Fully Metallic Flat Lens Based on Locally Twist-Symmetric Array of Complementary Split-Ring Resonators

Abstract

In this article, we demonstrate how twist symmetries can be employed in the design of flat lenses. A lens design is proposed, consisting of 13 perforated metallic sheets separated by an air gap. The perforation in the metal is a two-dimensional array of complementary split-ring resonators. In this specific design, the twist symmetry is local, as it is only applied to the unit cell of the array. Moreover, the twist symmetry is an approximation, as it is only applied to part of the unit cell. First, we demonstrate that, by varying the order of twist symmetry, the phase delay experienced by a wave propagating through the array can be accurately controlled. Secondly, a lens is designed by tailoring the unit cells throughout the aperture of the lens in order to obtain the desired phase delay. Simulation and measurement results demonstrate that the lens successfully transforms a spherical wave emanating from the focal point into a plane wave at the opposite side of the lens. The demonstrated concepts find application in future wireless communication networks where fully-metallic directive antennas are desired.

Dispersion Analysis of Periodically Loaded Transmission Lines with Twist Symmetry Using the Mode-Matching Technique

“…Specifically, the importance of the higher order modes on the dispersion properties of twist-symmetric structures is demonstrated, and it is shown that that the relevant higher order modes impact the propagation characteristics differently depending on the symmetry order. This insight is valuable for the design of filters [40], phase-shifters [42], antennas [43], lenses [44], and optical fibers [45].…”

“…Additionally, reduced frequency dispersion and an increased effective refractive index in twist-symmetric structures were demonstrated in [41]. These properties have been used to design twist-symmetric filters [40], phase-shifters [42], miniaturized helix antennas [43], lenses [44], and polarization filtering optical fibers [45]. Furthermore, in [42], a close relation between the twist symmetry and the existence of a broad pass-band was found in periodically loaded cylindrical waveguides.…”

Anisotropy Characterization of Metallic Lens Structures

“…The incidence angles from the source antenna beam to the lenses are not normal except at the center of the lens, especially in the case of multi-beam antenna sources. The incidence angle is oblique in one plane ( ϕ ) in circular metasurface lenses employed to design a waveguide crossover and multi-beam antennas [ 35 , 36 ] and oblique in two planes ( ϕ and θ ) in flat lenses [ 37 , 38 ]. The ϕ and θ represent the azimuthal angle and polar angle of the standard spherical coordinate system of the lens structure shown in Figure 1 .…”

Periodic Structures With Higher Symmetries: Their Applications in Electromagnetic Devices

“…Other applications of twist symmetry can be encountered in metasurface designs. For example, in [71], a multilayered configuration of metasurfaces based on split-ring resonators was proposed. The layered structure is locally twist-symmetric with respect to the stratification direction.…”

“…These properties can be used to tailor a flat lens, since different phase delays at each incidence point can be produced. In [71], these delays were designed to obtain a plane wave emerging from the lens. The design maintained the orientation of the split rings on the first and the last metasurfaces, so no depolarization is encountered.…”