This paper details a class of metal-based space-time metasurfaces for application in wireless communications scenarios. Concretely, we describe space-time metasurfaces that periodically alternate their properties in time between three spatial states: “air,” “conductor,” and “grating.” We analyze the physics of these metastructures via a computationally efficient analytical technique based on the use of Floquet-Bloch series, integral equations, and circuit models. By doing so, we reveal features of these spatiotemporal metasurfaces: scattering parameters, field profiles, diffraction angles, and the nature of the space-time harmonics. The results, corroborated with a self-implemented numerical finite-difference time-domain approach, show the potential application of these space-time metasurfaces as beamformers acting in reflection, in transmission or both. The amplitude and direction of the diffracted orders can be electronically controlled with the parameters of the metasurface. Moreover, the intrinsic ability of time-modulated diffractive metasurfaces to mix and multiply frequencies is tested. We show how two different modulations can lead to the same diffraction angle but with different mixed output frequencies.

In this paper, we study the scattering and diffraction phenomena in time-modulated metamaterials of metallic nature by means of Floquet equivalent circuits. Concretely, we focus on a time-periodic screen that alternates between “metal" and “air" states. We generalize our previous approaches by introducing the concepts of “macroperiod" and “duty cycle" to the time modulation. This allows to analyze time-periodic metallic metamaterials whose modulation ratios are, in general, rational numbers. Furthermore, with the introduction of the duty cycle, perfect temporal symmetry is broken within the time modulation as the time screen could remain a different amount of time in metal and air states. Previous statements lead to an enrichment of the diffraction phenomenon and to additional degrees of freedom that can be exploited in engineering to control the reflection and transmission of electromagnetic waves. Finally, we present some analytical results that are validated with a self-implemented finite-difference time-domain (FDTD) approach. Results show that the scattering level and diffraction modes can be controlled independently by means of the duty cycle and the modulation ratio, respectively, leading to an efficient design of time-based pulsed sources and beamformers.

Recently, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison with the analysis of their one-dimensional and two-dimensional counterparts due to the intricate geometries involved. In this paper, we present a fully analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared with full-wave solutions and also works under oblique-incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows one to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in cst. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarization states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated here.

A 3-D printed Fresnel-lens antenna formed by dielectric unit cells insensitive to polarization is presented in this article. The proposed unit cell can be implemented in any azimuth orientation, simplifying the design and the implementation of the Fresnel subzones, which is an advantage over the previous 3-D-printed Fresnel-lens designs. The unit cell exhibits a T-shaped geometry capable of providing no change in relative permittivity under TE polarizations orthogonal to each other. The novel design of the unit cell also provides robustness under oblique incidence and frequency. These features allow the radial arrangement of the unit cells to configure the subzones of the Fresnel lens, ensuring the desired relative permittivity. Additionally, the geometry of the printed unit cells enables self-supported subzones with the minimum number of unit cells per subzone. A 3-D-printed prototype of the proposed Fresnel lens was manufactured by stereolithography (SLA). The measurement results showed a good agreement with the simulated ones. The measured gain was 26.5 ± 0.5 dBi from 55 GHz to 65 GHz with a mean antenna efficiency of 79%.

This manuscript introduces an analytical framework for the design of reconfigurable metasurfaces rooted in 2D periodic structures. Incorporating lumped elements into the model streamlines the design process for such devices, offering significantly improved efficiency compared to designs reliant on full-wave software. The theoretical framework, founded on Floquet modal expansions, also guarantees robust performance even under oblique incidence conditions, far beyond the onset of the first grating lobe. In practical applications, an absorber has been devised by integrating a resistor into the model. Nevertheless, circuit analysis extends the possibilities for crafting more intricate structures utilizing diodes or tunable varactors, enabling the design of phase shifters, polarizers, and reconfigurable intelligent surfaces (RIS).

This paper presents a novel full-metal 3D periodic structure that allows for the excitation of phase resonance. This phenomenon, thanks to the 3D geometry, enables the individual control of the phase response of each polarization along the frequency band. This structure is inspired on square waveguides that have open resonators slotted on the walls. An extended study has been carried out by modelling the unit cell via equivalent circuits with the goal of developing a powerful optimization tool instead of using more resource-consuming software. An ultra-wideband electric-field rotator covering a bandwidth of 141.8% has been designed with the aid of the circuit model and has been manufactured via 3D-printing laser powder bed fusion (LPBF). The experimental results have been validated with success.