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Optical spin-dependent beam separation in cyclic group symmetric metasurface

Abstract : Cross-polarization scattering of a circularly polarized beam from nano-rod introduces a geometric phase to the outgoing beam with opposite circular polarization. By manipulating the spatial array of subwavelength nano-structure constituting metasurface, the geometric phase can be engineered to generate a variety of beam profiles, including vortex beam carrying orbital angular momentum via a process called spin-to-orbital angular momentum conversion. Here we introduce a cyclic group symmetric metasurface composed of tapered arc nano-rods and explore how azimuthal angular distribution of total phase determines the feature of spin-dependent beam separation. When scattered from a circular array of tapered arc nano-rods possessing varying width with a fixed length, a dynamical phase having non-constant azimuthal gradient is introduced to an incoming Gaussian beam. This leads to a spin-dependent beam separation in the outgoing vortex beam profile, which is attributed to an azimuthal angle dependent destructive interference between scatterings from two plasmonic excitations along the width and the length of tapered arc nano-rod. Relation of cyclic group symmetry property of metasurface and the generated vortex beam profile is examined in detail by experimental measurement and analysis in terms of partial-wave expansion and non-constant azimuthal gradient of total phase. Capability of spatial beam profiling by spin-dependent beam separation in vortex beam generation has an important implication for spatial demultiplexing in optical communication utilizing optical angular momentum mode division multiplexing as well as for optical vortex tweezers and optical signal processing employing vortex beams. Novelty and impact statement Vortex beam generation by geometric phase is known. However, a systematic study of relating symmetry property of cyclic group C_nh and vortex beam profile is never reported. This work shows that non-constant azimuthal gradient of total phase is the key in controlling azimuthal interference pattern of vortex beam with asymmetric helical wavefront. Noting the importance of vortex beam communications, this work opens a novel way to manipulate interplay between spin-and orbital angular momentum for vortex beam profiling.
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Yeon Ui Lee, Igor Ozerov, Frédéric Bedu, Ji Su Kim, Frédéric Fages, et al.. Optical spin-dependent beam separation in cyclic group symmetric metasurface. Nanophotonics, Walter de Gruyter, 2020, 9 (10), pp.3459-3471. ⟨10.1515/nanoph-2020-0160⟩. ⟨hal-02880755⟩

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