The Plasmon Resonance of a Multilayered Gold Nanoshell and its Potential Bioapplications

Dept. of Electr. Eng., Shanghai Jiao Tong Univ., Shanghai, China
IEEE Transactions on Nanotechnology (Impact Factor: 1.62). 08/2011; DOI: 10.1109/TNANO.2010.2079943
Source: IEEE Xplore

ABSTRACT The optical spectra and near-field enhancement of a multilayered gold nanoshell were theoretically studied in this paper to explore its potential biological applications. The mathematical model was developed within the framework of multipole expansion of a multilayered concentric sphere. Results show that compared with a conventional single-layered Au-SiO2 nanoshell, a multilayered Au-SiO2-Au nanoshell has an advantage of realizing the localized surface plasmon resonance at wavelength of 1300 nm or longer, which is believed to be more beneficial to ultrahigh resolution optical coherent imaging. With single-layered nanoshell, an extremely thin gold layer is required for resonance at long wavelength, and making such thin layer would be almost practically impossible within the current synthesis techniques.

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    ABSTRACT: We present a computational study of the plasmonic response of a gold–silica–gold multilayered nanostructure based on truncated nanocones. Symmetry breaking is introduced by rotating the nanostructure and by offsetting the layers. Nanocones with coaxial multilayers show dipole–dipole Fano resonances with resonance frequencies depending on the polarization of the incident light, which can be changed by rotating the nanostructure. By breaking the axial symmetry, plasmonic modes of distinct angular momenta are strongly mixed, which provide a set of unique and higher order tunable Fano resonances. The plasmonic response of the multilayered nanocones is compared to that of multishell nanostructures with the same volume and the former are discovered to render visible high-order dark modes and to provide sharp tunable Fano resonances. In particular, higher order tunable Fano resonances arising in non-coaxial multilayer nanocones can vary the plasmon lines at various spectral regions simultaneously, which makes these nanostructures greatly suitable for plasmon line shaping both in the extinction and near field spectra.
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    ABSTRACT: The plasmon hybridization theory is widely used to study the plasmon response of metallic nanostructures. In this work, we study the plasmon hybridization picture of the gold–silica–gold multilayer nanoshells from the viewpoint of the optical extinction spectrum and the charge density distribution. We find that reducing the distance between the Au core and the Au shell causes the conversion from |ω–+ to |ω+– modes of the high energy extinction peak. According to our opinion, it is because the increased plasmon interaction between the Au core and the Au shell induces the energy reversion of the |ω–+ and |ω+– plasmon modes. The interesting contrary shift effect of the high energy extinction peaks with different dielectric constants of the middle silica shell and outer surrounding is well-explained by the |ω+– modes. The energy reversion of hybrid plasmon modes we reported would give new insight into the plasmon hybridization theory. Moreover, our study could offer a modified way based on the charge interaction analysis, which is a necessary supplement to the plasmon hybridization theory, for studying plasmon responses in the optical spectrum of metal nanostructures.
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    ABSTRACT: Plasmonic Fano resonances arise in symmetric single-layer conical nanoshells, which can be switched on and off by changing the polarization of the incident electric field. By breaking the symmetry, higher-order dark hybridized modes emerge in the spectrum, which couple to the superradiant bright mode and induce higher-order plasmonic Fano resonances. From a comparison with spherical nanostructures, it comes out that single-layer conical nanoshells are found to be highly capable in the generation of higher-order Fano resonances with larger modulation depths in the optical spectra. Such nanostructures are also found to offer high values of figure of merit and contrast ratio due to which they are highly suitable for biological sensors.
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