The recently proposed digital coding metasurfaces make it possible to control electromagnetic (EM) waves in real time, and allow the implementation of many different functionalities in a programmable way. However, current configurations are only space-encoded, and do not exploit the temporal dimension. Here, we propose a general theory of space-time modulated digital coding metasurfaces to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously. As proof-of-principle application examples, we consider harmonic beam steering, beam shaping, and scattering-signature control. For validation, we realize a prototype controlled by a field-programmable gate array, which implements the harmonic beam steering via an optimized space-time coding sequence. Numerical and experimental results, in good agreement, demonstrate good performance of the proposed approach, with potential applications to diverse fields such as wireless communications, cognitive radars, adaptive beamforming, holographic imaging.

Metasurfaces enable a new paradigm to control electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low‐dimensionality, high transmission efficiency, real‐time continuous reconfigurability, and a wide variety of reprogrammable functions is still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. This study reports a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. The proposed design provides a unified solution to the aforementioned challenges of real‐time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, a reconfigurable metalens at the microwave frequencies is experimentally realized, which, to the best of the knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and reprogrammable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real‐time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging, and active emission control.

A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

The concept of coding metasurface makes a link between physically metamaterial particles and digital codes, and hence it is possible to perform digital signal processing on the coding metasurface to realize unusual physical phenomena. Here, this study presents to perform Fourier operations on coding metasurfaces and proposes a principle called as scattering‐pattern shift using the convolution theorem, which allows steering of the scattering pattern to an arbitrarily predesigned direction. Owing to the constant reflection amplitude of coding particles, the required coding pattern can be simply achieved by the modulus of two coding matrices. This study demonstrates that the scattering patterns that are directly calculated from the coding pattern using the Fourier transform have excellent agreements to the numerical simulations based on realistic coding structures, providing an efficient method in optimizing coding patterns to achieve predesigned scattering beams. The most important advantage of this approach over the previous schemes in producing anomalous single‐beam scattering is its flexible and continuous controls to arbitrary directions. This work opens a new route to study metamaterial from a fully digital perspective, predicting the possibility of combining conventional theorems in digital signal processing with the coding metasurface to realize more powerful manipulations of electromagnetic waves.

which can manipulate electromagnetic (EM) waves in unconventional ways, and have enabled many exotic physical phenomena and effects, also inspiring novel devices and engineering applications. [1] Their 2D versions, commonly referred to as metasurfaces, are experiencing a strong surge of interest owing to a number of attractive features, including ultrathin thickness, low loss, easy fabrication, and potential conformability. In the wake of the pioneering work by Yu et al. [2] on generalized Snell's laws enabled by imparting an abrupt phase shift, metasurfaces have demonstrated unprecedented capabilities in wavefront engineering, amplitude modulation, and polarization conversion, just to mention a few. [2][3][4][5][6] However, metasurfaces that only impart space-gradient phase discontinuities are inherently constrained by Lorentz reciprocity. This implies, for instance, that the time-reversed version of a reflected wave propagates along the same direction as the original incident wave at the same frequency.The quest for breaking reciprocity is of longstanding interest in EM engineering, and is currently eliciting renewed attention in view of its pivotal role in lifting some fundamental limitations in communication systems as well as energy harvesting and heat management. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] For instance, in wireless communication systems, a nonreciprocal antenna could radiate a very directive beam without being bound to receiving its reflected echo. [20] A common way to attain nonreciprocal effects, especially at microwave frequencies, is to break the time-reversal symmetry by means of biased magnetic materials (e.g., ferrites). These, however, are typically bulky, costly, and difficult to integrate and scale up to optical wavelengths, [10] which motivates the strong interest in magnetless approaches. Some of these are based on nonlinear materials, which are not bound by the reciprocity theorem, but are power-dependent and require sufficiently high signal intensity. [11,12] Other magnetless approaches rely on transistor-based devices [13] and moving media, [14,15] but are limited in terms of operating frequency, and are difficult to extend to the optical regime. Timevarying approaches have emerged as attractive alternatives based on time-modulated devices, [16][17][18][19][20][21][22][23][24] which have smaller size, lower cost, and better integrability. In 2015, Shaltout et al. [19] Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time-varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently...