Dense Arrays of Nanohelices: Raman Scattering from Achiral Molecules Reveals the Near-Field Enhancements at Chiral Metasurfaces.

Against the background of the current healthcare and climate emergencies, surface enhanced Raman scattering (SERS) is becoming a highly topical technique for identifying and fingerprinting molecules, e.g., within viruses, bacteria, drugs, and atmospheric aerosols. Crucial for SERS is the need for substrates with strong and reproducible enhancements of the Raman signal over large areas and with a low fabrication cost. Here, dense arrays of plasmonic nanohelices (≈100 nm in length), which are of interest for many advanced nanophotonics applications, are investigated, and they are shown to present excellent SERS properties. As an illustration, two new ways to probe near-field enhancement generated with circular polarization at chiral metasurfaces are presented, first using the Raman spectra of achiral molecules (crystal violet) and second using a single, element-specific, achiral molecular vibrational mode (i.e., a single Raman peak). The nanohelices can be fabricated over large areas at a low cost and they provide strong, robust and uniform Raman enhancement. It is anticipated that these advanced materials will find broad applications in surface enhanced Raman spectroscopies and material science.