Rigorous Modal Analysis of Chiral Nanoparticles

Abstract The physical origins of the chiroptical response from nanostructures are highly demanded for chiral photonics. The chiroptical response of a nanoparticle, whether measured along specific incident directions of exciting waves (when immobilized onto a substrate) or in solution through averaging responses across all structure orientations, is believed to stem from the interplay between three key factors: geometrical chirality, far‐field chirality, and near‐field chirality. This study aims to understand the relationship among these factors using a rigorous quasinormal mode (QNM) expansion method. By decomposing circular dichroism (CD) spectra into individual QNM contributions, it is observed that the chiroptical response with respect to incident directions depends on the extent of field overlapping between the QNM and light. In contrast, the near‐field optical chirality (OC) is determined by the QNMs of the nanoparticle, independent of external excitations. As a proof‐of‐concept, a nanohelix metasurface is proposed that employs lossless gold and exploits the physics of bound states in the continuum to maximize the CD signal.