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A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance

An Erratum to this article was published on 05 March 2013

This article has been updated


The lack of symmetry between electric and magnetic charges, a fundamental consequence of the small value of the fine-structure constant1, is directly related to the weakness of magnetic effects in optical materials2,3. Properly tailored plasmonic nanoclusters have been proposed recently to induce artificial optical magnetism4,5,6,7 based on the principle that magnetic effects are indistinguishable from specific forms of spatial dispersion of permittivity at optical frequencies1. In a different context, plasmonic Fano resonances have generated a great deal of interest, particularly for use in sensing applications that benefit from sharp spectral features and extreme field localization8,9,10,11,12. In the absence of natural magnetism, optical Fano resonances have so far been based on purely electric effects. In this Letter, we demonstrate that a subwavelength plasmonic metamolecule consisting of four closely spaced gold nanoparticles supports a strong magnetic response coupled to a broad electric resonance. Small structural asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Our findings are supported by excellent agreement with simulations and analytical calculations, and represent an important step towards the quest for artificial magnetism and negative refractive index metamaterials at optical frequencies13,14,15.

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Figure 1: Illustration of the scattering response of plasmonic nanorings.
Figure 2: Experimental set-up and scattering measurements versus simulations demonstrating the presence of a magnetic-based Fano resonance in the asymmetric nanoring.
Figure 3: Theoretical multipolar analysis of the total scattering cross-section (SCS) demonstrating the dominating magnetic dipole mode in the asymmetric nanoring.
Figure 4: Normalized electric and magnetic near-field distributions and induced electric dipole moments at different wavelengths.
Figure 5: Comparison between scattering spectra under different incident polarizations.

Change history

  • 01 February 2013

    In the version of this Letter originally published online, the surname of the fifth author should have read 'Hartsfield'. This error has been corrected in the HTML and PDF versions of the Letter.


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The authors thank Chihhui Wu and Jiming Bao for helpful discussions. This work was supported in part by the US Army Research Laboratory and the US Army Research Office (W911NF-11-1-0447), the National Science Foundation (NSF; DMR-0747822), the Office of Naval Research (ONR; N00014-08-1-0745), the Air force Office of Scientific Research (AFOSR; FA9550-10-1-0022), the Welch Foundation (F-1662), the Alfred P. Sloan Foundation, AFOSR with the Young Investigator Program (YIP) (award no. FA9550-11-1-0009) and an ONR MURI grant (no. N00014-10-1-0942).

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F.S. performed the experiments. F.M. and K.L. conducted the calculations and theoretical modelling. T.H. and X-X.L. facilitated the experimental and theoretical work, respectively. A.A and X.L. supervised the project. All authors discussed the results and commented on the paper.

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Correspondence to Andrea Alù or Xiaoqin Li.

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Shafiei, F., Monticone, F., Le, K. et al. A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance. Nature Nanotech 8, 95–99 (2013).

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