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Quantitatively analyzing the mechanism of giant circular dichroism in extrinsic plasmonic chiral nanostructures by the interplay of electric and magnetic dipoles

The plasmonic chirality has drawn a lot of attention because of the tunable circular dichroism (CD) and the enhancement for the signal of chiral molecules. Different mechanisms have been proposed for explaining the plasmonic CD, however, a quantitative one like ab initio mechanism in chiral molecules is still unavailable. In this work, a mechanism similar to the chiral molecules is analyzed. The giant extrinsic circular dichroism of plasmonic splitting rectangle ring is quantitatively investigated theoretically. The interplay of electric and magnetic modes of the meta-structure is proposed to explain the giant CD. The interplay is analyzed both in an analytical coupled electric-magnetic dipoles model and finite element method model. The surface charge distributions show that the circular current yielded in the splitting rectangle ring makes it behave like a magneton at some resonant modes, which interact with electric modes and results in a mixing of the two kinds of modes. The strong interplay of the two kinds of modes is mainly responsible for the giant CD.The analysis of the chiral near field of the structure shows potential applications in chiral molecule sensing.