Meso-chiral optical properties of plasmonic nanoparticles: uncovering hidden chirality

Molecular chirality plays an important role in chemistry and biology, allows control of biological interactions, affects drug efficacy and safety, and promotes synthesis of new materials. In general, chirality manifests itself in optical activity (circular dichroism and/or circular birefringence). Chiral plasmonic nanoparticles have been recently developed for molecular enantiomer separation, chiral sensing and chiral photocatalysis. Here, we show that optical chirality of plasmonic nanoparticles exhibiting strong scattering can remain completely undetected using standard characterisation techniques, such as circular dichroism measurements. This phenomenon, which we term meso-chiral in analogy to meso-compounds in chemistry, is based on mutual cancellation of absorption and scattering chiral responses. As a prominent example, the meso-chiral behaviour has been numerically demonstrated in multi-wound-SiO₂/Au nanoparticles over the entire visible spectral range and in other prototypical chiral nanoparticles in narrower spectral ranges. The meso-chiral property has been experimentally verified by demonstrating chiral absorption of gold helicoid nanoparticles at the wavelength where conventional circular dichroism measurements show absence of a chiral response. These findings demonstrate a valuable link between microscopic and macroscopic manifestations of chirality and can provide insights for interpreting a wide range of experimental results and designing chiral properties of plasmonic nanoparticles.