Large-Area Metallic Nanohelices for Engineering Optical Chirality.

Submicrometer helical structures possess unique properties and functionalities arising from their intrinsic twisting nature. However, existing fabrication methods are often limited to small sample sizes, hindering comprehensive characterization and practical implementation. In this study, a scalable self-assembly method for fabricating metallic helices through a transformation of planar microstrips into helical geometries upon their release from the substrate is presented. This technique exploits the nanoscale engineering of residual stress and gradient strains within the metallic strips to induce spontaneous folding and twisting. This method allows for large-scale production while providing precise control over structural parameters. Notably, the ability to produce centimeter-scale samples has facilitated the characterization of chiroptical properties using standard spectrometers. Experimental measurements and numerical simulations reveal pronounced chiroptical responses in the mid- and near-infrared regions for helices with radii on the order of hundreds of nanometers. These findings clarify the geometric factors that influence the excitation of chiral eigenmodes and thereby establish a straightforward framework for designing metallic helices as chiral plasmonic structures tailored to specific operating wavelengths. This study marks a significant advancement in the fabrication of helical and 3D nanostructures, with potential implications for photonics, stereochemistry, and chiroptical spectroscopy.