Optical Activity in Third‐Harmonic Rayleigh Scattering: A New Route for Measuring Chirality

In 3D isotropic liquids, optical third‐harmonic generation is forbidden, with circularly polarized light (CPL). Yet the associated nonlinear susceptibility directly influences the optical properties at the fundamental frequency by intensity dependence (Kerr effect). Here, the hidden third‐harmonic optical properties upon CPL illumination are revealed by demonstrating a new effect, in hyper‐Rayleigh scattering. This effect is succinctly enunciated: the intensity of light scattered at the third‐harmonic frequency of the CPL incident light depends on the chirality of the scatterers. It is referred to as third‐harmonic (hyper) Rayleigh scattering optical activity (THRS OA) and was observed from Ag nanohelices randomly dispersed in water. The first analytical theory model for the new effect in nanohelices is also provided, highlighting the role of localized transition dipoles along the helical length. THRS OA is remarkably user‐friendly. It offers access to intricate optical properties (hyperpolarizabilities) that have so far been more easily accessible by computation and that are essential for the understanding of light−matter interactions. The new effect could find applications in hyper‐sensitive characterization of the chirality in molecules and in nanostructures; this chirality plays a fundamental role in the function of bio/nano‐machinery, with promising applications in next generation technologies. A new chiroptical effect is reported: third‐harmonic Rayleigh scattering optical activity. A silver nanohelix is illuminated with a laser beam with wavelength λ, which leads to scattering at the third‐harmonic frequency (i.e., λ/3). The intensity of the scattered light depends on the direction of circularly polarized incident light.