Enhanced Circular Dichroism in 3D Helical-Shaped Chiral Plasmonic Metamaterials Assembled from Colloidal Nanocrystals

Abstract Circular dichroism (CD) in plasmonic metamaterials can be strengthened by engineering three-dimensional (3D) chiral geometry and electromagnetic coupling. We report 3D helical-shaped plasmonic metamaterials assembled from colloidal Au nanocrystal/bulk Au bilayer heterostructures. Arrays of planar spiral patterns defined by nanoimprint lithography are transformed into 3D helical metamaterials through strain-driven, out-of-plane folding induced by ligand exchange and annealing. Compared with multiple-short-arm (4C) references having the same total arm length, single-helical (1H) structures exhibit stronger differential transmittance between left- and right-handed circularly polarized light (ΔT). Adding a second helical arm to form double-helical (2H) structures further increases ΔT to nearly 70% in the mid-infrared range. Electromagnetic simulations reveal that intra-meta-atom coupling, antenna loading, and mode hybridization reshape the resonances and amplify CD beyond the response of isolated components. This work demonstrates that 3D helical architectures can significantly strengthen plasmonic CD responses and provides a scalable nanocrystal-based platform for designing compact mid-infrared chiroptical devices.