First Observation of Optical Activity in Hyper-Rayleigh Scattering

Chiral nano- or metamaterials and surfaces enable striking photonic properties, such as negative refractive index and superchiral light, driving promising applications in novel optical components, nanorobotics, and enhanced chiral molecular interactions with light. In characterizing chirality, although nonlinear chiroptical techniques are typically much more sensitive than their linear optical counterparts, separating true chirality from anisotropy is a major challenge. Here, we report the first observation of optical activity in second-harmonic hyper-Rayleigh scattering (HRS). We demonstrate the effect in a 3D isotropic suspension of Ag nanohelices in water. The effect is 5 orders of magnitude stronger than linear optical activity and is well pronounced above the multiphoton luminescence background. Because of its sensitivity, isotropic environment, and straightforward experimental geometry, HRS optical activity constitutes a fundamental experimental breakthrough in chiral photonics for media including nanomaterials, metamaterials, and chemical molecules

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

peer reviewedAgainst 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, we investigate dense arrays of plasmonic nanohelices (∼100 nm in length) that are of interest for many advanced nanophotonics applications, and we show that they present excellent SERS properties. As an illustration, we present two new ways to probe near-field enhancement generated with circular polarization at chiral metasurfaces, 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. We anticipate that these advanced materials will find broad applications in surface enhanced Raman spectroscopies and material science