Anomalous three-dimensional refraction in the microwave region by ultra-thin high efficiency metalens with phase discontinuities in orthogonal directions

Abstract

An ultrathin flat metalens that experimentally realizes three-dimensional microwave manipulation has been demonstrated as able to approach the theoretical limit of cross-polarization (cross-pol) conversion efficiency of the transmission, as predicted by Monticone et al (2013 Phys. Rev. Lett. 110 203903). The helicity-dependent phase change is introduced to the transmission and can be engineered by assembling the spatial orientation of each Pancharatnam-Berry phase element. By realizing the constant phase gradients in orthogonal directions, an anomalous non-coplanar refraction is unanimously demonstrated in the three-dimensional space under the circular-polarized incidence, and the refraction angle is well predicted with the generalized Snell's law, derived with phase gradients in orthogonal directions. More importantly, the maximum conversion efficiency of the cross-pol transmission is as high as 24%, which approaches the upper-bound of the theoretical limit. The proposed metalens has only a single layer as thin as 0.001 λ, which massively reduces the thickness of the microwave lens along the wave propagation direction. With the great improvements in efficiency and the thickness reduction, as well as the Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. excellent non-coplanar refraction, our design provides a promising approach to miniaturize, planarize and integrate multiple microwave components.The manipulation of electromagnetic (EM) waves has always been a hot topic. The traditional means, which includes using dielectric materials (lenses) and metallic surfaces (antennae), show their abilities to control the transmission of EM waves; however, they both hold inherent limitations in the microwave region. For lenses, the phase accumulation, which is the origin for the specific refraction effect, comes from either the specific surface topography or the spatial variation of the refractive index of the lens. Thickness is therefore an essential factor for realizing specific functions [1]. For antennae, both the size of a single antenna and the distance between the antenna cells for the array are of the same order of magnitude as the working wavelength, which means either a single antenna an or antenna array cannot be treated as effective media [2][3][4]. Furthermore, the microwave-phased array antenna, which is composed of a great number of bulky antennae, feeding networks and sources, can hardly be integrated with other equipment. Although the rise of metamaterials and transform optics provides new approaches for the design of lenses and antennae [5][6][7], the inherent limitations still cannot be conquered.The concept of an abrupt phase change at the interface provides a powerful solution to overcome the limitations mentioned above [8]. By introducing abrupt phase changes with subwavelength unit cells into the cross-polarized (cross...

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