Observation of polarization-maintaining near-field directionality

Abstract Directional and highly-efficient excitation of guided waves is closely related to the on-chip information processing and is of fundamental importance to plasmonics, nanophotonics, and chiral quantum optics. However, during the directional coupling between propagating waves and guided waves, there is a loss of information about the incident polarization state. It remains elusive and challenging to preserve the incident polarization information in the near-field directionality. Here we experimentally demonstrate polarization-maintaining and polarization-dependent near-field directionality by exploiting a reflection-free, anisotropic, and gradient metasurface. The s- and p- polarized guided waves are excited only by the s- and p-polarized components of incident waves, respectively, and they propagate predominantly to opposite designated directions. Remarkably, the measured coupling efficiency between propagating waves and guided waves exceeds 85% for arbitrary incident polarization states. Our work thus reveals a promising route to directly and efficiently convert the polarization-encoded photon qubits to polarization-encoded guided waves, a process that is highly sought after in the context of optical network and plasmonic circuitry.