Abstract
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.
Highlights
We introduce metasurface composed of circular array of tapered arc (TA), which belong to a well-defined cyclic group Cnh to study the relationship between cyclic group symmetry property of metasurface and the generated vortex beam profile
Tapered arc cyclic group symmetric metasurface is introduced to explore the details of optical spin-dependent beam separation in spin-to-orbital angular momentum conversion
Presence of non-constant azimuthal gradient of total phase is found to be responsible for azimuthal interference pattern in vortex beams with asymmetric helical wavefront
Summary
Since the pioneering work of Hasman group on Pancharatnam-Berry (PB) phase to manipulate wavefront of optical beam, [1,2,3,4,5] application of geometric phase has been expanded to metasufrace to open a research field of flat optics. [6,7,8,9,10] PB phase is a geometric phase, associated not with optical path but with polarization, which can be utilized for beam deflection or vortex beam generation, when a cross-polarization scattering of circular polarized light (CPL) takes place in an array of sub-wavelength nano-rods. [1,2,3]. When nano-rods of the same rectangular shape are arrayed head-to-tail in a circle, the geometric phase ΦPB(φ) introduced by two neighboring nano-rods increases linearly along azimuthal direction, leading to a helical wavefront formation to generate a vortex beam of topological charge ±2. There are several ways to relate the feature of a spindependent beam separation in vortex beam generation with the presence of a non-constant azimuthal gradient of total phase ∇φΦtot, which is the main topic of this work. Sample fabrication of Cnh TA-CGSM and spindependent beam profile measurement are presented in Section 4 including elucidation of symmetry properties of Cnh. In Section 5 we discuss spin-dependent beam separation in terms of partial-wave expansion, azimuthal interference, and a non-constant azimuthal gradient of total phase ∇φΦtot. Wavelength dispersion of spindependent beam separation of vortex beam is discussed in terms of photonic spin Hall effect
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