In this letter, we first analyze the dispersion relation for achromatic focusing and obtain the achromatic focusing conditions for discretized unit cells of flat lenses. Then, we propose to engineer the dispersion of spoof surface plasmon polaritons (SSPPs) to satisfy the achromatic focusing conditions. Metallic blades structures are utilized to achieve the linear dispersion response by tailoring the weak dispersion region of SSPPs. A broadband achromatic flat focusing lens (AFFL) is implemented with delicate combinations of the blade structures. A prototype was designed, fabricated, and measured. Both the simulated and experimental results demonstrate that the proposed AFFL can achieve achromatic focusing from 7.5 to 9.0 GHz under the normal incidence.

Metasurfaces have provided an unprecedented degree of freedom (DOF) in the manipulation of electromagnetic waves. A geometric phase can be readily obtained by rotating the meta-atoms of a metasurface. Nevertheless, such geometric phases are usually spin-coupled, with the same magnitude but opposite signs for left- and right-handed circularly polarized (LCP and RCP) waves. To achieve independent control of LCP and RCP waves, it is crucial to obtain spin-decoupled geometric phases. In this paper, we propose to obtain completely spin-decoupled geometric phases by engineering the surface current paths on meta-atoms. Based on the rotational Doppler effect, the rotation manner is first analyzed, and it is found that the generation of a geometric phase lies in the rotation of the surface current paths on meta-atoms. Since the induced surface current paths under the LCP and RCP waves always start oppositely and are mirror-symmetrical with each other, it is natural that the geometric phases have the same magnitude and opposite signs when the meta-atoms are rotated. To obtain spin-decoupled geometric phases, the induced surface current under one spin should be rotated by one angle while the current under the other spin is rotated by a different angles. In this way, LCP and RCP waves can acquire different geometric phase changes. Proof-of-principle prototypes were designed, fabricated, and measured. Both the simulation and experiment results verify spin-decoupled geometric phases. This work provides a robust means to obtain a spin-dependent geometric phase and can be readily extended to higher frequency bands such as the terahertz, IR, and optical regimes.

In this paper, we propose the design of wideband planar retro-reflective metasurface (WRRM) that can enhance backscattering under oblique incidence. By a delicate combination of narrow-band RRMs (NRRMs), the bandwidth with backscattering enhancement can be expanded significantly. As an example, a prototype operating in the X band is demonstrated. The modified metallic square loop structure is used as the sub-cell of four NRRMs. The operating bands of the four RRMs are tailored to act as quarter structures of the WRRM. A prototype was fabricated and measured. Both the simulation and experiment results verify that the WRRM can realize good backscattering enhancement performance in 8.0–12.0 GHz under incidence of angle θ = 20.0°.