Near-field inter-coupled cell-less metasurface fabrics and their applications

Abstract

Metasurfaces are subwavelength-thick artificial structures with engineered re-sponses, designed to provide functionalities that do not exist in the natural do-main. Their application areas are broad and vary in both functionality and operation regime. Across all functionalities and regimes, the fundamental pur-pose behind the metasurfaces is to manipulate the surrounding electromagnetic landscape to form devices with superior sensitivity and efficiency. With conven-tional design routines and available high-index materials, this was achieved across longer wavelengths in the past decade. However, the lack of suitable materials in the higher frequencies limit the design space for the mainstream approaches that discretize the phase surface with the help of independent nanostructures, dubbed as “meta-cells” or alternatively “meta-atoms”. With increasing frequencies, the discrepancy between a smoothly-changing effective index surface and a discretized step-index surface increase, resulting in unwanted scattering. Additionally, the methodology behind the uncoupled scatterers break up as the meta-atoms act collectively in sufficiently small scales, resulting in topology-induced errors in the generated phase response. In this thesis, a new class of highly efficient meta-surfaces relying on the near-field coupling of identical scatterers in a continuous fabric is proposed. Contrary to the conventional approach, which sees the inter-cell coupling as phase distortions, our proposed methodology utilizes near-field coupling between the nearest neighbors, enabling lattice generation schemes ap-plicable at broad scales that are insusceptible to topological errors. Owing to these, this methodology offers opportunities in further miniaturization of optical consumer products. One of the functionalities high in demand from metasurfaces is efficient and achromatic focusing with compact devices in the visible range, core to contact lenses and smartphone cameras. However, the examples in the litera-ture are not sufficient in terms of either broadband performance or efficiency. Our proposed phase acquisition scheme effectively eliminates inhomogeneous scatter-ing while reducing the design procedure to the tiling of the phase surface, subject to a function of the nearest-neighbor distances. Here, with this methodology, one cylindrical and one circular achromatic metasurface lenses (metalenses) with near-diffraction-limit focusing operating across the whole visible spectrum are demonstrated as a proof of concept. The validity of the utilized approach is con-firmed via both waveguide solutions and full electromagnetic computations. Both structures proved to be highly efficient, with the cylindrical one having superior efficiency in its preferred polarization, whereas the circular one is highly efficient while operating independent of polarization. These findings prove the applicabil-ity of our near-field inter-coupled cell-less metasurface fabrics as a compact and efficient optical device generation framework.