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Real-Space Mapping of the Chiral Near-Field Distributions in Spiral Antennas and Planar Metasurfaces

M. Schnell, P. Sarriugarte, T. Neuman, A. B. Khanikaev, G. Shvets, J. Aizpurua, and R. Hillenbrand

Nano Letters, 2016, 16 (1), pp 663–670 (2015)
Chiral antennas and metasurfaces can be designed to react differently to left- and right-handed circularly polarized light, which enables novel optical properties such as giant optical activity and negative refraction. Here, we demonstrate that the underlying chiral near-field distributions can be directly mapped with scattering-type scanning near-field optical microscopy employing circularly polarized illumination. We apply our technique to visualize, for the first time, the circular-polarization selective nanofocusing of infrared light in Archimedean spiral antennas, and explain this chiral optical effect by directional launching of traveling waves in analogy to antenna theory. Moreover, we near-field image single-layer rosette and asymmetric dipole–monopole metasurfaces and find negligible and strong chiral optical near-field contrast, respectively. Our technique paves the way for near-field characterization of optical chirality in metal nanostructures, which will be essential for the future development of chiral antennas and metasurfaces and their applications.