Polarization-Resolved Characterization of Antenna-Mediated Scattering in Transflection s-SNOM

Optical antennas are widely used to enhance light–matter interactions at the nanoscale, enabling applications in surface-enhanced infrared absorption, Raman scattering, and circular dichroism spectroscopy. These spectroscopies rely on the antenna’s ability to locally enhance incident electromagnetic fields and, conversely, to enhance scattering from nanoscale emitters within the near field. Scattering-type scanning near-field optical microscopy (s-SNOM) provides a means to study this antenna-mediated scattering, with the s-SNOM tip acting as a local scatterer. However, in conventional setups with top-side illumination and detection, interference between tip and antenna scattering challenges studies beyond linearly polarized illumination. Here, we use transflection s-SNOM with normal-incidence illumination and detection to demonstrate polarization-resolved measurements of antenna-mediated tip scattering. We derive an analytical relation between the measured signal and the antenna’s near-field enhancement tensor, and validate it experimentally with mid-infrared metallic disk and spiral antennas. Particularly, we show that from four measurements using only linear polarizers it is possible to obtain results corresponding to illumination with circular polarization. Generally, our approach links the local near-field enhancement of complex antenna structures to the antenna-mediated polarized scattering of a nanoscale object (a tip in this work) placed in their vicinity, thereby establishing a new route for broadband spectroscopy studies of field-enhanced chiral and anisotropic light–matter interactions at the nanoscale.