Near-field polarization shaping by a near-resonant plasmonic cross antenna

The optical phase in the feed gap of a plasmonic dipole antenna shows a transition from in-phase to counter-phase response, when its length is varied across the resonance length. We exploit this behavior in an asymmetric cross antenna structure, constituted of two perpendicular dipole antennas with different lengths, sharing the same feed gap, in order to shape the local polarization state. As an application of this concept, we propose a lambda/4 nanowaveplate, able to shape and confine linearly polarized propagating waves into circularly polarized fields localized in the feed gap.