Cylindrical Vector Beam Generation Using Spatially-Dispersive, Mode-Converting Metasurfaces

Abstract

The year 1972 marks the date of the first experimental demonstration of what is known today as a Cylindrical Vector Beam (CVB). Since 1972, multiple passive antennas have been proposed to generate Cylindrical Vector Beams (CVBs). However, up to date there is one common downside between all the proposed passive CVBs antennas. Current passive CVBs antennas can be used to generate only standard beams, such as Gaussian beams or Bessel beams, and arbitrarily defined CVBs are generated only using active antennas. In this thesis, the mode converting capabilities of metasurfaces are used to design passive antennas that can generate arbitrarily defined, axially symmetric CVBs in the radiative near field. Generating arbitrarily defined CVBs, opens the door to a new class of CVBs that can be optimized for specific applications or functions. For example, using this method, the long-standing problem of maximum coupling between circular apertures within the Fresnel zone is tackled. The problem of maximum coupling between apertures is considered to demonstrate the potential of the proposed CVB antenna over standard CVB antennas.

The proposed CVB antenna consists of a mode-converting metasurface atop a coaxially-fed radial cavity. The mode-converting metasurface is a passive and lossless device that can be designed to transform a set of incident modes to a desired set of reflected/transmitted modes. Notably, the normal power density does not have to be conserved locally across a mode-converting metasurface. Mode-converting metasurfaces can abruptly change the amplitude and the phase profiles of incident fields, which allows for extreme control over the transmitted and reflected fields. Consequently, a mode-converting metasurface can shape the field radiated by a radial cavity at very close distances from the cavity’s aperture, or even directly at the cavity's aperture. At the same time, a mode-converting metasurface can control the reflected fields to match a coaxial feed to the antenna, eliminating the need of an additional matching circuitry.

Finally, the design of two CVB antennas at 30 GHz is reported in this thesis to demonstrate the utility and versatility of mode-converting metasurfaces in antenna design. Moreover, measured results of a fabricated CVB antenna are reported. The fabricated CVB antenna is designed to generate a radial Gaussian (RG) beam.