Extrinsic Trochoidal Dichroism is Modulated by Nanoparticle Symmetry

The polarization properties of near-field confined light significantly diverge from their freely propagating counterparts. Polarized evanescent waves can support cycloidlike trochoidal field motion generated by transverse oscillations that are out-of-phase with longitudinal oscillations, which are absent from free-space light. We have recently observed that trochoidal waves with opposite rotational directions preferentially excite hybridized plasmon modes in gold nanoparticle dimers (McCarthy et al. Proc. Natl. Acad. Sci. U. S. A. 2020, 117, 16143-16148). However, the symmetry properties responsible for this effect, named trochoidal dichroism, have not been investigated. Here, we lithographically fabricate nanoparticle assemblies with varying symmetry to uncover the geometric parameters driving sensitivity to trochoidal polarizations. We find that while symmetric structures, such as single nanorods, do not exhibit trochoidal dichroism, asymmetric nanoparticle arrangements facilitating a planar rotation of dipole orientations, with one or fewer planes of mirror symmetry, are trochoidal active. In particular, fan-shaped nanorod assemblies tracing out the arch of the trochoidal field exhibit polarization-selective promotion of their bonding and antibonding hybridized plasmons. By characterizing trochoidal dichroism as a function of nanoparticle symmetry, we gain a deeper understanding of efficient nanoantenna design principles and molecular geometries that can be probed using this novel light-matter interaction.