Effects of plasmon coupling on circular dichroism of chiral nanoparticle arrays

Arrays and ensembles of chiral nanostructures have potential applications in the field of enantioselective sensors, metamaterials, and metasurfaces. In particular, chiral nanostructures fabricated through chemical and bottom-up approaches have attracted much attention from the viewpoint of cost and scalability, but the heterogeneity of the unit nanostructure constituting the array or ensemble often deteriorates its chiroptical responses. Here, we report that their deteriorated responses can be recovered or even enhanced further by interparticle plasmon coupling. We employed chiral silver (Ag) hexamers as models for electromagnetic simulations and investigated the effect of their parameters, such as interparticle spacing, chiral purity, and enantiomeric excess, on their g-factor, which is an index for chiroptical responses. The maximum value of g-factor (g_max) of the Ag hexamer surpasses that of the chiral monomer and augments with decreasing interparticle spacing. This enhancement in g-factor is observed even when chiral purity and enantiomeric excess are less than 100%, showing the potent role of plasmon coupling in amplifying chiroptical responses. Furthermore, our research highlights the amplification of the effect of plasmon coupling on the g_max value of infinite periodic chiral nanostructures. These results corroborate the potential of plasmon coupling to improve chiroptical responses by precisely controlling the interparticle spacing of chiral plasmonic nanostructures, thus mitigating the loss of g-factor caused by low purity and enantiomeric excess of the nanostructures fabricated by chemical and bottom-up approaches.