Multipole Analysis of the Extinction Cross Section and Circular Dichroism of Chiral Metamolecules with Optical Theorem

Plasmonic chirality of metamolecules attracts increasing attention because of the tunable circular dichroism (CD) and near-field enhancement for chiral molecule detection. Although various mechanisms have been proposed to explain the plasmonic CD, the quantitative analysis of particles with sub-wavelength dimensions remains to be refined. Unlike biomolecules and small particles for which only absorption effects are considered, both multipolar scattering and absorption of sub-wavelength metamolecules need to be analyzed. Here, the extinction and CD of chiral metamolecules are experimentally measured, and the contribution of excited multipole moments is theoretically investigated via combining multipole scattering and optical theorem. It is found that for the low-order resonance mode, the absorption dominates the CD signal. While for high-order resonance modes, both scattering and absorption significantly influence the CD, and the high-order electric quadrupole moment that is commonly ignored plays a crucial role. Moreover, the intrinsic mechanism of the chiral response is also intuitively explained with linear polarization analysis. The mechanism proposed here is not only applicable for biomolecules and tiny particles with low-order resonance modes but also works well for sub-wavelength particles with high-order modes, where conventional mixed electric and magnetic polarizability mechanisms usually do not work very well.