Microwave Space-Time-Modulated Metasurfaces

Abstract

Over the past decade, static metasurfaces have proved to be low-profile and efficient apparatuses for transformation of electromagnetic waves. However, such metasurfaces are restricted by their reciprocal and time-and frequency-invariant responses. To overcome these restrictions, space-time-modulated metasurfaces have recently been introduced for versatile, reciprocal/ nonreciprocal, and frequency translation of electromagnetic waves. These are capable of changing both the momentum and energy of the incident wave and provide functionalities that are far beyond the capabilities of conventional static and reciprocal metasurfaces. This Perspective provides a discussion on the unique functionalities of a microwave space-time-modulated metasurface. In particular, we review various techniques that have been recently used for the realization of metasurfaces introducing spatiotemporal decomposition, scattering and diffraction, digital coding, nonreciprocal transmission, serrodyne frequency translation, pure frequency conversion, parametric wave amplification, and multifunctional operations. Although the paper focuses on microwave space-time metasurfaces, the described concepts can inspire realization of their optical counterparts.

Time-varying Metallic Interfaces

“…7(c)]. This means that great amount of power could be transferred from the incident wave to (−1)-th harmonic, fact that is of potential interest to be exploited in engineering for beamforming purposes and the creation of analog mixers [34]. Furthermore, the symmetry of the considered tangential field E(t) causes that higher-order even harmonics are null, fact that is corroborated by the FDTD simulation.…”

“…Circuit-model interpretations have also been reported based on shunt topologies derived from admittance matrices [32], [33]. In most cases, direct comparisons with results provided by FDTD are needed to validate the models [34].…”

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

“…They offer a good performance in most of the features highlighted in Table 1. Actually, Floquet modal expansions (together with FDTD and homogenization methods) are the first techniques to be implemented for the analysis of spacetime metamaterials (periodic configurations modulated in space and time) [187][188][189], which is one of the current hot topics in electromagnetism [190][191][192]. In fact, current commercial software fail to consider time modulations in the computation, so inhouse codes are needed to model spacetime structures.…”

Time-Modulated Transmissive Programmable Metasurface for Low Sidelobe Beam Scanning

“…As for programmable metasurfaces with multiple-bit phase resolutions, the reconfigurable unit configurations and the corresponding control circuits would be extremely complicated for practical applications, especially in a large-scale metasurfaces. To achieve a more sophisticated EM manipulation, the time-modulated array technologies [30][31][32] were introduced and combined with programmable metasurfaces, and the space-time coding digital metasurfaces were thus proposed and verified to provide flexible beam steering, beam shaping, and scattering-signature control over optimized space-time coding sequences [33][34][35]. Many other promising applications are also conducted creatively through space-time coding digital metasurfaces, for example, implementing efficient harmonic control for far-field radiation or near-field distribution [36,37], manipulating the nonreciprocal effects dynamically for frequency conversion [38][39][40], realizing smart Doppler cloaks in the microwave domain for selfadaptive anti-Doppler shift [41,42], executing dynamic and arbitrary polarization syntheses [43,44], extracting the accurate electromagnetic parameters of complex lossy dispersive materials [45], and more importantly, constructing the minimalist or secure wireless communication architectures through space-and frequency-division multiplexing [46][47][48].…”