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Realization of superchiral surface lattice resonances in three-dimensional bipartite nanoparticle arrays

Optical chirality (OC) is a fundamental property of electromagnetic waves that plays a key role in governing chiral light-matter interaction. Here, we demonstrate how to obtain superchiral surface lattice resonances (SLRs), which arise from the hybridization between localized surface plasmons (LSPs) and diffractive Rayleigh anomalies (RAs), in nanoparticle arrays. We first study the coupling constants between LSPs and RAs in 2D Au monopartite nanorod arrays by angle-resolved reflectivity spectroscopy and finite-difference time-domain (FDTD) simulations. The complex dispersion relations and the near-fields of SLRs are then analyzed by temporal coupled-mode theory (CMT) for formulating the dependence of the coupling constants on the dipole orientation of the nanorod. The TE and TM coupling constants are found to depend strongly on the orientation of the dipole lying in the plane perpendicular to the propagation direction of RA. By using two orthogonally oriented nanorods, TE- and TM-SLRs can be excited independently. We then extend the CMT approach to rationally design superchiral SLRs based on 3D bipartite nanorods. The OC is shown to depend on the relative displacement between two nanorods, the near-field strength, and the interplay between the coupling constants and the Q-factor of the SLR resonance. We have achieved an averaged OC of 27 times stronger than that of the circularly polarized plane wave over the entire surface, featuring large area chiral surface waves that are useful for chirality-based applications.