Decoding chirality at the nanoscale with momentum-space polarimetry.

Engineering optical chirality at the nanoscale has unlocked a wide range of light-matter interactions, with implications for the controlled manipulation of photonic degrees of freedom, ultrasensitive enantiomer detection, structured illumination microscopy, and quantum communication. Efficient characterization of chiral nanostructures is therefore of paramount importance, as it provides direct insights into their chiro-optical responses and guides the rational design of next-generation nanodevices. Conventional chiro-optical techniques, however, often fall short due to intrinsic limitations, such as their inability to probe spatially and angularly inhomogeneous chirality or to disentangle coexisting linear and circular anisotropies. Here, we present a Fourier-domain polarimetric framework to investigate the chiro-optical responses of plasmonic gammadion nanoarrays. By mapping scattered polarization states in momentum space through Stokes-Mueller polarimetry, we capture inhomogeneous radiation patterns that encode the underlying electromagnetic modes and diffraction features governing the observed chirality within the nanostructured system. The momentum-resolved Mueller matrix not only enables simultaneous quantification of circular birefringence and circular diattenuation but also facilitates their decoupling from linear anisotropies, thereby providing a comprehensive characterization of intrinsic chiro-optical behavior. We further show how structural thickness modulates the chiral response and demonstrate the sensitivity of this approach in detecting subtle chiro-optical signals. Finally, we combine gammadions arrays with momentum-domain chiral measurements as a sensitive platform for molecular enantiomer detection, opening new opportunities for advanced chiral sensing applications.