Generation of arbitrary cylindrical vector beams using mode-converting metasurfaces

Cylindrical vector beams (CVBs) are radially polarized or azimuthally polarized beams in the radiative near field. They resist diffraction within the Fresnel zone and exhibit a tighter focusing spot compared to linearly polarized beams. Therefore, CVBs have found several applications, and multiple passive methods have been proposed to generate them. To date, passive antennas have been limited to generating standard CVBs like Gaussian or Bessel beams at microwave frequencies. Therefore, the topic of this paper is twofold. First, a passive method to generate arbitrarily defined CVBs is proposed. Second, the method is applied to realize two experimental CVB antennas. One antenna generates an orthogonal Bessel summation (OBS) beam, while the other generates the well-known radial Gaussian (RG) beam. The OBS beam is a superposition of multiple, weighted Bessel beams. Therefore, it cannot be readily realized using standard CVB antennas. It is unique in that it exhibits zero diffraction loss at a given plane tangential to the aperture, within its Fresnel zone, resulting in maximum coupling between circular apertures within the Fresnel zone. Key to the proposed method of generating CVBs is the spatially dispersive mode-converting metasurface. Unlike conventional metasurfaces, the normal power density does not have to be locally conserved across this metasurface.