Microwave Metasurfaces

Metasurfaces are planar arrays of subwavelength elements that can efficiently control electromagnetic reflection, refraction, and scattering, being able to replace bulky components with ultrathin, engineered interfaces. This article traces their evolution in the past years with an emphasis on their applications in microwave engineering settings. We feature three classes of microwave metasurfaces: local surfaces, characterized by a tailored surface impedance that enables full control over the phase, amplitude, and polarization of the scattered waves; nonlocal architectures, which have been expanding this paradigm by introducing engineered coupling among the constituent elements of the metasurface and engineered dispersion, enabling angular and spectral selectivity, analog signal processing, and spatial compression; and time-varying and space-time-modulated metasurfaces, which further extend the design space, enabling broadband absorption and magnet-free nonreciprocal propagation, among other features that linear, time-invariant designs cannot support. Finally, digital space-time-coding metasurfaces (STCMs) unify these advances within programmable hardware, offering real-time reconfigurability, multifunctional beam control, and dynamically reconfigurable analog processing. Metasurfaces are rapidly expanding to become an essential tool for microwave engineering, thanks to their agility and ubiquitous relevance in many emerging application spaces.