radiation that is fueled by toroidal plasmonic oscillations in the nanostructure. Finally, we experimentally demonstrate the toroidal dipolar resonance at optical frequencies in the U-shaped metamolecules, which are manufactured using a double exposure e-beam lithographic process.
Subjects
Metamaterials
Toroidal multipoles
Surface plasmons
Plasmonics
Optical resonators
Split-ring resonators
Type
thesis
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